MP2228 – SYNCHRONOUS STEP-DOWN CONVERTER

MP2228 Rev. 1.1

www.MonolithicPower.com

9

8/20/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP2228 is a high-frequency, synchronous,

rectified, step-down, switch-mode converter

with built-in power MOSFETs. It offers a very

compact solution that achieves a 2A continuous

output current with excellent load and line

regulation over a wide input supply range.

The MP2228 operates in a fixed-frequency,

peak-current–control mode to regulate the

output voltage. An internal clock initiates a

PWM cycle. The integrated high-side power

MOSFET turns on and remains on until the

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 95% of one PWM period, the current in

the power MOSFET does not reach the value

set by the COMP value, the power MOSFET is

forced off.

Internal Regulator

A 5V internal regulator powers most of the

exceeds 5.0V, the output of the regulator is in

output decreases, and the part requires a 0.1µF

ceramic decoupling capacitor.

Error Amplifier

The error amplifier compares the FB pin voltage

outputs a current proportional to the difference

between the two. This output current then

charges

or

discharges

the

internal

compensation network to form 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.

Under-Voltage Lockout (UVLO)

The MP2228 has under-voltage lock-out

protection (UVLO). When the VCC voltage

exceeds the UVLO rising threshold voltage, the

MP2228 powers up. It shuts off when the VCC

voltage drops below the UVLO falling threshold

voltage. This is non-latch protection.

Soft-Start

Adjust the soft-start time by connecting a

capacitor from SS pin to ground. When the soft-

start begins, an internal 11µA current source

charges the external capacitor. During soft-start,

the soft-start capacitor connects to the non-

inverting input of the error amplifier. The soft-

start period continues until the voltage on the

soft-start capacitor exceeds the 0.8V reference.

Then the non-inverting amplifier uses the

reference voltage takes as the input. Use the

following equation to calculate the soft-start

time:

SS

0.8V Css(nF)

t(ms)

11 A

×

=

μ

Over-Current-Protection and Hiccup

The MP2228 has a cycle-by-cycle over-current

limit when the inductor current peak value

exceeds the set current limit threshold.

below the Under-Voltage (UV) threshold—

typically 50% below the reference. Once UV is

triggered, the MP2228 enters hiccup mode to

periodically restart the part. This protection

mode is especially useful when the output is

dead-shorted to ground, and greatly reduces

the average short circuit current to alleviate

thermal issues and protect the regulator. The

MP2228 exits the hiccup mode once the over-

current condition is removed.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die reaches temperatures that

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

When the temperature drops below its lower

threshold, typically 130°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. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor