MP174A – NON-ISOLATED OFF-LINE REGULATOR

MP174A Rev. 1.0

www.MonolithicPower.com

13

2/27/2019

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

© 2019 MPS. All Rights Reserved.

Use Equation (2) to determine the output voltage:





R1 R2

Vo 2.55V

R2

(2)

Frequency

Foldback

and

Peak

Current

Compression

The MP174A remains highly efficient under a

light-load condition by reducing the switching

frequency automatically.

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

voltage drops very slowly, which increases the

MOSFET

off

time.

Thus,

the

frequency

decreases as the load decreases.

Determine the switching frequency with Equation

(3) and Equation (4):

in

o

o

s

peak

o

in

(V

V )

V

f

2L(I

I ) V







, for CCM

(3)





in

O

o

o

s

2

peak

in

2(V

V ) I V

f

LI

V

, for DCM

(4)

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. The peak current limit can be

estimated with Equation (5).

τOff is the power

module’s off time.



    

Peak

Off

I

660mA (4.8mA / s) (

12 s) (5)

EA Compensation

-

+

VFB

M

-

+

Vref

2.55V

EA

-

+

FB

Comparator

Vramp

Vramp

Ipeak

+

+

Figure 4: EA and Ramp Compensation

The MP174A has an internal error amplifier (EA)

compensation loop. It samples the feedback

voltage 6μs 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. There is an exponential

voltage signal added to pull down the reference

voltage of the feedback comparator (see Figure

4). The ramp compensation is a function of the

load conditions: the compensation is 1mV/µs

under

full-load

conditions;

compensation

increases exponentially as the peak current

decreases.

Over-Load Protection (OLP)

The maximum output power of the MP174A is

limited by the maximum switching frequency and

peak current limit. If the load current is too large,

the output voltage drops, so the FB voltage drops.

When the FB voltage drops below 1.7V it is

considered an error flag and the timer starts. If

the timer reaches 220ms (fS=28kHz), OLP occurs.

This timer duration avoids triggering OLP when

the power supply starts up or the load transitions.

The power supply should start up in less than

220ms (fS=28kHz). The OLP delay time is

calculated using Equation (6):





Delay

28kHz

220ms

fs

(6)

Short-Circuit Protection (SCP)

The MP174A monitors the peak current and

shuts down when the peak current rises above

the SCP threshold through short-circuit protection.

The power supply resumes operation with the

removal of the fault. During soft start and the

following 512 switching cycles, the SCP is

blanked, in order to guarantee successful start-

up with a large output capacitor.

Thermal Shutdown (OTP)

To prevent thermal induced damage, the

MP174A stops switching when the junction

temperature exceeds 150°C. During the thermal

shutdown

(OTP),

the

VCC

capacitor

is

discharged to 2.4V, and the internal high-voltage

regulator re-charges. The MP174A recovers

when the junction temperature drops below

120°C.