MP4575

– 5A, 55V, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4575 Rev. 1.0

www.MonolithicPower.com

15

8/16/2016

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does not have another logic signal acting as the

enable signal. Ensure that the pull-up resistor is

high enough to ensure that the sink current

going into EN is less than 150µA to avoid

damaging the Zener diode. For example, when

6.5V) ÷ 150µA = 37

kΩ.

Soft Start (SS)

Soft start (SS) is implemented to ensure a

smooth start-up of the output voltage during

power-on and power-off. The soft start function

also helps to reduce inrush current at start-up.

The soft start function is achieved by ramping

SS up slowly and overriding the internal

reference (REF) when SS - 900mV is lower

than REF. When SS - 900mV is higher than

REF, REF regains control. 900mV is the offset

voltage of SS, which means that 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 30µA of

average current pulls SS up to 600mV first.

Then use a 4µA constant current to charge SS

until it reaches 2.5V. When SS is in the range of

0.9V to 1.9V, it overrides REF as the reference

voltage of the error amplifier. During this period,

the output voltage ramps up from 0 to the

regulated value following SS rising. The soft-

can be calculated with Equation (1):

A)

(

I

(V)

V

(nF)

C

(ms)

t

SS

REF

SS

SS

μ





(1)

the 4µA SS charge current.

The delay time for SS reaching 900mV can be

estimated with Equation (2):

A

4

0.3V

(nF)

C

A

30

0.6V

(nF)

C

(ms)

t

SS

SS

_delay

SS

μ

μ









(2)

There is also an internal, fixed, 500µs soft start.

The final SS time is determined by the longer

time between 500

μs and the external SS setting

time.

When the output voltage is shorted to GND, the

feedback voltage is pulled low, and then SS is

discharged. The MP4575 soft starts again when

the short at the output is removed.

Internal Regulator and BIAS

An internal 2.5V regulator powers all of the

internal control circuits. This regulator uses VIN

as the power supply when BIAS is lower than

3.2V and uses BIAS as the supply when BIAS

is higher than 3.2V.

The VDD regulator powers the low-side driver

and the BST regulator when the VDD voltage is

BIAS is floating and is regulated at 3.6V. When

BIAS is higher than 4.2V, it powers VDD. VDD

increases as BIAS rises with a 600mV voltage

drop and is regulated at 4.8V when BIAS is

higher than 5.4V. A 1µF decoupling capacitor is

needed at VDD to make the capacitor as close

to VDD as possible.

Using BIAS to power the internal regulator can

improve

efficiency.

It

is

recommended

to

connect BIAS to an external power supply in

the range of 3.3V to 5.5V. The output voltage is

a good choice for this power supply if it is in

above range. A 0.1µF to 1µF decoupling

capacitor at BIAS is recommended.

Over-Voltage Protection (OVP)

The MP4575 monitors the feedback output

voltage

to

achieve

over-voltage

protection

(OVP). If the FB voltage is higher than

103%xREF, the MP4575 switches to sleep

mode, the HS-FET turn offs, and the LS-FET

turns on to discharge the output energy. The

MP4575 returns to normal after the FB voltage

drops below 103%xREF.

If the FB voltage is higher than 110%xREF, the

HS-FET and LS-FET are turned off immediately.

Both MOSFETs are latched, and the PG signal

is asserted to indicate the fault status and if EN

or VIN must be recycled to clear the protection.

Over-Current Protection (OCP)

The MP4575 has a cycle-by-cycle peak-current-

limit protection and valley current detection

protection. The inductor current is monitored

during the HS-FET on state. If the inductor

current exceeds the current limit value set by

the LS-FET is turned on to discharge the

energy, and the inductor current decreases.

The HS-FET does not turn on again until the

inductor valley current is below a certain current