MP9151

– 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP9151 Rev. 1.0

www.MonolithicPower.com

11

3/20/2013

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External Soft-Start

The soft

start

time can

be

adjusted by

connecting a capacitor from this pin to ground.

When the soft-start period starts, an internal

8

μA current source begins charging the

external capacitor. During soft-start, the voltage

on the soft-start capacitor is connected to the

non-inverting input of the error amplifier. The

soft-start period lasts until the voltage on the

soft-start

capacitor

exceeds

the

reference

voltage of 0.795V. At this point the reference

voltage takes over at the no inverting error

amplifier input. The soft-start time can be

calculated as follows:

SS

SS

0.795V C (nF)

t (ms)

8A

If the output of the MP9151 is pre-biased to a

certain voltage during startup, the IC will disable

the switching of both high-side and low-side

switches until the voltage on the internal soft-

start capacitor exceeds the sensed output

voltage at the FB pin.

Power Save Mode for Light Load Condition

The

MP9151

has

AAM

(Advanced

Asynchronous Modulation) power save mode

for light load. The AAM voltage is set at 0.4V

internally. Under the heavy load condition, the

high, the high-side power MOSFET turns on

set by the COMP voltage. The internal clock

Under the light load condition, the value of

clock is blocked, thus the MP9151 skips some

pulses for PFM (Pulse Frequency Modulation)

mode and achieves the light load power save.

Figure 3: Simplified AAM Control Logic

When the load current is light, the inductor peak

current set internally is about 630mA and the

load current threshold exit AAM is about 0.3A

Over-Current-Protection

The MP9151 has hiccup over current limit when

the inductor current peak value exceeds the set

current limit threshold.

When output voltage drops below 70% of the

reference, and inductor current exceeds the

current limit at the meantime, MP9151 will be

hiccup. This is especially useful to ensure

system safety under fault condition.

The hiccup function is disabled during soft-start

duration.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from operating at exceedingly high

temperatures. When the silicon die temperature

is higher than 150 C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 120 C, the chip is

enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered

by an external bootstrap capacitor. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN through

more than 5V, U1 will regulate M1 to maintain a

5V BST voltage across C4.