MAX1951A データシートの表示（PDF） - Maxim Integrated

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MAX1951A

1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable

Maxim Integrated 

1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC

Step-Down Regulator with Enable

VCC

COMP

EN

OSC

RAMP GEN

CLAMP

POSITIVE AND NEGATIVE CURRENT LIMITS

CURRENT SENSE

SLOPE

COMP

ERROR

SIGNAL

CLOCK

PWM

CONTROL

THERMAL

SHUTDOWN

PREBIAS

ZERO-

CROSSING

DETECTOR

SOFT-START/

DAC

UVLO

BANDGAP

REF

1.25V

gm

MAX1951A

IN

LX

PGND

FB

GND

Figure 1. Functional Diagram

VCC Decoupling

Due to the high switching frequency and tight output

tolerance (1.5%), decouple VCC with a 0.1µF capacitor

connected from VCC to GND, and a 10Ω resistor con-

nected from VCC to IN. Place the capacitor as close as

possible to VCC.

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 VCC drops below 2.07V, the UVLO circuit inhibits

switching. Once VCC 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 VCC 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

TJ = +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.

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