MP174 – NON-ISOLATED OFF-LINE REGULATOR

MP174 Rev. 1.01

www.MonolithicPower.com

12

8/18/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.





R1 R2

Vo 2.55V

R2

Frequency

Foldback

and

Peak

Current

Compression

The MP174 remains highly efficient under light-

load

condition

by

reducing

the

switching

frequency automatically.

Under light-load or no-load conditions, the output

voltage drops very slowly, which increasing the

MOSFET off time. Thus the frequency decreases

as the load decreases.

Determine the switching frequency as:

in

o

o

s

peak

o

in

(V

V )

V

f

2L(I

I ) V







, for CCM





in

O

o

o

s

2

peak

in

2(V

V ) I V

f

LI

V

, for DCM

At the same time, the peak current limit

decreases from 660mA as the off-time increases.

In standby mode, the frequency and the peak

current are both minimized, allowing for a smaller

dummy

load.

As

a

result,

peak-current

compression

helps

further

reduce

no-load

consumption. Peak current limit can be estimated

from the following equation (

module’s off time):

Peak

off

I

660mA (2.4mA / s) (

12 s)



    

EA Compensation

-

+

M

-

+

V

ref

2.55V

EA

-

+

FB

Comparator

+

+

Figure 4: EA and Ramp Compensation

MP174

has

internal

error

amplifier

(EA)

compensation loop. It samples the feedback

voltage 6us after the MOSFET turns off, and

regulates the output based on the 2.55V

reference voltage.

Ramp Compensation

An internal ramp compensation circuit improves

the load regulation. As shown in Figure 4, an

exponential voltage signal added to pull down the

reference voltage of the feedback comparator.

The ramp compensation is a function of the load

conditions: the compensation is about the

1mV/µs under full-load conditions compensation

increases exponentially as the peak current

decreases.

Over-Load Protection (OLP)

Maximum output power of MP174 is limited by

maximum switching frequency and peak current

limit. If the load current is too large, output

voltage drops, so that the FB voltage drops.

When the FB voltage drops below 1.7V it is

considered as an error flag and timer starts. If the

This timer duration avoids triggering OLP when

the power supply starts up or load transition. The

power supply should start up in less than 220ms

per the following equation:





Delay

28kHz

220ms

fs

Short-Circuit Protection (SCP)

The MP174 monitors the peak current, and shuts

down when the peak current rises above SCP

threshold through short-circuit protection. The

power supply resumes operation with the

removal of the fault.

Thermal Shutdown (OTP)

To prevent any thermal induced damage, the

MP174 shuts down switching when the junction

shutdown

(OTP),

the

VCC

capacitor

is

discharged to 2.4V, and then the internal high

voltage regulator re-charges. MP174 recovers

when junction temperature drops below 120°C.