MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR

MP155 Rev. 1.12

www.MonolithicPower.com

10

10/28/2013

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© 2013 MPS. All Rights Reserved.

MOS

Diode

2.5V

Monitoring the sampling capacitor regulates the

output voltage can be regulated, as per the

following equation:

+

=×

R1 R2

Vo 2.5V

R2

Frequency Foldback

Under light load or no load conditions, the output

drops very slowly, which increases the time for

the MOSFET to turn ON again; i.e., frequency

decreases as the load decreases. So the MP155

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

decreases from 290mA as the OFF time

increases. In standby mode, the frequency and

the peak current are both minimized, allowing for

a small dummy load. As a result, the peak-

current-compression function helps to reduce no-

load consumption. Determine the peak current

the power module OFF time:

=−

μ × τ − μ

Peak

off

I

290mA (1mA / S) (

18 S)

Minimum off time limitation

The MP155 implements a minimum OFF time

limit. During the normal operation, the minimum

OFF time limit is 18µs; during start up, the

minimum OFF time limit gradually drops from

72µs, to 36µs, then to 18 µs (see Figure 4). Each

minimum OFF time has 128 switching cycles.

This soft-start function allows for safe start-up.

128 Switching cycle

72us

≥

128 Switching cycle

36us

≥

18us

≥

Driver

EA Compensation

-

+

-

+

2.5V

EA

+

+

-

+

FB

Comparator

Figure 5: EA and Ramp Compensation

To

improve

load

regulation,

the

MP150

implements an error amplifier (EA) compensation

function ( Figure 5 ). The MP155 samples the

feedback voltage 6µs after the MOSFET turns off.

EA compensation regulates the 2.5V reference

voltage with the load, thus improving the power

module regulation.

RAMP Compensation

An internal ramp compensation circuit precisely

maintains the output voltage. An additional

exponential voltage sinking source pulls down

the feedback comparator’s reference voltage as

shown in Figure 5. The ramp compensation is

relative to the load conditions: Under full-load

conditions, the compensation is ~1mV/µs; with a

decreasing load, the compensation increases

exponentially.