MP28251 – 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP28251 Rev. 1.16

www.MonolithicPower.com

10

4/17/2013

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When the LPM pin connects to VCC, the

inductor peak current in set internally which is

about 240mA for VIN=12V, VOUT=3.3V and

L=4.7uH. Peak inductor current also can be set

by connecting LPM pin to a voltage divider from

VCC with 2 external resistors. By program LPM

voltage, efficiency and output ripple can be

traded off and optimize. Higher LPM gives

higher peak inductor current thus more efficient

but more output ripple. To help select the LPM

voltage, the curve of inductor peak current vs.

low power mode voltage is shown in Page 5,

Typical Performance Characteristics section.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at insufficient supply voltage.

The MP28251’s UVLO comparator monitors the

output voltage of the internal regulator, VCC.

The UVLO rising threshold is 4.25V while its

falling threshold is 3.38V.

External Soft-Start

Connect a capacitor from the soft-start pin to

ground to adjust the soft-start time. When soft-

start begins, an internal 8μA current source

charges the external capacitor. The soft-start

capacitor connects to the non-inverting input of

the error amplifier. The soft-start period lasts

until the voltage on the soft-start capacitor

exceeds the 0.815V reference. At this point the

reference voltage takes over at the non-

inverting error-amplifier input. The soft-start

time can be calculated as follows:

SS

SS

0.815V C (nF)

t(ms)

8A







If the output of the MP28251 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.

Over-Current-Protection

The MP28251 has a hiccup mode over-current

limit for when the inductor current peak value

exceeds the set current limit threshold.

When the output voltage drops below 70% of

the reference, and inductor current exceeds the

current limit, the MP28251 will enter soft-start

mode and retry until the fault is removed. This

is especially useful to ensure system safety

under fault conditions.

The latch-off function is disabled during soft-

start duration.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds

150°C, it shuts down the whole chip. When the

temperature falls below its lower threshold,

typically 120°C, the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. The

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN through

exceeds 5V, U1 regulates M1 to maintain a 5V

BST voltage across C4.

D1

M1

BST

C4

L1

SW

U1

5V

C2

Figure 3: Internal Bootstrap Charging Circuit

Startup and Shutdown

If both VIN and EN exceed their respective

thresholds, the chip is enabled. The reference

block starts first, generating a stable reference

voltage and currents, and then the internal

regulator is enabled. The regulator provides a

stable supply for the remaining circuitries.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In shutdown, the

signaling path is first blocked to avoid any fault

triggering. The COMP voltage and the internal

supply rail are then pulled down. The floating

driver is not subject to this shutdown command.