MP157 –SMALLEST ENERGY EFFICIENT OFF-LINE REGULATOR WITH EXTENDED POWER RANGE

MP157 Rev. 1.04

www.MonolithicPower.com

12

6/27/2016

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MOS

Diode

2.5V

Figure 3: VFB Vs Vout

Thus by monitoring the sampling capacitor, the

output voltage can be regulated, and the output

voltage is determined by the following equation:

R1 R2

Vout

2.5V

R2





Frequency Foldback

At the light load or no load conditions, the

output drops very slowly. This makes the

MOSFET OFF time increase. The frequency

decreases as the load decreases. So MP157

can maintain a high efficiency under light load

condition by reducing the switching frequency

automatically.

The switching frequency can be obtained 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 starts to

decrease from 640mA as the off-time increases.

At standby mode, the frequency and the peak

current are both minimum, so smaller dummy

load could be added. As a result, peak current

compression function helps to save no load

consumption. The peak current limit can be

off time of the power module):



 





peak

off

I

640mA (3mA / s) (T

13.1 s)

Minimum off time limitation

A minimum off time limitation is implemented.

During the normal operation, the minimum off

time limit is 13.1

s, and during the start up

period, the minimum off time limit is shorten

gradually from 52.4

s, 26.2s to 13.1s

(Shown as Figure 4). Each minimum off time

retains 128 switching cycle. This soft start

function provides safe start-up.

128 Switching cycle

52.4us



128 Switching cycle

26.2us



13.1us



Driver

EA Compensation

-

+

-

+

2.5V

EA

+

+

-

+

FB

Comparator

Figure 5: EA and Ramp Compensation

To obtain a better load regulation, MP157

features EA (Error Amplifier) Compensation

function (Shown as Figure 5). After 6

s delay of

MOSFET turning off, MP157 samples the

feedback

voltage.

So

with

the

EA

Compensation, the reference voltage— 2.5V

can be regulated with the load. This can make

better regulation of power module.

Ramp Compensation

To maintain the precise output voltage, an

internal ramp compensation circuit is added ---

an exponential voltage sinking source will be

added to pull down the reference voltage of the