MP28266 – 21V, 3A, 500kHz, SYNCHRONOUS, STEP-DOWN CONVERTER

MP28266 Rev. 1.0

www.MonolithicPower.com

8

3/16/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

The MP28266 is a high-frequency, synchronous,

rectified, step-down, switch-mode converter with

built-in power MOSFETs. It offers a very compact

solution to achieve more than 3A continuous

output current over a wide input supply range,

with excellent load and line regulation.

The MP28266 operates in a fixed-frequency,

peak-current–control mode to regulate the output

voltage. An internal clock initiates the PWM cycle

to turn on the integrated high-side power

MOSFET. This MOSFET remains on until its

current reaches the value set by the COMP

voltage. When the power switch is OFF, it

remains off until the next clock cycle starts. If

within 90% of one PWM period the power

MOSFET current does not reach the COMP-set

current value, the power MOSFET will be forced

to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

against the internal 0.6V reference (REF) and

outputs a current proportional to their difference.

This output current charges or discharges the

internal compensation network for the COMP

voltage, which controls the power MOSFET

current. The optimized internal compensation

network minimizes the external component

counts and simplifies the control loop design.

Internal Regulator

The 5V internal regulator powers most of the

exceeds 5.0V, the output of the regulator is in full

output decreases and requires a 0.1μF ceramic

decoupling capacitor.

Frequency Synchronizing

The MP28266 can be synchronized through the

SYNC pin to an external clock with a range from

300kHz up to 2MHz. The internal clock rising

edge is synchronized to the external clock rising

edge.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

against operating at an insufficient supply

voltage.

The MP28266’s UVLO comparator monitors the

output voltage of the internal regulator, VCC. The

UVLO rising threshold is about 5.5V while its

falling threshold is a consistent 4.4V.

External Soft-Start

Adjust the soft start time through the capacitor

connected from SS to ground. When the soft-

start period starts, an internal 10µA current

source charges the external capacitor. During

soft-start, the voltage on 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

reference voltage of 0.6V. 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

0.6V C (nF)

t(ms)

10 A







If the output of the MP28266 is pre-biased to a

certain voltage during startup, the IC will disable

both the 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 MP28266 has a hiccup 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 while

the inductor current exceeds the current limit, the

MP28266 enters hiccup mode. This is especially

useful to ensure system safety under fault

conditions. The latch-off function is disabled

during the 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 is below its lower threshold—

typically 140°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 with a rising