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MAX783CBX-MAX783EBX
Triple-Output Power-Supply Controller for Notebook Computers
_______________General DescriptionThe MAX783 is a system-engineered power-supply controller
for notebook computers or similar battery-powered equipment.
It provides two high-performance step-down (buck) pulse-
width modulators (PWMs) for +3.3V/+5V and dual PCMCIA
VPP outputs powered by an integral flyback winding controller.
Other functions include dual, low-dropout, micropower linear
regulators for CMOS/RTC back up, and two precision low-
battery-detection comparators.
High efficiency (95% at 2A, greater than 80% at loads from
5mA to 3A) is achieved through synchronous rectification
and PWM operation at heavy loads, and Idle-ModeTMoper-
ation at light loads. The MAX783 uses physically small
components, thanks to high operating frequencies
(300kHz/200kHz) and a new current-mode PWM architec-
ture that allows for output filter capacitors as small as 30µF
per ampere of load. Line- and load-transient responses
are terrific, with a high 60kHz unity-gain crossover frequen-
cy that allows 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 con-
troller provides a low-cost, +15V high-side output that regu-
lates 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 synchronizable
oscillator for noise-sensitive applications such as electro-
magnetic pen-based systems and communicating comput-
ers. The MAX783 is similar to the MAX782, except the fly-
back winding is on the 3.3V inductor instead of the 5V
inductor, the VPP outputs can be optionally programmed to
3.3V, and the device may be completely shut down.
________________________ApplicationsNotebook Computers
Portable Data Terminals
Communicating Computers
Pen-Entry Systems
____________________________FeaturesDual PWM Buck Controllers (+3.3V and +5V)Dual PCMCIA VPP Outputs (0V/3.3V/5V/12V)Two Precision Comparators or Level TranslatorsPower-Ready Status Output (RDY5
)95% EfficiencyOptimized for 6-Cell Applications420µA Quiescent Current;
70µA in Standby (linear regulators alive)
25µA Shutdown Current5.5V to 30V Input RangeSmall SSOP PackageFixed Output Voltages Available:
3.3V (standard)
3.45V (High-Speed Pentium™)
3.6V (PowerPC™)
______________Ordering Information
MAX783
for Notebook Computers
________________________________________________________________Maxim Integrated Products1
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.
_______Typical Application Diagram
__________________Pin Configuration
Ordering Information continued on last page.
MAX783riple-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, D2, S—H—D—N–, ON5, REF, SYNC, DA1, DA0, DB1, DB0, ON5,
SS5, CS5, FB5, R—D—Y—5–, 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, Q2 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)...................762mW
Operating Temperature Ranges:
MAX783CBX/MAX783_CBX.................................0°C to +70°C
MAX783EBX/MAX783_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, S—H—D—N–= ON3 = ON5 = 5V, other digital input levels are 0V or +5V, = 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
MAX783riple-Output Power-Supply Controller
for Notebook Computers
_______________________________________________________________________________________3
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0mA, S—H—D—N–= ON3 = ON5 = 5V, other digital input levels are 0V or +5V, = TMINto TMAX, unless otherwise noted.)
MAX783riple-Output Power-Supply Controller
for Notebook Computers_______________________________________________________________________________________
Note 1:Output current is further limited by maximum allowable package power dissipation.
Note 2:Because the reference uses VL as its supply, the REF line regulation error is insignificant.
Note 3:The main switching outputs track the reference voltage. Loading the reference reduces the main outputs slightly according
to the closed-loop gain (AVCL) and the reference voltage load regulation error. AVCLfor the +3.3V supply is unity gain.
AVCLfor the +5V supply is 1.54.
MAX183 5
10m1
100m
EFFICIENCY (%)
+5V OUTPUT CURRENT (A)
EFFICIENCY vs.
+5V OUTPUT CURRENTMAX183 6
10m1
100m
EFFICIENCY (%)
+3.3V OUTPUT CURRENT (A)
EFFICIENCY vs.
+3.3V OUTPUT CURRENT50020
MAXIMUM +15V VDD OUTPUT CURRENT vs.
SUPPLY VOLTAGESUPPLY VOLTAGE (V)
MAXIMUM +15V LOAD CURRENT (mA)15
QUIESCENT SUPPLY CURRENT vs.
SUPPLY VOLTAGE
QUIESCENT SUPPLY CURRENT (mA)612182430
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT vs.
SUPPLY VOLTAGESTANDBY SUPPLY CURRENT (mA)
SUPPLY VOLTAGE (V)
__________________________________________Typical Operating Characteristics(Circuit of Figure 1, Transpower TTI5902 transformer, TA = +25°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0mA, S—H—D—N–= ON3 = ON5 = 5V, other digital input levels are 0V or +5V, = TMINto TMAX, unless otherwise noted.)
MAX783riple-Output Power-Supply Controller
for Notebook Computers
_______________________________________________________________________________________5
MINIMUM VIN TO VOUT DIFFERENTIAL
vs. +5V OUTPUT CURRENTMINIMUM V
TO V
OUT
DIFFERENTIAL (V)
+5V OUTPUT CURRENT (A)
1.010m100m110
100μ10m1
SWITCHING FREQUENCY vs.
LOAD CURRENTLOAD CURRENT (A)
SWITCHING FREQUENCY (kHz)
100100m
____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, Transpower TTI5902 transformer, TA = +25°C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT vs.
SUPPLY VOLTAGESHUTDOWN SUPPLY CURRENT (
SUPPLY VOLTAGE (V)
MAX783riple-Output Power-Supply Controller
for Notebook Computers_______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, Transpower TTI5902 transformer, TA = +25°C, unless otherwise noted.)
MAX783riple-Output Power-Supply Controller
for Notebook Computers
_______________________________________________________________________________________7
______________________________________________________________Pin Description
MAX783
_______________Detailed DescriptionThe MAX783 converts a 5.5V to 30V input to six outputs
(Figure 1). It produces two high-power, PWM switch-
mode supplies, one at +5V and the other at +3.3V. The
two supplies operate at either 300kHz or 200kHz, allow-
ing for small external components. Output current capa-
bility depends on external components, and can exceed
6A on each supply. Two 12V VPP outputs, an internal 5V,
25mA supply (VL) and a 3.3V, 5mA reference voltage
are also generated via linear regulators (Figure 2). Fault-
protection circuitry shuts off the PWM and high-side sup-
ply when the internal supplies lose regulation.
Two precision voltage comparators are also included.
Their output stages permit them to be used as level
translators for driving external N-channel MOSFETs in
load-switching applications, or for more conventional
logic signals.
The MAX783 is capable of accepting input voltages
from 5.5V to 30V, but is optimized for the lower end of
this range because the +15V flyback winding controller
is appended to the +3.3V buck supply. This architecture
allows for lower input voltages than are possible with the
MAX782 sister chip, which puts the winding on the +5V
side, while maintaining high +15V load capability.
However, the MAX783’s transformer has a higher turns
ratio (4:1 vs. 2:1), which leads to higher interwinding
capacitance as well as higher switching noise ampli-
tudes at the transformer secondary when the input volt-
age is high. Therefore, the MAX783 standard applica-
tion circuit is optimized with external components for
low-voltage (6-8 cell) designs with maximum input volt-
ages of 20V and less. The MAX783 itself can easily
accept 30V inputs, but expect to see more noise and
higher voltage swings at the transformer secondary
under these conditions. The inductor and filter capacitor
values may also require some adjustment for inputs
greater than 20V; see the Design Proceduresection.
+5V Switch-Mode SupplyThe +5V supply is generated by a current-mode PWM
step-down regulator using two N-channel MOSFETs, a
rectifier, plus an LC output filter (Figure 1). The gate-
drive signal to the high-side MOSFET, which must
exceed the battery voltage, is provided by a boost cir-
cuit that uses a 100nF capacitor connected to BST5.
The +5V supply’s dropout voltage, as configured in
Figure 1, is typically 400mV at 2A. As V+ approaches
5V, the +5V output falls with V+ until the VL regulator
output hits its undervoltage lockout threshold at 4V. At
this point, the +5V supply turns off.
A synchronous rectifier at LX5 keeps efficiency high by
effectively clamping the voltage across the rectifier
diode. Maximum current limit is set by an external low-
value sense resistor, which prevents excessive inductor
current during start-up or under short-circuit conditions.
Programmable soft-start is set by an optional external
capacitor; this reduces in-rush surge currents upon
start-up and provides adjustable power-up times for
power-supply sequencing purposes.
+3.3V Switch-Mode SupplyThe +3.3V output is produced by a current-mode PWM
step-down regulator similar to the +5V supply. The +3.3V
supply uses a transformer primary winding as its induc-
tor; the secondary is used for the 15V VDD supply.
The default switching frequency for both PWM controllers
is 200kHz (with SYNC connected to GND or VL), but
300kHz may be used by connecting SYNC to REF.
+3.3V and +5V PWM Buck ControllersThe two current-mode PWM buck controllers are nearly
identical except for different preset output voltages and
the addition of a flyback winding control loop to the
3.3V side. Each PWM is independent, except both are
synchronized to a master oscillator and share 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 exter-
nal feedback compensation components if you follow
the filter capacitor ESR guidelines in the Design
Procedure.
The main gain block is an open-loop comparator that
sums four input signals: output voltage error signal,
current-sense signal, slope-compensation ramp, and
precision reference voltage. 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 factor
(approximately VOUT/VIN). As the high-side switch turns
off, the synchronous rectifier latch is set; 60ns later, the
low-side switch turns on. The low-side switch stays on
until the beginning of the next clock cycle (in continu-
ous mode) or until the inductor current crosses through
riple-Output Power-Supply Controller
for Notebook Computers_______________________________________________________________________________________
zero (in discontinuous mode). Under fault conditions
when the inductor current exceeds the 100mV current-
limit threshold, the high-side latch resets and the high-
side switch turns 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 3.3V out-
put. If VDD falls below the preset +13V VDD regulation
threshold, a 1µs one-shot is triggered that extends the
low-side switch’s on-time 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 for-
ward mode. The low impedance presented by the
transformer secondary in forward mode allows the
+15V filter capacitor to be quickly charged up again,
bringing VDD into regulation.
MAX783riple-Output Power-Supply Controller
for Notebook Computers
_______________________________________________________________________________________9Figure 1. Standard Application Circuit
MAX783riple-Output Power-Supply Controller
for Notebook Computers______________________________________________________________________________________Figure 2. Block Diagram
MAX783riple-Output Power-Supply Controller
for Notebook Computers
______________________________________________________________________________________11Figure 3. PWM Controller Block Diagram
MAX783riple-Output Power-Supply Controller
for Notebook Computers______________________________________________________________________________________
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 MAX783's boost
gate-drive and VDD supplies.
When the external high-side power MOSFET 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 and
degrades efficiency. A synchronous rectifier MOSFET
parallels the diode and is turned on by DL3 (or DL5)
shortly after the diode conducts. Since the on resis-
tance (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”) occurs if the high-
side switch turns on at the same time as the synchronous
rectifier. Internal break-before-make timing ensures that
shoot-through does not occur. The Schottky rectifier con-
ducts during the time that neither MOSFET is on, which
improves efficiency by preventing the synchronous-rectifi-
er MOSFET’s lossy body diode from conducting.
The synchronous rectifier works under all operating condi-
tions, including discontinuous-conduction and idle-mode.
The +3.3V synchronous rectifier also controls the 15V VDD
voltage (see the High-Side Supply (VDD)section).
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 continu-
ous-current 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 discharged 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
MAX783riple-Output Power-Supply Controller
for Notebook Computers
______________________________________________________________________________________13continuously receiving current from the inductor. This
minimizes output ripple and maximizes inductor use,
allowing very small physical and electrical sizes. Output
ripple is primarily a function of the filter capacitor effec-
tive 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
inductor, allowing the inductor current to be continuously
measured 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 cir-
cuits or temporary load surges. The current-limiting
resistor R1 (R2) is typically 25mΩ(20mΩ) for a 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.
300kHz operation is used to minimize the inductor and
filter capacitor sizes, but 200kHz may be necessary for
low input voltages (see Low-Voltage Operation).
High-Side Supply (VDD)The 15V VDD supply is obtained from the rectified and
filtered secondary of transformer L2. VDD is enabled
whenever the +3.3V supply is on (ON3 = high). The
primary and secondary of L2 are connected so that,
during the flyback (discharge) portion of each cycle,
energy stored in the core is transferred into the +3.3V
load through the primary and into VDD through the sec-
ondary, as determined by the turns ratio. The sec-
ondary voltage is added to the +3.3V 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 +3.3V output. (Most coupled-inductor converters
can only support the auxiliary output when the main
output is loaded.) When the +3.3V 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 +3.3V supply’s
output capacitor 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 prima-
ry). These forward-converter pulses are interspersed
with normal synchronous-rectifier pulses, and they only
occur at light loads on the +3.3V 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. It can also be used as the VH power
supply for the comparators or any external MOSFET
drivers.
When the input voltage is above 12V, or when the
+3.3V supply is heavily loaded and VDD is lightly
loaded, L2’s interwinding capacitance and leakage
inductance can produce voltages above that calculat-
ed from the turns ratio. A 2.5mA shunt regulator limits
VDD to 19V. If the battery voltage can rise above 12V,
VDD must either be externally clamped with an 18V
zener diode, or there must be a 1mA minimum load on
VDD (or VPPA/VPPB).
Clock-frequency noise on the VDD rail of up to 3VP-Pis
a facet of normal operation, and can be reduced by
adding more output capacitance.
PCMCIA-Compatible,
Programmable VPP SuppliesTwo independent linear regulators furnish PCMCIA VPP
supplies. The VPPA and VPPB outputs can be pro-
grammed to deliver 0V, 3.3V, 5V, or 12V. The 0V out-
put mode has a 250Ωpull-down to discharge external
filter capacitors and ensure that flash EPROMs cannot
be accidentally programmed. These linear regulators
draw their power from the high-side supply (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.
MAX783These codes are compatible with many popular PCMCIA
digital controllers such as the Intel 82365SL. 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 two VPP outputs can
be safely connected in parallel for increased load capabil-
ity 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.
ComparatorsTwo 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-D2) are high imped-
ance, and the inverting input is internally connected to
a 1.650V reference. Each output (Q1-Q2) 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-Q2 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.
Internal VREF and VL 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 VL linear
regulator is turned off and the high-efficiency +5V
switch-mode supply output is internally connected to VL.
The 3.3V precision reference (REF) is powered from the
internal 5V VL supply. It can furnish up to 5mA for exter-
nal loads. Bypass REF to GND with 0.22µF, plus 1µF/mA
of load current. The main switch-mode outputs track the
reference voltage. Loading the reference reduces the
main output voltages slightly, according to the reference
voltage load regulation error.
Both the VL and REF supplies can remain active—even
when the switch-mode regulators are turned off—to supply
memory keep-alive power (see Shutdown Mode section).
These linear regulator outputs can be directly connected
to the corresponding switch-mode regulator outputs (i.e.,
REF to +3.3V, VL to +5V) to hold up the main supplies in
standby mode. However, to ensure start-up, standby
load currents must not exceed 5mA on each supply.
Shutdown ModeShutdown (S—H—D—N–= low) forces both PWMs off and dis-
ables the REF output and the auxiliary comparators
including R—D—Y—5–. Supply current in shutdown mode is
typically 25µA. The VL supply remains active and can
source 25mA for external loads. VL load capability is
higher in shutdown and standby modes than when the
PWMs are operating (25mA vs. 5mA).
Standby mode is achieved by holding ON3 and ON5
low while S—H—D—N–is high. This disables both PWMs, but
keeps VL, REF, and the precision comparators alive.
Supply current in standby mode is typically 70µA.
Other ways to shut down the MAX783 are suggested in
the applications section of the MAX782 data sheet.
__________________Design ProcedureFigure 1’s predesigned application circuit contains the
correct component values for 3A output currents and a
6V to 20V input range. Use the design procedure that
follows to optimize this basic schematic for different
voltage or current requirements.
Before beginning a design, firmly establish the following:
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 6V, the filter capacitance required to
maintain good AC load regulation increases, and the
current limit for the +5V supply has to be increased
for the same load level.
riple-Output Power-Supply Controller
for Notebook Computers______________________________________________________________________________________
