MP4423 – 36V, 3A SYNCHRONOUS STEP-DOWN CONVERTER

MP4423 Rev. 1.0

www.MonolithicPower.com

12

8/18/2014

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OPERATION

The MP4423 is a high-frequency, synchronous,

rectified, step-down, switch-mode converter with

built-in power MOSFETs. It offers a very compact

solution to achieve 3A continuous output current

with excellent load and line regulation over a

wide input supply range.

The MP4423 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 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 the current in the power MOSFET

does not reach the current value set by COMP

within 95% of one PWM period, the power

MOSFET will be forced to turn off.

Internal Regulator

The 5V internal regulator power most of the

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

of the regulator decreases following the VIN. A

0.1uF decoupling ceramic capacitor is needed at

the pin.

Error Amplifier

The error amplifier compares the FB pin voltage

against the internal 0.8V reference (REF) and

outputs a COMP voltage—this COMP voltage

controls the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component count and

simplifies the control loop design.

Enable/SYNC Control

EN/SYNC is a digital control pin that turns the

regulator on and off: Drive EN high to turn on the

regulator, drive it low to turn it off. An internal

500kΩ resistor from EN/SYNC to GND allows

EN/SYNC to be floated to shut down the chip.

The EN pin is clamped internally using a 6.5V

series Zener diode, as shown in Figure 2.

Connect the EN input pin through a pullup

the pullup resistor limits the EN input current to

less than 150µA.

≥ (12V – 6.5V) ÷ 150µA = 36.7kΩ.

Connecting the EN pin directly to a voltage

source without any pullup resistor requires

limiting voltage amplitude to ≤6V to prevent

damage to the Zener diode.

Figure 2: 6.5V-Type Zener Diode

Connect an external clock with a range of

200kHz to 2.2MHz 2ms after output voltage is set

to synchronize the internal clock rising edge to

the external clock rising edge. The pulse width of

external clock signal should be less than 1.7μs.

Under-Voltage Lockout

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP4423 UVLO comparator monitors the

output voltage of the internal regulator, VCC. The

UVLO rising threshold is about 3.5V while its

falling threshold is 3.3V.

Internal Soft-Start

The soft-start prevents the converter output

voltage from overshooting during startup. When

the chip starts, the internal circuitry generates a

soft-start voltage (SS) that ramps up from 0V to

1.2V. When SS is lower than REF, SS overrides

REF so the error amplifier uses SS as the

reference. When SS exceeds REF, the error

amplifier uses REF as the reference. The SS

time is internally set to 1.5ms.

Over-Current Protection and Hiccup

The MP4423 has cycle-by-cycle over current limit

when the inductor current peak value exceeds

the set current limit threshold. If the output

voltage starts to drop until FB is below the Under-

Voltage (UV) threshold—typically 84% below the

reference—the MP4423 enters hiccup mode to

periodically restart the part. This protectionmode

is especially useful when the output is dead-

shorted to ground. The average short-circuit

current is greatly reduced to alleviate the thermal

issue

and

to

protect

the

regulator.