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

MP28251 Rev. 1.16

www.MonolithicPower.com

9

4/17/2013

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OPERATION

The MP28251 is a high-frequency, synchronous,

rectified, step-down, switch-mode converter

with internal power MOSFETs. It offers a very

compact solution to achieve 4A of continuous

output current over a wide input supply range

with excellent load and line regulation.

The MP28251 operates in a fixed-frequency,

peak-current–mode control to regulate the

output voltage. The internal clock initiates the

PWM cycle, turning on the integrated high-side

power MOSFET. The high-side 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 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

against the internal 0.8V reference (REF) and

outputs a current proportional to the difference

between the two. This output current 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 count and simplifies the control loop

design.

Internal Regulator

The 5V internal regulator powers most of the

internal circuitries. This regulator takes the VIN

input and operates in the full VIN range. When

VIN exceeds 5.0V, the output of the regulator is

in full regulation. When VIN is less than 5.0V,

the output decreases. The part requires a 0.1µF

ceramic decoupling capacitor.

Enable Control

The MP28251 has a dedicated enable control

pin (EN): pulling it high enables the IC, pulling it

low disables it.. Tie EN to VIN through a resistor

for automatic start up. EN must be pulled low to

disable the part.

The EN pin is clamped internally using a 6.7V

series-Zener-diode as shown in Figure 2.

Connect the EN input pin through a pullup

resistor to any voltage connected to the VIN pin

such that the pullup resistor limits the EN input

current to less than 100µA.

(12V – 6.7V)/100µA = 53kΩ.

Connecting the EN pin is directly to a voltage

source without any pullup resistor requires

limiting the amplitude of the voltage source to

below 6V to prevent damage to the Zener diode.

EN

EN LOGIC

Zener

6.7V typ.

GND

Figure 2: Zener Diode between EN and GND

The EN pin also features an internal 2.3μA

current source. Connect a capacitor to the EN

pin for delayed startup. When VIN exceeds the

input UVLO, an internal 2.3μA current source

charges the external capacitor. The external

capacitor connects to the non-inverting input of

a comparator. The part is enabled once the

capacitor voltage exceeds the 1.5V internal

reference voltage.

Frequency Synchronizing

The MP28251 can be synchronized to an

external clock with a range from 300kHz to

2MHz through the SYNC pin. The internal clock

rising edge is synchronized to the external clock

rising edge.

Low Power Mode

The MP28251 can operate in discontinuous

mode (DCM) and Pulse skip mode (PSM) under

light load to improve efficiency, featured as the

Low Power Mode. To enable Low Power Mode,

connect LPM pin to VCC pin or to a voltage

divider from VCC. To disable Low Power Mode,

connect LPM pin to ground and the converter

will always operate in CCM.