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MAX16922ATPB/V

2.2MHz, Dual, Step-Down DC-DC Converters, Dual LDOs, and RESET

Maxim Integrated 

MAX16922

2.2MHz, Dual, Step-Down DC-DC

Converters, Dual LDOs, and RESET

Detailed Description

The MAX16922 PMIC is designed for medium-power

-level automotive applications requiring multiple supplies

in a small footprint. As shown in the Typical Applications

Circuit, the MAX16922 integrates one high-voltage power

supply and three low-voltage cascaded power supplies.

OUT1 and OUT2 are step-down DC-DC converters, and

OUT3 and OUT4 are linear regulators. The device also

includes a reset output (RESET) and a high-voltage -com-

patible enable input (EN). The operating input voltage

range is from 3.5V to 28V, tolerant of transient voltages

up to 45V.

OUT1 Step-Down DC-DC Regulator

Step-Down Regulator Architecture

OUT1 is a high-input-voltage, high-efficiency 2.2MHz

PWM current-mode step-down DC-DC converter that

delivers up to 1.2A. OUT1 has an internal high-side

n-channel switch and uses a low-forward-drop free-

wheeling diode for rectification. Under normal operating

conditions, OUT1 is fixed-frequency to prevent unwanted

AM radio interference. However, under light loads and

high-input voltage, the step-down regulator skips cycles to

maintain regulation. The output voltage is factory select-

able from 3.0V to 5.5V in 50mV increments.

Soft-Start

When initially powered up or enabled with EN, the OUT1

step-down regulator soft-starts by gradually ramping up

the output voltage for approximately 2.2ms. This reduces

inrush current during startup. During soft-start, the full

output current is available. Before a soft-start sequence

begins, the outputs of both DC-DC regulators discharge

below 1.25V through an internal resistor. See the start-

up waveforms in the Typical Operating Characteristics

section.

Current Limit

The MAX16922 limits the peak inductor current sourced

by the n-channel MOSFET. When the peak current limit is

reached, the internal n-channel MOSFET turns off for the

remainder of the cycle. If the current limit is exceeded for

16 consecutive cycles and the output voltage is less than

1.25V, the n-channel MOSFET is turned off for 256 clock

cycles to allow the inductor current to discharge and then

initiate a soft-start sequence for all four outputs.

Dropout

The high-voltage step-down converter (OUT1) of the

MAX16922 is designed to operate near a 100% duty

cycle. When the input voltage is close to the output volt-

age, the device tries to maintain the high-side switch on

with a 100% duty cycle. However, to maintain proper

gate charge, the high-side switch must be turned off peri-

odically so that the LX pin can go to ground and charge

the BST capacitor. As the input voltage approaches the

output voltage, the effective duty cycle of the n-channel

MOSFET approaches 94%. Every 4th cycle is limited to a

maximum duty cycle of 75% (recharge period is approxi-

mately 112ns), while the remaining cycles can go to 100%

duty cycle. As a result, when the MAX16922 is in dropout,

the switching frequency is reduced by a factor of 4.

During dropout conditions under light load, the load

current may not be sufficient to enable the LX pin to reach

ground during the recharge period. To ensure that the LX

pin is pulled to ground and proper BST capacitor recharge

occurs, an internal load is applied to OUTS1 when PV1

falls below approximately 6.5V. This load is approxi-

mately 70Ω and is connected between OUTS1 and GND3

through an internal switch.

OUT2 Step-Down DC-DC Regulator

Step-Down Regulator Architecture

OUT2 is a low-input-voltage, high-efficiency 2.2MHz PWM

current-mode step-down DC-DC converter that outputs

up to 600mA. OUT2 has an internal high-side p-channel

switch and a low-side n-channel switch for synchronous

rectification. The DC-DC regulator supports auto-PWM

operation so that under light loads the device automatically

enters high-efficiency skip mode. The auto-PWM mode

can be disabled by connecting the PWM input to OUTS1.

The output voltage is factory selectable from 1.0V to 3.9V

in 50mV increments.

Soft-Start

OUT2 enters soft-start when OUT1 finishes its soft-start

sequence to prevent high startup current from exceed-

ing the maximum capability of OUT1. The step-down

regulator executes a soft-start by gradually ramping up

the output voltage for approximately 1.5ms. This reduces

inrush current during startup. During soft-start, the full

output current is available. The soft-start sequence on

OUT2 begins after the soft-start sequence is completed

on OUT1. See the startup waveforms in the Typical

Operating Characteristics section.

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