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

MP4570 Rev. 1.01

www.MonolithicPower.com

15

1/4/2017

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up resistor requires limiting voltage amplitude to

≤6V to prevent damage to the zener diode. EN

pin can be connected to higher voltage (e.g. VIN)

through pull-up resistor if the system doesn’t

have another logic signal acting as enable signal.

Just make sure the pull-up resistor is high

enough to make sure the sink current into EN pin

less than 150uA to avoid damaging the zener

diode.

For example, when connecting EN to

Soft Start

The soft start is implemented to ensure the

smooth up output voltage during the power on

and off. In addition, the soft start function also

helps to reduce the inrush current value at the

startup.

The soft start function is achieved by ramping SS

up slowly and using SS to override the internal

reference (REF) when SS-900mV is lower than

REF. When SS-900mV is higher than REF, REF

regains the control. The 900mV above is the

offset voltage of SS which means SS is detected

as 0 internally when it is lower than 900mV. To

minimize the delay for SS to reach 0.9V, an

internal pull-up circuit with about 30uA average

current pulls SS up to 600mV first. Then use 4uA

constant current to charge SS until it reaches to

2.5V. When SS is in the range of 0.9V to 1.9V, it

overrides the REF as reference voltage of the

error amplifier. During this period, output voltage

ramps up from 0 to the regulated value following

external SS capacitor can be calculated by below

formula:

A)

(

I

(V)

V

(nF)

C

(ms)

t

SS

REF

SS

SS

μ





4

The delay time for SS reaches to 900mV can be

estimated as below:

A

4

0.3V

(nF)

C

A

30

0.6V

(nF)

C

(ms)

t

SS

SS

_delay

SS

μ

μ









There is also an internal fixed 500us soft start.

The final SS time is determined by the longer

time between 500μs and external SS setting time.

When output voltage is shorted to GND,

feedback voltage is pulled to low then the SS will

be discharged. The part will soft start again when

the short at output is removed.

Internal Regulator and BIAS

There is an internal 2.5V regulator powers all the

internal control circuits. The 2.5V regulator takes

VIN as power supply when BIAS pin is lower than

3.2V while supplies from BIAS when BIAS is

higher than 3.2V.

VDD regulator powers LS driver and also BST

regulator when it is higher than 4.5V. VDD is

powered from VIN when BIAS is floating and

regulated at 3.6V. When BIAS is higher than

4.2V, it starts to power VDD. VDD will increases

as BIAS rising with 600mV voltage drop and

regulated at 4.8V when BIAS is higher than 5.4V.

A 1µF decoupling capacitor is needed at the pin

and make the capacitor as close to the pin as

possible.

Using BIAS to power internal regulator can

improve the efficiency. It is recommended to

connect BIAS to an external power supply that is

in the range of 3.3V to 5.5V. Output voltage is a

good choice for this power supply if it is in above

range. A 0.1µF to 1µF decoupling capacitor at

the pin is recommended.

Over Voltage Protection

The MP4570 monitors the feedback output

voltage to achieve the over voltage protection. If

the FB voltage is higher than the 115%xREF, the

high side and low side MOSFET will be turned off

immediately, the PG signal will be asserted to

inform the fault status. High side and low side

MOSFET will resume switching following close

loop operation after FB voltage drops back to

lower than 103%xREF.

Over Current Protection

The MP4570 has cycle-by-cycle peak current

limit protection and valley current detection

protection. The inductor current is monitored

during the high side MOSFET on state. If the

inductor current exceeds the current limit value

set by COMP voltage, the high side MOSFET

turns off immediately. Then, the low side

MOSFET will be turned on to discharge the

energy and inductor current will decrease. The

high side MOSFET will not be on again unless