MP28256 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MP28256 Rev. 1.0

www.MonolithicPower.com

8

11/3/2011

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© 2011 MPS. All Rights Reserved.

OPERATION

The MP28256 is a high frequency synchronous

rectified step-down switch mode converter with

built in internal 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 MP28256 operates in a fixed frequency,

peak current control mode to regulate the output

voltage. A PWM cycle is initiated by the internal

clock. The integrated high-side power MOSFET

is turned on and 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, in 90% of one PWM

period, the current in the power MOSFET 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

with the internal 0.6V reference (REF) and

outputs a current proportional to the difference

between the two. This output current is then used

to charge or discharge the internal compensation

network to form the COMP voltage, which is used

to control the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component counts and

simplifies the control loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes the

VIN input and operates in the full VIN range.

When VIN is greater than 5.0V, the output of the

regulator is in full regulation. When VIN is lower

than 5.0V, the output decreases, a 0.1uF ceramic

capacitor for decoupling purpose is required.

Enable Control

The MP28256 has a dedicated Enable control pin

(EN). By pulling it high or low, the IC can be

enabled and disabled by EN. Tie EN to VIN

through a resistor for automatic start up. To

disable the part, EN must be pulled low.

Frequency Synchronizing

The MP28256 can be synchronized to external

clock range from 300 kHz up to 2MHz through

the SYNC pin. The internal clock rising edge is

synchronized to the external clock rising edge.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The MP28256 UVLO comparator

monitors the output voltage of the internal

regulator, VCC. The UVLO rising threshold is

about 4.0V while its falling threshold is a

consistent 3.2V.

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 10uA 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 0.6V. 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

0.6V C (nF)

t(ms)

10 A







If the output of the MP28256 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 MP28256 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,

MP28256 will be hiccup. This is especially useful

to ensure system safety under fault condition.

The latch-off function is disabled during soft-start

duration.