MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER

MP4570 Rev. 1.01

www.MonolithicPower.com

14

1/4/2017

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OPERATION

The MP4570 is a step-down switching regulator

with integrated high-side high voltage power

MOSFETs. It features as wide input voltage

range, high efficiency, external internal soft-start,

frequency programmable and comprehensive

protection mode, i.e. OVP, OCP, OTP.

PWM Control

The MP4570 uses peak current mode control to

regulate the output voltage.

A PWM cycle is initiated by the internal clock at

the beginning of every cycle. After the high side

MOSFET turns on, the inductor current will rise

linearly to provide the energy to the load. The

high side MOSFET remains on until its current

hits the COMP voltage which is the output of the

internal error amplifier (EA). The output voltage

of error amplifier depends on the difference of

output feedback voltage and the internal high

precision reference and it will decide how much

energy should be transferred to the load. The

higher load current, the higher COMP voltage.

After the high switch is off, the low side switch is

on and the inductor current will flow through the

low side switch. In order to avoid shoo-through

issue, the dead time is inserted to avoid the high

side and low side MOSFETS to turn on at the

same time. For each turn on and off in a

switching cycle, the high side MOSFET will keep

on and off with minimum on and off time limit.

Light Load Operation

In order to get high efficiency, MP4570 has two

features during light load: 1).When the load

current decreases, the inductor current will drop

at same time. The low side MOSFET will turn off

in order to save driver loss when inductor current

drops to zero. 2) When the load decreases, the

switching frequency will be scaled down in order

to reduce switching loss after COMP voltage

drops down lower than certain threshold.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal reference and outputs a current

proportional to the difference between the two.

This current is used to charge the external

compensation networks to form the COMP

voltage, which is used to control the high side

MOSFET peak current and to regulate the output

voltage.

Oscillator and SYNC Function

The internal oscillator frequency is set by a single

FREQ pin and GND. The frequency setting

resistor should be located close to the device.

The relationship between oscillator frequency

INFORMATION section.

During light load, the switching frequency will be

scalded down according to the COMP voltage.

The switching frequency will start to decrease

when the COMP voltage is lower than around

0.8V. And the switching will be disabled when the

COMP voltage drops lower than around 0.7V.

In order to reduce the switching loss and the

thermal dissipation, the switching frequency will

be decreased according to the FB voltage. When

the FB is lower than 25%xRFE, the switching

frequency starts to decrease from the normal

value, and finally drops to 5% of the normal value

when the FB is zero.

The FREQ pin can be used to synchronize the

internal oscillator rising edge to an external clock

falling edge. Make sure the HIGH amplitude of

SYNC clock is higher than 1.5V and LOW

amplitude is lower than 1V to drive the internal

logic.

The

recommended

external

SYNC

frequency is in the range of 100kHz and 1MHz.

There is no pulse width requirement but please

note that there is always parasitic capacitance of

the pad there, so, if the pulse width is too short, a

clear rising and falling edge may not be seen due

to the parasitic capacitance. The pulse longer

than 100ns is recommended in application.

EN Control

EN is a control pin that turns the regulator on and

off: Drive EN higher than 1.6V to turn on the

regulator, drive it lower than 1.3V to turn off.

There is no internal pull-up or pull-down at EN,

so when it is floating, the EN status is uncertain.

The EN pin is clamped internally using a 6.5V

zener diode between EN and GND, as shown in

the functional block diagram. Connecting the EN

pin directly to a voltage source without any pull-