MAX782CBX-T ,Triple-Output Power Supply Controller for Notebook ComputersApplications Q36 31 BST3Notebook Computers7 30Q2 DL3Portable Data TerminalsQ1 8 29V+Communicating C ..
MAX782RCBX ,Triple-Output Power-Supply Controller for Notebook ComputersFeatures' Dual PWM Buck Controllers (+3.3V and +5V)The MAX782 is a system-engineered power-supply c ..
MAX783CBX ,Triple-Output Power-Supply Controller for Notebook ComputersFeatures' Dual PWM Buck Controllers (+3.3V and +5V)The MAX783 is a system-engineered power-supply c ..
MAX783EBX ,Triple-Output Power-Supply Controller for Notebook ComputersELECTRICAL CHARACTERISTICS———–(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0mA, SHDN = ON3 = ON5 = 5V, ..
MAX7841 ,Octal, 14-Bit Voltage-Output DAC with Parallel InterfaceApplicationsREFAB+ 4 30 REFGH+V 5 29 CLRDDV 6 28 DB13SSMX7841LDAC 7 27 DB12A2 8 26 DB11A1 9 25 DB10 ..
MAX786CAI ,Dual-Output Power-Supply Controller for Notebook ComputersMAX78619-0160; Rev 2; 4/97Dual-Output Power-Supply Controller for Notebook Computers_______________ ..
MB6M ,MINIATURE GLASS PASSIVATED SINGLE-PHASE BRIDGE RECTIFIERThermal Characteristics (TA = 25°C unless otherwise noted)Parameter Symbol MB2M MB4M MB6M UnitDevic ..
MB6S ,Bridge RectifiersThermal Characteristics (T = 25°C unless otherwise noted)AParameter Symbol MB2S MB4S MB6S UnitDevic ..
MB7117E , Schottky TTL 2048-Bit Bipolar Programmable Read-Only Memory
MB71A38-25 , PROGRAMMABLE SCHOTTKY 16384-BIT READ ONLY MEMORY
MB8117800A-60 ,2 M X 8 BIT FAST PAGE MODE DYNAMIC RAMapplications where very low power dissipation and high bandwidth are basic requirements of the desi ..
MAX782CBX-MAX782RCBX
Triple-Output Power-Supply Controller for Notebook Computers
_______________General DescriptionThe MAX782 is a system-engineered power-supply con-
troller for notebook computers or similar battery-powered
equipment. It provides two high-performance step-down
(buck) pulse-width modulators (PWMs) for +3.3V and +5V,
and dual PCMCIA VPP outputs powered by an integral fly-
back winding controller. Other functions include dual, low-
dropout, micropower linear regulators for CMOS/RTC back-
up, and three precision low-battery-detection comparators.
High efficiency (95% at 2A; greater than 80% at loads
from 5mA to 3A) is achieved through synchronous recti-
fication and PWM operation at heavy loads, and Idle-
ModeTMoperation at light loads. It uses physically
small components, thanks to high operating frequen-
cies (300kHz/200kHz) and a new current-mode PWM
architecture that allows for output filter capacitors as
small as 30µF per ampere of load. Line- and load-tran-
sient response are terrific, with a high 60kHz unity-gain
crossover frequency allowing output transients to be
corrected within four or five clock cycles. Low system
cost is achieved through a high level of integration and
the use of low-cost, external N-channel MOSFETs. The
integral flyback winding controller provides a low-cost,
+15V high-side output that regulates even in the
absence of a load on the main output.
Other features include low-noise, fixed-frequency PWM
operation at moderate to heavy loads and a synchroniz-
able oscillator for noise-sensitive applications such as
electromagnetic pen-based systems and communicat-
ing computers. The MAX782 is a monolithic BiCMOS IC
available in fine-pitch, SSOP surface-mount packages.
_______________________ApplicationsNotebook Computers
Portable Data Terminals
Communicating Computers
Pen-Entry Systems
___________________________FeaturesDual PWM Buck Controllers (+3.3V and +5V)Dual PCMCIA VPP Outputs (0V/5V/12V)Three Precision Comparators or Level Translators95% Efficiency420µA Quiescent Current;
70µA in Standby (linear regulators alive)5.5V to 30V Input RangeSmall SSOP PackageFixed Output Voltages Available:
3.3 (standard)
3.45 (High-Speed Pentium™)
3.6 (PowerPC™)
______________Ordering Information
MAX782riple-Output Power-Supply
Controller for Notebook Computers
________________________________________________________________Maxim Integrated Products1
__________________Pin Configuration
______Typical Application Diagram
Call toll free 1-800-998-8800 for free samples or literature.™Idle-Mode is a trademark of Maxim Integrated Products.Pentium is a trademark of Intel. PowerPC is a trademark of IBM.
Ordering Information continued on last page.
MAX782riple-Output Power-Supply
Controller for Notebook Computers_______________________________________________________________________________________V+ to GND.................................................................-0.3V, +36V
PGND to GND........................................................................±2V
VL to GND...................................................................-0.3V, +7V
BST3, BST5 to GND..................................................-0.3V, +36V
LX3 to BST3.................................................................-7V, +0.3V
LX5 to BST5.................................................................-7V, +0.3V
Inputs/Outputs to GND
(D1-D3, ON5, REF, SYNC, DA1, DA0, DB1, DB0, ON5,
SS5, CS5, FB5, CS3, FB3, SS3, ON3)..........-0.3V, (VL + 0.3V)
VDD to GND.................................................................-0.3V, 20V
VPPA, VPPB to GND.....................................-0.3V, (VDD + 0.3V)
VH to GND...................................................................-0.3V, 20V
Q1-Q3 to GND.................................................-0.3V, (VH + 0.3V)
DL3, DL5 to PGND...........................................-0.3V, (VL + 0.3V)
DH3 to LX3..................................................-0.3V, (BST3 + 0.3V)
DH5 to LX5..................................................-0.3V, (BST5 + 0.3V)
REF, VL, VPP Short to GND........................................Momentary
REF Current.........................................................................20mA
VL Current...........................................................................50mA
VPPA, VPPB Current.........................................................100mA
Continuous Power Dissipation (TA= +70°C)
SSOP (derate 11.76mW/°C above +70°C)...................941mW
Operating Temperature Ranges:
MAX782CBX/MAX782__CBX...............................0°C to +70°C
MAX782EBX/MAX782__EBX............................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
ELECTRICAL CHARACTERISTICS(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0mA, ON3 = ON5 = 5V, other digital input levels are 0V or +5V, TA = TMINto TMAX,
unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings‘” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ABSOLUTE MAXIMUM RATINGS
MAX782riple-Output Power-Supply
Controller for Notebook Computers
_______________________________________________________________________________________3
Note 1:Output current is further limited by maximum allowable package power dissipation.
Note 2:Since the reference uses VL as its supply, V+ line regulation error is insignificant.
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0mA, ON3 = ON5 = 5V, other digital input levels are 0V or +5V, TA = TMINto TMAX,
unless otherwise noted.)
MAX782riple-Output Power-Supply
Controller for Notebook Computers_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics(Circuit of Figure 1, Transpower transformer type TTI5870, TA= +25°C, unless otherwise noted.)
EFFICIENCY vs.
+3.3V OUTPUT CURRENT, 200kHz
+3.3V OUTPUT CURRENT (A)
EFFICIENCY (%)
IDD OUTPUT CURRENT vs. INPUT VOLTAGE
COILTRONIX CTX03-12062 TRANSFORMER
INPUT VOLTAGE (V)
IDD
LOAD CURRENT (A)25
EFFICIENCY vs.
+5V OUTPUT CURRENT, 200kHz
+5V OUTPUT CURRENT (A)
EFFICIENCY (%)
QUIESCENT INPUT CURRENT vs.
INPUT VOLTAGE
INPUT VOLTAGE (V)
INPUT CURRENT (2030
EFFICIENCY vs.
+5V OUTPUT CURRENT, 300kHz
+5V OUTPUT CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs.
+3.3V OUTPUT CURRENT, 300kHz
+3.3V OUTPUT CURRENT (A)
EFFICIENCY (%)
+5V OUTPUT CURRENT vs.
MINIMUM INPUT VOLTAGE, 200kHz
+5V LOAD CURRENT (A)
MINIMUM INPUT VOLTAGE (V)
+5V OUTPUT CURRENT vs.
MINIMUM INPUT VOLTAGE, 300kHz
+5V OUTPUT CURRENT (A)
MINIMUM INPUT VOLTAGE (V)
100μA10mA1A
SWITCHING FREQUENCY vs.
LOAD CURRENTLOAD CURRENT
SWITCHING FREQUENCY (kHz)
1mA100mA
MAX782riple-Output Power-Supply
Controller for Notebook Computers
_______________________________________________________________________________________5
_____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, Transpower transformer type TTI5870, TA= +25°C, unless otherwise noted.)
MAX782riple-Output Power-Supply
Controller for Notebook Computers_______________________________________________________________________________________
_____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, Transpower transformer type TTI5870, VDD ‡13V, TA= +25°C, unless otherwise noted.)
MAX782riple-Output Power-Supply
Controller for Notebook Computers
_______________________________________________________________________________________7
______________________________________________________________Pin Description
MAX782riple-Output Power-Supply
Controller for Notebook Computers_______________________________________________________________________________________
_________________________________________________Pin Description (continued)
Table 1. Truth Table for VPP Control Pins
_______________Detailed DescriptionThe MAX782 converts a 5.5V to 30V input to five outputs
(Figure 1). It produces two high-power, switch-mode,
pulse-width modulated (PWM) supplies, one at +5V and
the other at +3.3V. These two supplies operate at either
200kHz or 300kHz, allowing extremely small external
components to be used. Output current capability
depends on external components, and can exceed 5A
on each supply. A 15V high-side (VDD) supply is also
provided, delivering an output current that can exceed
300mA, depending on the external components chosen.
Two linear regulators supplied by the 15V VDD line cre-
ate programmable VPP supplies for PCMCIA slots.
These supplies (VPPA, VPPB) can be programmed to be
grounded or high impedance, or to deliver 5V or 12V at
up to 60mA.
An internal 5V, 25mA supply (VL) and a 3.3V, 5mA ref-
erence voltage (REF) are also generated, as shown in
Figure 2. Fault-protection circuitry shuts off the PWM
and high-side supply when the internal supplies lose
regulation.
Three precision comparators are included. Their out-
put stages permit them to be used as level translators
for driving high-side external power MOSFETs: For
example, to facilitate switching VCC lines to PCMCIA
slots.
MAX782riple-Output Power-Supply
Controller for Notebook Computers
_______________________________________________________________________________________9
+3.3V SupplyThe +3.3V supply is produced by a current-mode PWM
step-down regulator using two small N-channel MOSFETs,
a catch diode, an inductor, and a filter capacitor.
Efficiency is greatly enhanced by the use of the second
MOSFET (connected from LX3 to PGND), which acts as
a synchronous rectifier. A 100nF capacitor connected
to BST3 provides the drive voltage for the high-side
(upper) N-channel MOSFET.
A current limit set by an external sense resistor prevents
excessive inductor current during start-up or under
short-circuit conditions. A soft-start capacitor can be
chosen to tailor the rate at which the output ramps up.
This supply can be turned on by connecting ON3 to
logic high, or can be turned off by connecting ON3 to
GND. All logic levels are TTL and CMOS compatible.
+5V SupplyThe +5V output is produced by a current-mode PWM
step-down regulator similar to the +3.3V supply. This
supply uses a transformer primary as its inductor, the
secondary of which is used for the high-side (VDD)
supply. It also has current limiting and soft-start. It can
be turned off by connecting ON5 to GND, or turned on
by connecting ON5 to logic high.
The +5V supply’s dropout voltage, as configured in
Figure 1, is typically 400mV at 2A. As VINapproaches
5V, the +5V output gracefully falls with VINuntil the VL
regulator output hits its undervoltage lockout threshold.
At this point, the +5V supply turns off.
The default frequency for both PWM controllers is
300kHz (with SYNC connected to REF), but 200kHz
may be used by connecting SYNC to GND or VL.
Figure 1. MAX782 Application Circuit
MAX782riple-Output Power-Supply
Controller for Notebook Computers______________________________________________________________________________________Figure 2. MAX782 Block Diagram
MAX782riple-Output Power-Supply
Controller for Notebook Computers
______________________________________________________________________________________11Figure 3. PWM Controller Block Diagram
MAX782riple-Output Power-Supply
Controller for Notebook Computers______________________________________________________________________________________
+3.3V and +5V PWM Buck ControllersThe two current-mode PWM controllers are identical
except for different preset output voltages and the
addition of a flyback winding control loop to the +5V
side (see Figure 3, +3.3V/+5V PWM Controller Block
Diagram). Each PWM is independent except for being
synchronized to a master oscillator and sharing a com-
mon reference (REF) and logic supply (VL). Each PWM
can be turned on and off separately via ON3 and ON5.
The PWMs are a direct-summing type, lacking a tradi-
tional integrator-type error amplifier and the phase shift
associated with it. They therefore do not require any
external feedback compensation components if the fil-
ter capacitor ESR guidelines given in the Design
Procedureare followed.
The main gain block is an open-loop comparator that
sums four input signals: an output voltage error signal,
current-sense signal, slope-compensation ramp, and
precision voltage reference. This direct-summing
method approaches the ideal of cycle-by-cycle control
of the output voltage. Under heavy loads, the controller
operates in full PWM mode. Every pulse from the oscil-
lator sets the output latch and turns on the high-side
switch for a period determined by the duty cycle
(approximately VOUT/VIN). As the high-side switch turns
off, the synchronous rectifier latch is set and, 60ns later,
the low-side switch turns on (and stays on until the
beginning of the next clock cycle, in continuous mode,
or until the inductor current crosses through zero, in
discontinuous mode). Under fault conditions where the
inductor current exceeds the 100mV current-limit
threshold, the high-side latch is reset and the high-side
switch is turned off.
At light loads, the inductor current fails to exceed the
25mV threshold set by the minimum current compara-
tor. When this occurs, the PWM goes into idle-mode,
skipping most of the oscillator pulses in order to reduce
the switching frequency and cut back switching losses.
The oscillator is effectively gated off at light loads
because the minimum current comparator immediately
resets the high-side latch at the beginning of each
cycle, unless the FB_ signal falls below the reference
voltage level.
A flyback winding controller regulates the +15V VDD
supply in the absence of a load on the main +5V out-
put. If VDD falls below the preset +13V VDD regulation
threshold, a 1µs one-shot is triggered that extends the
on-time of the low-side switch beyond the point where
the inductor current crosses zero (in discontinuous
mode). This causes inductor (primary) current to
reverse, pulling current out of the output filter capacitor
and causing the flyback transformer to operate in the
forward mode. The low impedance presented by the
transformer secondary in forward mode allows the
+15V filter capacitor to be quickly charged again,
bringing VDD into regulation.
Soft-Start/SS_ InputsConnecting capacitors to SS3 and SS5 allows gradual
build-up of the +3.3V and +5V supplies after ON3 and
ON5 are driven high. When ON3 or ON5 is low, the
appropriate SS capacitors are discharged to GND.
When ON3 or ON5 is driven high, a 4µA constant cur-
rent source charges these capacitors up to 4V. The
resulting ramp voltage on the SS_ pins linearly increas-
es the current-limit comparator setpoint so as to
increase the duty cycle to the external power MOSFETs
up to the maximum output. With no SS capacitors, the
circuit will reach maximum current limit within 10µs.
Soft-start greatly reduces initial in-rush current peaks
and allows start-up time to be programmed externally.
Synchronous RectifiersSynchronous rectification allows for high efficiency by
reducing the losses associated with the Schottky recti-
fiers. Also, the synchronous rectifier MOSFETS are
necessary for correct operation of the MAX782's boost
gate-drive and VDD supplies.
When the external power MOSFET N1 (or N2) turns off,
energy stored in the inductor causes its terminal volt-
age to reverse instantly. Current flows in the loop
formed by the inductor, Schottky diode, and load, an
action that charges up the filter capacitor. The Schottky
diode has a forward voltage of about 0.5V which,
although small, represents a significant power loss,
degrading efficiency. A synchronous rectifier, N3 (or
N4), parallels the diode and is turned on by DL3 (or
DL5) shortly after the diode conducts. Since the on
resistance (rDS(ON)) of the synchronous rectifier is very
low, the losses are reduced.
The synchronous rectifier MOSFET is turned off when
the inductor current falls to zero.
Cross conduction (or “shoot-through”) is said to occur
if the high-side switch turns on at the same time as the
synchronous rectifier. The MAX782’s internal break-
before-make timing ensures that shoot-through does not
occur. The Schottky rectifierconducts during the time
that neither MOSFET is on, which improves efficiency
by preventing the synchronous-rectifier MOSFET’s
lossy body diode from conducting.
The synchronous rectifier works under all operating condi-
tions, including discontinuous-conduction and idle-mode.
The +5V synchronous rectifier also controls the 15V VDD
voltage (see the High-Side Supply (VDD)section).
MAX782riple-Output Power-Supply
Controller for Notebook Computers
______________________________________________________________________________________13
Boost Gate-Driver SupplyGate-drive voltage for the high-side N-channel switch is
generated with a flying-capacitor boost circuit as shown
in Figure 4. The capacitor is alternately charged from
the VL supply via the diode and placed in parallel with
the high-side MOSFET’s gate-source terminals. On start-
up, the synchronous rectifier (low-side) MOSFET forces
LX_ to 0V and charges the BST_ capacitor to 5V. On the
second half-cycle, the PWM turns on the high-side
MOSFET by connecting the capacitor to the MOSFET
gate by closing an internal switch between BST_ and
DH_. This provides the necessary enhancement voltage
to turn on the high-side switch, an action that “boosts”
the 5V gate-drive signal above the battery voltage.
Ringing seen at the high-side MOSFET gates (DH3 and
DH5) in discontinuous-conduction mode (light loads) is
a natural operating condition caused by the residual
energy in the tank circuit formed by the inductor and
stray capacitance at the LX_ nodes. The gate driver
negative rail is referred to LX_, so any ringing there is
directly coupled to the gate-drive supply.
Modes of Operation
PWM ModeUnder heavy loads – over approximately 25% of full load
– the +3.3V and +5V supplies operate as continuous-cur-
rent PWM supplies (see Typical Operating
Characteristics). The duty cycle (%ON) is approximately:
%ON = VOUT/VIN
Current flows continuously in the inductor: First, it
ramps up when the power MOSFET conducts; then, it
ramps down during the flyback portion of each cycle
as energy is put into the inductor and then dis-
charged into the load. Note that the current flowing
into the inductor when it is being charged is also
flowing into the load, so the load is continuously
receiving current from the inductor. This minimizes
output ripple and maximizes inductor use, allowing
very small physical and electrical sizes. Output rip-
ple is primarily a function of the filter capacitor (C7 or
C6) effective series resistance (ESR) and is typically
under 50mV (see the Design Proceduresection).
Output ripple is worst at light load and maximum
input voltage.
Idle ModeUnder light loads (<25% of full load), efficiency is fur-
ther enhanced by turning the drive voltage on and off
for only a single clock period, skipping most of the
clock pulses entirely. Asynchronous switching, seen as
“ghosting” on an oscilloscope, is thus a normal operating
condition whenever the load current is less than
approximately 25% of full load.
At certain input voltage and load conditions, a transition
region exists where the controller can pass back and
forth from idle-mode to PWM mode. In this situation,
short bursts of pulses occur that make the current
waveform look erratic, but do not materially affect the
output ripple. Efficiency remains high.
Current LimitingThe voltage between CS3 (CS5) and FB3 (FB5) is contin-
uously monitored. An external, low-value shunt resistor is
connected between these pins, in series with the induc-
tor, allowing the inductor current to be continuously mea-
sured throughout the switching cycle. Whenever this
voltage exceeds 100mV, the drive voltage to the external
high-side MOSFET is cut off. This protects the MOSFET,
the load, and the battery in case of short circuits or tem-
porary load surges. The current-limiting resistor R1 (R2)
is typically 25mΩ(20mΩ) for 3A load current.
Oscillator Frequency; SYNC InputThe SYNC input controls the oscillator frequency.
Connecting SYNC to GND or to VL selects 200kHz opera-
tion; connecting to REF selects 300kHz operation. SYNC
can also be driven with an external 240kHz to 350kHz
CMOS/TTL source to synchronize the internal oscillator.
Normally, 300kHz is used to minimize the inductor and
filter capacitor sizes, but 200kHz may be necessary for
low input voltages (see Low-Voltage (6-cell) Operation).
MAX782riple-Output Power-Supply
Controller for Notebook Computers______________________________________________________________________________________
High-Side Supply (VDD)The 15V VDD supply is obtained from the rectified and
filtered secondary of transformer L2. VDD is enabled
whenever the +5V supply is on (ON5 = high). The pri-
mary and secondary of L2 are connected so that, dur-
ing the flyback portion of each cycle (when MOSFET
N2 is off and N4 is on), energy stored in the core is
transferred into the +5V load through the primary and
into VDD through the secondary, as determined by the
turns ratio. The secondary voltage is added to the +5V
to make VDD. See the Typical Operating
Characteristicsfor the VDD supply’s load capability.
Unlike other coupled-inductor flyback converters, the
VDD voltage is regulated regardless of the loading on
the +5V output. (Most coupled-inductor converters can
only support the auxiliary output when the main output
is loaded.) When the +5V supply is lightly loaded, the
circuit achieves good control of VDD by pulsing the
MOSFET normally used as the synchronous rectifier.
This draws energy from the +5V supply’s output capac-
itor and uses the transformer in a forward-converter
mode (i.e., the +15V output takes energy out of the
secondary when current is flowing in the primary).
Note that these forward-converter pulses are inter-
spersed with normal synchronous-rectifier pulses, and
they only occur at light loads on the +5V rail.
The transformer secondary’s rectified and filtered out-
put is only roughly regulated, and may be between 13V
and 19V. It is brought back into VDD, which is also the
feedback input, and used as the source for the PCMCIA
VPP regulators (see Generating Additional VPP Outputs
Using External Linear Regulators). It can also be used
as the VH power supply for the comparators or any
external MOSFET drivers.
When the input voltage is above 20V, or when the +5V
supply is heavily loaded and VDD is lightly loaded, L2’s
interwinding capacitance and leakage inductance can
produce voltages above that calculated from the turns
ratio. A 3mA shunt regulator limits VDD to 19V.
Clock-frequency noise on the VDD rail of up to 3Vp-p is
a facet of normal operation, and can be reduced by
adding more output capacitance.
PCMCIA-Compatible
Programmable VPP SuppliesTwo independent regulators are provided to furnish
PCMCIA VPP supplies. The VPPA and VPPB outputs
can be programmed to deliver 0V, 5V, 12V, or to be high
impedance. The 0V output mode has a 250Ωpull-down
to discharge external filter capacitors and ensure that
flash EPROMs cannot be accidentally programmed.
These linear regulators operate from the high-side sup-
ply (VDD), and each can furnish up to 60mA. Bypass
VPPA and VPPB to GND with at least 1µF, with the
bypass capacitors less than 20mm from the VPP pins.
The outputs are programmed with DA0, DA1, DB0 and
DB1, as shown in Table 2.
These codes are Intel 82365 (PCMCIA digital controller)
compatible. For other interfaces, one of the inputs can
be permanently wired high or low and the other toggled
to turn the supply on and off. The truth table shows that
either a “0” or “1” can be used to turn each supply on.
The high-impedance state is to accomodate external
programming voltages. The two VPP outputs can be
safely connected in parallel for increased load capability
if the control inputs are also tied together (i.e., DA0 to
DB0, DA1 to DB1). If VPAA and VPPB are connected in
parallel, some devices may exhibit several milliamps of
increased quiescent supply current when enabled, due
to slightly mismatched output voltage set points.
ComparatorsThree noninverting comparators can be used as preci-
sion voltage comparators or high-side drivers. The
supply for these comparators (VH) is brought out and
may be connected to any voltage between +3V and
+19V. The noninverting inputs (D1-D3) are high imped-
ance, and the inverting input is internally connected to
a 1.650V reference. Each output (Q1-Q3) sources
20µA from VH when its input is above 1.650V, and
sinks 500µA to GND when its input is below 1.650V.
The Q1-Q3 outputs can be fixed together in wired-OR
configuration since the pull-up current is only 20µA.
Connecting VH to a logic supply (5V or 3V) allows the
comparators to be used as low-battery detectors. For dri-
ving N-channel power MOSFETs to turn external loads on
and off, VH should be 6V to 12V higher than the load volt-
age. This enables the MOSFETs to be fully turned on and
results in low rDS(ON). VDD is a convenient source for VH.
MAX782riple-Output Power-Supply
Controller for Notebook Computers
______________________________________________________________________________________15The comparators are always active when V+ is above
+4V, even when VH is 0V. Thus, Q1-Q3 will sink current
to GND even when VH is 0V, but they will only source
current from VH when VH is above approximately 1.5V.
If Q1, Q2, or Q3 is externally pulled above VH, an inter-
nal diode conducts, pulling VH a diode drop below the
output and powering anything connected to VH. This
voltage will also power the other comparator outputs.
Internal VL and REF SuppliesAn internal linear regulator produces the 5V used by the
internal control circuits. This regulator’s output is avail-
able on pin VL and can source 5mA for external loads.
Bypass VL to GND with 4.7µF. To save power, when
the +5V switch-mode supply is above 4.5V, the internal
linear regulator is turned off and the high-efficiency +5V
switch-mode supply output is connected to VL.
The internal 3.3V bandgap reference (REF) is powered
by the internal 5V VL supply, and is always on. It can
furnish up to 5mA. Bypass REF to GND with 0.22µF,
plus 1µF/mA of load current.
Both the VL and REF outputs remain active, even when
the switching regulators are turned off, to supply mem-
ory keep-alive power.
These linear-regulator ouputs can be directly connected
to the corresponding step-down regulator outputs (i.e.,
REF to +3.3V, VL to +5V) to keep the main supplies alive
in standby mode. However, to ensure start-up, standby
load currents must not exceed 5mA on each supply.
Fault ProtectionThe +3.3V and +5V PWM supplies, the high-side sup-
ply, and the comparators are disabled when either of
two faults is present: VL < +4.0V or REF < +2.8V (85%
of its nominal value).
__________________Design ProcedureFigure 1’s schematic and Table 2’s component list
show values suitable for a 3A, +5V supply and a 3A,
+3.3V supply. This circuit operates with input voltages
from 6.5V to 30V, and maintains high efficiency with
output currents between 5mA and 3A (see the Typical
Operating Characteristics). This circuit’s components
may be changed if the design guidelines described in
this section are used – but before beginning thedesign,
the following information should be firmly established:
VIN(MAX), the maximum input (battery) voltage.This
value should include the worst-case conditions under
which the power supply is expected to function, such
as no-load (standby) operation when a battery charger
is connected but no battery is installed. VIN(MAX)can-
not exceed 30V.
VIN(MIN), the minimum input (battery) voltage.This
value should be taken at the full-load operating cur-
rent under the lowest battery conditions. If VIN(MIN)
is below about 6.5V, the power available from the
VDD supply will be reduced. In addition, the filter
capacitance required to maintain good AC load reg-
ulation increases, and the current limit for the +5V
supply has to be increased for the same load level.
+3.3V Inductor (L1)Three inductor parameters are required: the inductance
value (L), the peak inductor current (ILPEAK), and the
coil resistance (RL). The inductance is:
VOUTx (VIN(MAX)- VOUT)
L = ————————————-
VIN(MAX)x f x IOUTx LIR
where: VOUT= output voltage, 3.3V;
VIN(MAX)= maximum input voltage (V);
f = switching frequency, normally 300kHz;
IOUT= maximum +3.3V DC load current (A);
LIR = ratio of inductor peak-to-peak AC
current to average DC load current, typically 0.3.
A higher value of LIR allows smaller inductance, but
results in higher losses and higher ripple.
The highest peak inductor current (ILPEAK) equals the
DC load current (IOUT) plus half the peak-to-peak AC
inductor current (ILPP). The peak-to-peak AC inductor
current is typically chosen as 30% of the maximum DC
load current, so the peak inductor current is 1.15 times
IOUT.
The peak inductor current at full load is given by:
VOUTx (VIN(MAX)- VOUT)
ILPEAK= IOUT+ —————————————.
2 x f x L x VIN(MAX)
The coil resistance should be as low as possible,
preferably in the low milliohms. The coil is effectively in
series with the load at all times, so the wire losses alone
are approximately:
Power loss = IOUT2x RL
In general, select a standard inductor that meets the L,
ILPEAK, and RLrequirements (see Tables 3 and 4). If a
standard inductor is unavailable, choose a core with an
LI2parameter greater than L x ILPEAK2, and use the
largest wire that will fit the core.
MAX782riple-Output Power-Supply
Controller for Notebook Computers______________________________________________________________________________________
+5V Transformer (T1)Table 3 lists two commercially available transformers
and parts for a custom transformer. The following
instructions show how to determine the transformer
parameters required for a custom design:
LP, the primary inductance value
ILPEAK, the peak primary current
LI2, the core’s energy ratingand RS, the primary and secondary resistances
N, the primary-to-secondary turns ratio.
The transformer primary is specified just as the +3.3V
inductor, using VOUT= +5.0V; but the secondary output
(VDD) powermust be added in as if it were part of the
primary. VDD current (IDD) usually includes the VPPA
and VPPB output currents. The total +5V power, PTOTAL,
is the sum of these powers:
PTOTAL= P5 + PDD
where: P5 = VOUTx IOUT;
PDD= VDD x IDD;
and: VOUT= output voltage, 5V;
IOUT= maximum +5V load current (A);
VDD = VDD output voltage, 15V;
IDD= maximum VDD load current (A);
so: PTOTAL= (5V x IOUT) + (15V x IDD)
and the equivalent +5V output current, ITOTAL, is:
ITOTAL= PTOTAL/ 5V
= [(5V x IOUT) + (15V x IDD)] / 5V.
The primary inductance, LP, is given by:
VOUTx (VIN(MAX)- VOUT)= ———————————————
VIN(MAX)x f x ITOTALx LIR
where: VOUT= output voltage, 5V;
VIN(MAX)= maximum input voltage;
f = switching frequency, normally 300kHz;
ITOTAL= maximum equivalent load current (A);
LIR = ratio of primary peak-to-peak AC
current to average DC load current, typically 0.3.
The highest peak primary current (ILPEAK) equals the
total DC load current (ITOTAL) plus half the peak-to-peak
AC primary current (ILPP). The peak-to-peak AC primary
current is typically chosen as 30% of the maximum DC
load current, so the peak primary current is 1.15 times
ITOTAL. A higher value of LIR allows smaller inductance,
but results in higher losses and higher ripple.
The peak current in the primary at full load is given by:
VOUTx (VIN(MAX) - VOUT)
ILPEAK= ITOTAL+ —————————————.
2 x f x LPx VIN(MAX)
Choose a core with an LI2parameter greater than LPx
ILPEAK2.
The winding resistances, RPand RS, should be as low
as possible, preferably in the low milliohms. Use the
largest gauge wire that will fit on the core. The coil is
effectively in series with the load at all times, so the
resistive losses in the primary winding alone are
approximately (ITOTAL)2x RP.
The minimum turns ratio, NMIN, is 5V:(15V-5V). Use 1:2.2
to accommodate the tolerance of the +5V supply. A
greater ratio will reduce efficiency of the VPP regulators.
Minimize the diode capacitance and the interwinding
capacitance, since they create losses through the
VDD shunt regulator. These are most significant when
the input voltage is high, the +5V load is heavy, and
there is no load on VDD.
Ensure the transformer secondary is connected with the
right polarity: A VDD supply will be generated with either
polarity, but proper operation is possible only with the cor-
rect polarity. Test for correct connection by measuring the
VDD voltage when VDD is unloaded and the input voltage
(VIN) is varied over its full range. Correct connection is
indicated if VDD is maintained between 13V and 20V.
Current-Sense Resistors (R1, R2)The sense resistors must carry the peak current in the
inductor, which exceeds the full DC load current.
The internal current limiting starts when the voltage
across the sense resistors exceeds 100mV nominally,
80mV minimum. Use the minimum value to ensure
adequate output current capability: For the +3.3V
supply, R1 = 80mV / (1.15 x IOUT); for the +5V supply,
R2 = 80mV/(1.15 x ITOTAL), assuming that LIR = 0.3.
Since the sense resistance values (e.g. R1 = 25mΩfor
IOUT= 3A) are similar to a few centimeters of narrow
traces on a printed circuit board, trace resistance can
contribute significant errors. To prevent this, Kelvin
connect the CS_ and FB_ pins to the sense resistors;
i.e., use separate traces not carrying any of the induc-
tor or load current, as shown in Figure 5.
Run these traces parallel at minimum spacing from one
another. The wiring layout for these traces is critical for
stable, low-ripple outputs (see the Layout and
Groundingsection).
MOSFET Switches (N1-N4)The four N-channel power MOSFETs are usually iden-
tical and must be “logic-level” FETs; that is, they must
be fully on (have low rDS(ON)) with only 4V gate-
source drive voltage. The MOSFET rDS(ON)should
ideally be about twice the value of the sense resistor.
MOSFETs with even lower rDS(ON)have higher gate
capacitance, which increases switching time and
transition losses.
