MP171 –NON-ISOLATED OFFLINE REGULATOR

MP171 Rev. 1.0

www.MonolithicPower.com

10

9/30/2015

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© 2015 MPS. All Rights Reserved.

Frequency

Foldback

and

Peak

Current

Compression

The MP171 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 105 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

I105mA (0.4mA / s) (

10 s)



    

(5)

EA Compensation

-

+

M

-

+

V

ref

2.55V

EA

-

+

FB

Comparator

+

+

Figure 4—EA and ramp compensation

MP171 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. As shown in Figure 4, a

voltage sinking source is added to pull down the

reference voltage of the feedback comparator.

The ramp compensation is relative to the

MOSFET off time, and increases exponentially

as the off time increases. The compensation is

about 1mV/µs under min off time switching

condition.

Over-Load Protection (OLP)

The maximum output power of the MP171 is

limited by the maximum switching frequency and

the 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

36kHz

170ms

f





(6)

Short-Circuit Protection (SCP)

The MP171 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 MP171

stops switching when the junction temperature

the the internal high-voltage regulator re-charges.

MP171 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.