MP173– NON-ISOLATED OFFLINE REGULATOR

MP173 Rev. 1.0

www.MonolithicPower.com

10

8/19/2015

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© 2015 MPS. All Rights Reserved.

Frequency

Foldback

and

Peak

Current

Compression

The MP173 remains highly efficient at light-load

conditions 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 along with the load.

The switching frequency is determined 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)

As the peak current limit decreases from 420 mA,

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 from Equation (5) where

Peak

off

I

420mA (1.6mA / s) (

12 s)



    

(5)

EA Compensation

-

+

M

-

+

V

ref

2.55V

EA

-

+

FB

Comparator

+

+

Figure 4—EA and ramp compensation

MP173 has an internal error amplifier (EA)

compensation loop. It samples the feedback

voltage 6 µs after the MOSFET turns off and

regulates the output based on the 2.55 V

reference voltage.

Ramp Compensation

An internal ramp compensation circuit improves

the load regulation. An exponential voltage signal

is 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 about the 1 mV/µs under

full-load conditions. The compensation increases

exponentially as the peak current decreases.

Over-Load Protection (OLP)

The maximum output power of the MP173 is

limited by the maximum switching frequency and

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

the output voltage drops, causing the FB voltage

to drop.

considered an error flag, and the timer starts. If

occurs. This timer duration avoids triggering OLP

when the power supply starts up or the load

transitions. The power supply should start up in

time is calculated using Equation (6):

Delay

s

28kHz

220ms

f





(6)

Short-Circuit Protection (SCP)

The MP173 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.

Thermal Shutdown (TSD)

To prevent thermal induced damage, the MP173

stops switching when the junction temperature

the the internal high-voltage regulator re-charges.

MP173 recovers when the junction temperature

drops below 120°C.

Open-Loop Detection

and a re-start cycle begins. During a soft start,

the open-loop detection is blanked.