MAX1902EAI ,500kHz Multi-Output / Low-Noise Power-Supply Controllers for Notebook ComputersMAX1901/MAX1902/MAX190419-2224; Rev 3; 12/03500kHz Multi-Output, Low-Noise Power-SupplyControllers ..
MAX1902EAI ,500kHz Multi-Output / Low-Noise Power-Supply Controllers for Notebook ComputersFeaturesThe MAX1901/MAX1902/MAX1904 are buck-topology, 97% Efficiencystep-down, switch-mode, powe ..
MAX1902EAI ,500kHz Multi-Output / Low-Noise Power-Supply Controllers for Notebook ComputersMAX1901/MAX1902/MAX190419-2224; Rev 3; 12/03500kHz Multi-Output, Low-Noise Power-SupplyControllers ..
MAX1902EAI ,500kHz Multi-Output / Low-Noise Power-Supply Controllers for Notebook ComputersApplicationsINPUTNotebook and Subnotebook Computers5V (RTC) 12VPDAs and Mobile Communicators5V 12VL ..
MAX1902EAI+ ,500kHz Multi-Output, Low-Noise Power-Supply Controllers for Notebook ComputersFeaturesThe MAX1901/MAX1902/MAX1904 are buck-topology, ♦ 97% Efficiencystep-down, switch-mode, powe ..
MAX1902EAI+T ,500kHz Multi-Output, Low-Noise Power-Supply Controllers for Notebook ComputersApplicationsINPUTNotebook and Subnotebook Computers5V (RTC) 12VPDAs and Mobile Communicators5V 12VL ..
MAX485ECSA ,15kV ESD-Protected / Slew-Rate-Limited / Low-Power / RS-485/RS-422 TransceiversFeaturesThe MAX481E, MAX483E, MAX485E, MAX487E–MAX491E, ' ESD Protection: ±15kV—Human Body Modelan ..
MAX485ECSA ,15kV ESD-Protected / Slew-Rate-Limited / Low-Power / RS-485/RS-422 Transceiversapplications that are not ESD sen-MAX481ECPA 0°C to +70°C 8 Plastic DIPsitive see the pin- and func ..
MAX485ECSA ,15kV ESD-Protected / Slew-Rate-Limited / Low-Power / RS-485/RS-422 TransceiversGeneral Description ________
MAX485ECSA ,15kV ESD-Protected / Slew-Rate-Limited / Low-Power / RS-485/RS-422 TransceiversGeneral Description ________
MAX485ECSA+ ,±15kV ESD-Protected, Slew-Rate-Limited, Low-Power, RS-485/RS-422 TransceiversApplications:ate from a single +5V supply.MAX3293/MAX3294/MAX3295: 20Mbps, +3.3V,Drivers are short- ..
MAX485ECSA+T ,±15kV ESD-Protected, Slew-Rate-Limited, Low-Power, RS-485/RS-422 TransceiversApplications:ceivers on the bus. The MAX488E–MAX491E areMAX3483E/MAX3485E/MAX3486E/MAX3488E/designe ..
MAX1901EAI-MAX1901ETJ-MAX1902EAI-MAX1904EAI-MAX1904ETJ
500kHz Multi-Output / Low-Noise Power-Supply Controllers for Notebook Computers
General DescriptionThe MAX1901/MAX1902/MAX1904 are buck-topology,
step-down, switch-mode, power-supply controllers that
generate logic-supply voltages in battery-powered sys-
tems. These high-performance, dual/triple-output devices
include on-board power-up sequencing, power-good sig-
naling with delay, digital soft-start, secondary winding
control, low dropout circuitry, internal frequency-compen-
sation networks, and automatic bootstrapping.
Up to 97% efficiency is achieved through synchronous
rectification and Maxim’s proprietary Idle Mode™ control
scheme. Efficiency is greater than 80% over a 1000:1
load-current range, which extends battery life in system
suspend or standby mode. Excellent dynamic response
corrects output load transients within five clock cycles.
Strong 1A on-board gate drivers ensure fast external N-
channel MOSFET switching.
These devices feature a logic-controlled and synchroniz-
able, fixed-frequency, pulse-width modulation (PWM)
operating mode. This reduces noise and RF interference
in sensitive mobile communications and pen-entry appli-
cations. Asserting the SKIPpin enables fixed-frequency
mode, for lowest noise under all load conditions.
The MAX1901/MAX1902/MAX1904 include two PWM reg-
ulators, adjustable from 2.5V to 5.5V with fixed 5.0V and
3.3V modes. All these devices include secondary feed-
back regulation, and the MAX1902 contains a 12V/120mA
linear regulator. The MAX1901/MAX1904 include a sec-
ondary feedback input (SECFB), plus a control pin
(STEER) that selects which PWM (3.3V or 5V) receives the
secondary feedback signal. SECFB provides a method
for adjusting the secondary winding voltage regulation
point with an external resistor divider, and is intended to
aid in creating auxiliary voltages other than fixed 12V.
The MAX1901/MAX1902 contain internal output overvolt-
age and undervoltage protection features.
________________________ApplicationsNotebook and Subnotebook Computers
PDAs and Mobile Communicators
Desktop CPU Local DC-DC Converters
Features97% Efficiency4.2V to 30V Input Range2.5V to 5.5V Dual Adjustable OutputsSelectable 3.3V and 5V Fixed or Adjustable
Outputs (Dual Mode™)12V Linear RegulatorAdjustable Secondary Feedback
(MAX1901/MAX1904)5V/50mA Linear Regulator OutputPrecision 2.5V Reference OutputProgrammable Power-Up SequencingPower-Good (RESET) OutputOutput Overvoltage Protection
(MAX1901/MAX1902)Output Undervoltage Shutdown
(MAX1901/MAX1902)333kHz/500kHz Low-Noise, Fixed-Frequency
OperationLow-Dropout, 98% Duty-Factor Operation2.5mW Typical Quiescent Power (12V input, both
SMPSs on)4µA Typical Shutdown Current
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Functional Diagram19-2224; Rev 3; 12/03
Ordering InformationIdle Mode is a trademark of Maxim Integrated Products, Inc.
Dual Mode is a trademark of Maxim Integrated Products, Inc.
Pin Configurations appear at end of data sheet.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = 15V, both PWMs on, SYNC = VL, VLload = 0, REF load = 0, SKIP= 0, TA
= 0°C to +85°C, unless otherwise noted. TypicalStresses 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.
V+ to GND..............................................................-0.3V to +36V
PGND to GND.....................................................................±0.3Vto GND ................................................................-0.3V to +6V
BST3, BST5 to GND ..............................................-0.3V to +36V
CSH3, CSH5 to GND................................................-0.3V to +6V
FB3 to GND..............................................-0.3V to (CSL3 + 0.3V)
FB5 to GND...............................................-0.3V to (CSL5 +0.3V)
LX3 to BST3..............................................................-6V to +0.3V
LX5 to BST5..............................................................-6V to +0.3V
REF, SYNC, SEQ, STEER, SKIP,
TIME/ON5, SECFB, RESETto GND ........-0.3V to (VL+ 0.3V)
VDDto GND............................................................-0.3V to +20V
RUN/ON3, SHDNto GND.............................-0.3V to (V+ + 0.3V)
12OUT to GND ..........................................-0.3V to (VDD+ 0.3V)
DL3, DL5 to PGND........................................-0.3V to (VL+ 0.3V)
DH3 to LX3 ..............................................-0.3V to (BST3 + 0.3V)
DH5 to LX5 ..............................................-0.3V to (BST5 + 0.3V)
VL, REF Short to GND ................................................Momentary
12OUT Short to GND..................................................Continuous
REF Current...........................................................+5mA to -1mACurrent.........................................................................+50mA
12OUT Current ..............................................................+200mA
VDDShunt Current............................................................+15mA
Continuous Power Dissipation (TA= +70°C)
28-Pin SSOP (derate 9.52mW/°C above +70°C) ......762mW
32-Pin Thin QFN (derate 21.3mW/°C above +70°C) ..1702mW
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range ............................-65°C to +160°C
Lead Temperature (soldering, 10s) ................................+300°C
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, both PWMs on, SYNC = VL, VLload = 0, REF load = 0, SKIP= 0, TA
= 0°C to +85°C, unless otherwise noted. Typical
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, both PWMs on, SYNC = VL, VLload = 0, REF load = 0, SKIP= 0, TA
= 0°C to +85°C, unless otherwise noted. Typical
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)
Note 1:Each of the four digital soft-start levels is tested for functionality; the steps are typically in 20mV increments.
Note 2:High duty-factor operation supports low input-to-output differential voltages, and is achieved at a lowered operating frequency
(see theDropout Operation section).
Note 3:MAX1902 only.
Note 4:Off mode for the 12V linear regulator occurs when the SMPS that has flyback feedback (VDD) steered to it is disabled. In situa-
tions where the main outputs are being held up by external keep-alive supplies, turning off the 12OUT regulator prevents a leak-
age path from the output-referred flyback winding, through the rectifier, and into VDD.
Note 5:Since the reference uses VLas its supply, the reference’s V+ line-regulation error is insignificant.
Note 6:Production testing limitations due to package handing require relaxed maximum on-resistance specifications for the thin
QFN package. The SSOP and thin QFN package contain the same die, and the thin QFN package imposes no additional
resistance incircuit.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
EFFICIENCY vs. 5V OUTPUT CURRENTMAX1901 toc01
5V OUTPUT CURRENT (A)
EFFICIENCY (%)70
EFFICIENCY vs. 3.3V OUTPUT CURRENT
MAX1901 toc02
3.3V OUTPUT CURRENT (A)
EFFICIENCY (%)70
MAXIMUM VDD OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX1901 toc03
INPUT VOLTAGE (V)
MAXIMUM V
OUTPUT CURRENT (mA)
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
MAX1901 toc04
INPUT VOLTAGE (V)
INPUT CURRENT (mA)25
10,000
V+ STANDBY INPUT CURRENT
vs. INPUT VOLTAGE
MAX1901 toc05
INPUT VOLTAGE (V)
INPUT CURRENT (250
SHUTDOWN INPUT CURRENT
vs. INPUT VOLTAGE
MAX1901 toc06
INPUT VOLTAGE (V)
INPUT CURRENT (
MINIMUM VIN TO VOUT DIFFERENTIAL
vs. 5V OUTPUT CURRENT
MAX1901 toc07
5V OUTPUT CURRENT (A)
MINIMUM V
TO V
OUT
DIFFERENTIAL (mV)
SWITCHING FREQUENCY
vs. LOAD CURRENT
MAX1901 toc08
LOAD CURRENT (A)
SWITCHING FREQUENCY (kHz)
VL REGULATOR OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX1901 toc09
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
Typical Operating Characteristics(Circuit of Figure 1, Table 1, 6A/500kHz components, TA = +25°C, unless otherwise noted.)
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Typical Operating Characteristics (continued)(Circuit of Figure 1, Table 1, 6A/500kHz components, TA = +25°C, unless otherwise noted.)
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersFigure 1. Standard 3.3V/5V Application Circuit (MAX1901/MAX1904)
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Standard Application CircuitThe basic MAX1901/MAX1904 dual-output 3.3V/5V
buck converter (Figure 1) is easily adapted to meet a
wide range of applications with inputs up to 28V by
substituting components from Table 1. These circuits
represent a good set of tradeoffs between cost, size,
and efficiency, while staying within the worst-case
specification limits for stress-related parameters, such
as capacitor ripple current. Don’t change the frequency
of these circuits without first recalculating component
values (particularly inductance value at maximum bat-
tery voltage). Adding a Schottky rectifier across each
synchronous rectifier improves the efficiency of these
circuits by approximately 1%, but this rectifier is other-
wise not needed because the MOSFETs required for
these circuits typically incorporate a high-speed silicon
diode from drain to source. Use a Schottky rectifier
rated at a DC current equal to at least one-third of the
load current.
Detailed DescriptionThe MAX1901/MAX1902/MAX1904 are dual, BiCMOS,
switch-mode power-supply controllers designed pri-
marily for buck-topology regulators in battery-powered
applications where high-efficiency and low-quiescent
supply current are critical. Light-load efficiency is
enhanced by automatic Idle-Mode operation, a vari-
able-frequency pulse-skipping mode that reduces tran-
sition and gate-charge losses. Each step-down,
power-switching circuit consists of two N-channel
MOSFETs, a rectifier, and an LC output filter. The out-
put voltage is the average AC voltage at the switching
node, which is regulated by changing the duty cycle of
the MOSFET switches. The gate-drive signal to the
N-channel high-side MOSFET must exceed the battery
voltage, and is provided by a flying-capacitor boost cir-
cuit that uses a 100nF capacitor connected to BST_.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersThe MAX1901/MAX1902/MAX1904 contain ten major
circuit blocks (Figure 2).
The two pulse-width-modulation (PWM) controllers
each consist of a Dual Mode feedback network and
multiplexer, a multi-input PWM comparator, high-side
and low-side gate drivers, and logic. MAX1901/
MAX1902 contain fault-protection circuits that monitor
the main PWM outputs for undervoltage and overvolt-
age. A power-on sequence block controls the power-
up timing of the main PWMs and determines whether
one or both of the outputs are monitored for undervoltage
faults. The MAX1902 includes a secondary feedback net-
work and 12V linear regulator to generate a 12V output
from a coupled-inductor flyback winding. The
MAX1901/MAX1904 have a secondary feedback input
(SECFB) instead, which allows a quasi-regulated,
adjustable output, coupled-inductor flyback winding to be
attached to either the 3.3V or the 5V main inductor. Bias
generator blocks include the 5V IC internal rail (VL) linear
regulator, 2.5V precision reference, and automatic boot-
strap switchover circuit. The PWMs share a common
333kHz/500kHz synchronizable oscillator.
These internal IC blocks aren’t powered directly from
the battery. Instead, the 5V VLlinear regulator steps
down the battery voltage to supply both VLand the
gate drivers. The synchronous-switch gate drivers are
directly powered from VL, while the high-side switch
gate drivers are indirectly powered from VLvia an
external diode-capacitor boost circuit. An automatic
bootstrap circuit turns off the 5V linear regulator and
powers the IC from the 5V PWM output voltage if the
output is above 4.5V.
PWM Controller BlockThe two PWM controllers are nearly identical. The only
differences are fixed output settings (3.3V vs. 5V), the
VL/CSL5 bootstrap switch connected to the 5V PWM,
and SECFB. The heart of each current-mode PWM con-
troller is a multi-input, open-loop comparator that sums
three signals: the output-voltage error signal with
respect to the reference voltage, the current-sense sig-
nal, and the slope-compensation ramp (Figure 3). The
PWM controller is a direct-summing type, lacking a tra-
ditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.
When SKIP= low, Idle Mode circuitry automatically
optimizes efficiency throughout the load current range.
Idle Mode dramatically improves light-load efficiency
by reducing the effective frequency, which reduces
switching losses. It keeps the peak inductor current
above 25% of the full current limit in an active cycle,
allowing subsequent cycles to be skipped. Idle Mode
transitions seamlessly to fixed-frequency PWM opera-
tion as load current increases.
With SKIP= high, the controller always operates in fixed-
frequency PWM mode for lowest noise. Each pulse from
the oscillator sets the main PWM latch that turns on the
high-side switch for a period determined by the duty fac-
tor (approximately VOUT/ VIN). As the high-side switch
turns off, the synchronous rectifier latch sets; 60ns later,
the low-side switch turns on. The low-side switch stays on
until the beginning of the next clock cycle.
In PWM mode, the controller operates as a fixed-fre-
quency current-mode controller where the duty ratio is
set by the input/output voltage ratio. The current-mode
feedback system regulates the peak inductor current
value as a function of the output-voltage error signal. In
continuous-conduction mode, the average inductor
current is nearly the same as the peak current, so the
circuit acts as a switch-mode transconductance ampli-
fier. This pushes the second output LC filter pole, nor-
mally found in a duty-factor-controlled (voltage-mode)
PWM, to a higher frequency. To preserve inner-loop
stability and eliminate regenerative inductor current
“staircasing”, a slope-compensation ramp is summed
into the main PWM comparator to make the apparent
duty factor less than 50%.
The MAX1901/MAX1902/MAX1904 use a relatively low
loop gain, allowing the use of lower-cost output capaci-
tors. The relative gains of the voltage-sense and cur-
rent-sense inputs are weighted by the values of current
sources that bias three differential input stages in the
main PWM comparator (Figure 4). The relative gain of
the voltage comparator to the current comparator is
internally fixed at K = 2:1. The low loop gain results in
the 2% typical load-regulation error. The low value of
loop gain helps reduce output filter capacitor size and
cost by shifting the unity-gain crossover frequency to a
lower level.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersFigure 2. MAX1902 Functional Diagram
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersFigure 3. PWM Controller Functional Block Diagram
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersThe output filter capacitors (Figure1, C1 and C2) set a
dominant pole in the feedback loop that must roll off the
loop gain to unity before encountering the zero intro-
duced by the output capacitor’s parasitic resistance
(ESR) (see the Design Procedure section). A 50kHz
pole-zero cancellation filter provides additional rolloff
above the unity-gain crossover. This internal 50kHz
low-pass compensation filter cancels the zero due to fil-
ter capacitor ESR. The 50kHz filter is included in the
loop in both fixed-output and adjustable-output modes.
Synchronous Rectifier Driver (DL)Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky catch
diode with a low-resistance MOSFET switch. Also, the
synchronous rectifier ensures proper startup of the
boost gate-driver circuit.
If the circuit is operating in continuous-conduction
mode, the DL drive waveform is simply the complement
of the DH high-side drive waveform (with controlled
dead time to prevent cross-conduction or “shoot
through”). In discontinuous (light-load) mode, the syn-
chronous switch is turned off as the inductor current falls
through zero. The synchronous rectifier works under all
operating conditions, including Idle Mode.
The SECFB signal further controls the synchronous switch
timing in order to improve multiple-output cross-regulation
(see the Secondary Feedback Regulation Loopsection).
Internal VL and REF SuppliesAn internal regulator produces the 5V supply (VL) that
powers the PWM controller, logic, reference, and other
blocks within the IC. This 5V low-dropout linear regula-
tor supplies up to 25mA for external loads, with a
reserve of 25mA for supplying gate-drive power.
Bypass VLto GND with 4.7µF.
Important:Ensure that VLdoes not exceed 6V.
Measure VLwith the main output fully loaded. If it is
pumped above 5.5V, either excessive boost-diode
capacitance or excessive ripple at V+ is the probable
cause. Use only small-signal diodes for the boost cir-
cuit (10mA to 100mA Schottky or 1N4148 are pre-
ferred), and bypass V+ to PGND with 4.7µF directly at
the package pins.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook ComputersThe 2.5V reference (REF) is accurate to ±2% over tem-
perature, making REF useful as a precision system ref-
erence. Bypass REF to GND with 1µF minimum. REF
can supply up to 5mA for external loads. (Bypass REF
with a minimum 1µF/mA reference load current.)
However, if extremely accurate specifications for both
the main output voltages and REF are essential, avoid
loading REF more than 100µA. Loading REF reduces
the main output voltage slightly, because of the refer-
ence load-regulation error.
When the 5V main output voltage is above 4.5V, an
internal P-channel MOSFET switch connects CSL5 to
VL, while simultaneously shutting down the VL linear
regulator. This action bootstraps the IC, powering the
internal circuitry from the output voltage, rather than
through a linear regulator from the battery.
Bootstrapping reduces power dissipation due to gate
charge and quiescent losses by providing that power
from a 90%-efficient switch-mode source, rather than
from a much less efficient linear regulator.
Boost High-Side Gate-Drive Supply
(BST3 and BST5)Gate-drive voltage for the high-side N-channel switches
is generated by a flying-capacitor boost circuit (Figure 2).
The capacitor between BST_ and LX_ is alternately
charged from the VL supply and placed parallel to the
high-side MOSFET’s gate-source terminals. On startup,
the synchronous rectifier (low-side MOSFET) forces LX_
to 0V and charges the boost capacitors to 5V. On the
second half-cycle, the SMPS turns on the high-side MOS-
FET 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 at the high-side MOSFET gate (DH3 and DH5)
in discontinuous-conduction mode (light loads) is a nat-
ural operating condition. It is caused by residual ener-
gy in the tank circuit, formed by the inductor and stray
capacitance at the switching node, LX. The gate-drive
negative rail is referred to LX, so any ringing there is
directly coupled to the gate-drive output.
Current-Limiting and Current-Sense
Inputs (CSH and CSL)The current-limit circuit resets the main PWM latch and
turns off the high-side MOSFET switch whenever the
voltage difference between CSH and CSL exceeds
100mV. This limiting is effective for both current flow
directions, putting the threshold limit at ±100mV. The
tolerance on the positive current limit is ±20%, so the
external low-value sense resistor (R1) must be sized for
80mV/ IPEAK, where IPEAKis the required peak-inductor
current to support the full load current, while compo-
nents must be designed to withstand continuous-
current stresses of 120mV/R1.
For breadboarding or for very-high-current applica-
tions, it may be useful to wire the current-sense inputs
with a twisted pair, rather than PC traces. (This twisted
pair need not be special; two pieces of wire-wrap wire
twisted together is sufficient.) This reduces the possible
noise picked up at CSH_ and CSL_, which can cause
unstable switching and reduced output current. The
CSL5 input also serves as the IC’s bootstrap supply
input. Whenever VCSL5> 4.5V, an internal switch con-
nects CSL5 to VL.
Oscillator Frequency and
Synchronization (SYNC)The SYNC input controls the oscillator frequency. Low
selects 333kHz; high selects 500kHz. SYNC can also
be used to synchronize with an external 5V CMOS or
TTL clock generator. SYNC has a guaranteed 400kHz
to 583kHz capture range. A high-to-low transition on
SYNC initiates a new cycle.
500kHz operation optimizes the application circuit for
component size and cost. 333kHz operation provides
increased efficiency, lower dropout, and improved
load-transient response at low input-output voltage dif-
ferences (see the Low-Voltage Operation section).
Shutdown ModeHoldingSHDNlow puts the IC into its 4µA shutdown
mode. SHDNis logic input with a threshold of about 1V
(the VTHof an internal N-channel MOSFET). For automat-
ic startup, bypass SHDNto GND with a 0.01µF capacitor
and connect it to V+ through a 220kΩresistor.
Power-Up Sequencing and
ON/OFFControlsStartup is controlled by RUN/ON3 and TIME/ON5 in
conjunction with SEQ. With SEQ tied to REF, the two
control inputs act as separate ON/OFFcontrols for
each supply. With SEQ tied to VL or GND, RUN/ON3
becomes the master ON/OFFcontrol input and
TIME/ON5 becomes a timing pin, with the delay
between the two supplies determined by an external
capacitor. The delay is approximately 800µs/nF. The
3.3V supply powers up first if SEQ is tied to VL, and the
5V supply is first if SEQ is tied to GND. When driving
TIME/ON5 as a control input with external logic, always
place a resistor (>1kΩ) in series with the input. This
prevents possible crowbar current due to the internal
discharge pulldown transistor, which turns on in stand-
by mode and momentarily at the first power-up or in
shutdown mode.
