1MHz, All-Ceramic, 2.6V to 5.5V Input,

2A PWM Step-Down DC-to-DC Regulators

8

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through the high-side MOSFET body diode, back to the

input, until the beginning of the next cycle or until the

inductor current drops to zero. The MAX1951/MAX1952

utilize a pulse-skip mode to prevent overheating during

short-circuit output conditions. The device enters pulse-

skip mode when the FB voltage drops below 300mV, lim-

iting the current to 3A (typ) and reducing power

dissipation. Normal operation resumes upon removal of

the short-circuit condition.

Due to the high switching frequency and tight output

Ω resistor con-

Soft-Start

The MAX1951/MAX1952 employ digital soft-start circuitry

to reduce supply inrush current during startup conditions.

When the device exits undervoltage lockout (UVLO), shut-

down mode, or restarts following a thermal-overload

event, or the external pulldown on COMP is released, the

digital soft-start circuitry slowly ramps up the voltages at

REF and FB (see the Soft-Start Waveforms in the Typical

Operating Characteristics).

Undervoltage Lockout

clears, and the soft-start sequence activates.

Compensation

and Shutdown Mode

The output of the internal transconductance voltage

error amplifier connects to COMP. The normal operation

voltage for COMP is 1V to 2.2V. To shut down the

MAX1951/MAX1952, use an NPN bipolar junction

transistor or a very low output capacitance open-drain

MOSFET to pull COMP to GND. Shutdown mode causes

the internal MOSFETs to stop switching, forces LX to a

high-impedance state, and shorts REF to GND.

Release COMP to exit shutdown and initiate the soft-

start sequence.

Thermal-Overload Protection

Thermal-overload protection limits total power dissipation

= +160°C, a thermal sensor forces the device into shut-

down, allowing the die to cool. The thermal sensor turns

the device on again after the junction temperature cools

by 15°C, resulting in a pulsed output during continuous

overload conditions. Following a thermal-shutdown condi-

tion, the soft-start sequence begins.

MAX1951

COMP

CURRENT SENSE

SLOPE

COMP

ERROR

SIGNAL

CLOCK

POSITIVE AND NEGATIVE CURRENT LIMITS

GND

REF

FB

PGND

LX

IN

DAC

SOFT-START/

UVLO

REF

2V

PWM

CONTROL

THERMAL

SHUTDOWN

BANDGAP

REF

1.25V

CLAMP

OSC

RAMP GEN

Figure 1. Functional Diagram