Datasheet

MAX1951A
1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC
Step-Down Regulator with Enable
_______________________________________________________________________________________ 7
V
CC
Decoupling
Due to the high switching frequency and tight output
tolerance (1.5%), decouple V
CC
with a 0.1µF capacitor
connected from V
CC
to GND, and a 10Ω resistor con-
nected from V
CC
to IN. Place the capacitor as close as
possible to V
CC
.
Soft-Start
The MAX1951A employs digital soft-start circuitry to
reduce supply inrush current during startup conditions.
When the device exits undervoltage lockout (UVLO)
shutdown mode, or restarts following a thermal-over-
load event, or EN is driven high, the digital soft-start cir-
cuitry slowly ramps up the voltage to the error-amplifier
noninverting input.
Undervoltage Lockout
If V
CC
drops below 2.07V, the UVLO circuit inhibits
switching. Once V
CC
rises above 2.19V, the UVLO
clear and the soft-start sequence activates.
Shutdown Mode
Use the enable input, EN, to turn on or off the MAX1951A.
Connect EN to V
CC
for normal operation. Connect EN to
GND to place the device in shutdown. Shutdown causes
the internal switches to stop switching and forces LX into
a high-impedance state. In shutdown, the MAX1951A
draws 500µA of supply current. The device initiates a
soft-start sequence when brought out of shutdown.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the device. When the junction temperature exceeds
T
J
= +165°C, a thermal sensor forces the device into
shutdown, allowing the die to cool. The thermal sensor
turns the device on again after the junction temperature
cools by 9°C, resulting in a pulsed output during continu-
ous overload conditions. Following a thermal-shutdown
condition, the soft-start sequence begins.
Safe Startup into Prebiased Output
The MAX1951A can start up safely even with a prebi-
ased output. A zero crossover detection (ZCD) circuit
turns on the switches only after the soft-start ramping
voltage equals the prebiased output voltage. If the pre-
biased output voltage is greater than the set voltage,
the ZCD circuit turns on the low-side switch (after the
soft-start period is over) to discharge the output capac-
itor until its voltage equals the set voltage.
MAX1951A
V
CC
COMP
EN
CURRENT SENSE
SLOPE
COMP
ERROR
SIGNAL
CLOCK
POSITIVE AND NEGATIVE CURRENT LIMITS
GND
FB
PGND
LX
IN
DAC
g
m
SOFT-START/
UVLO
PWM
CONTROL
PREBIAS
THERMAL
SHUTDOWN
BANDGAP
REF
1.25V
ZERO-
CROSSING
DETECTOR
CLAMP
OSC
RAMP GEN
Figure 1. Functional Diagram