MP156 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR

MP156 Rev. 1.15

www.MonolithicPower.com

10

4/21/2016

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MOS

Diode

2.5V

Thus monitoring the sampling capacitor regulates

the output voltage. Use the following equation to

determine the output voltage:

+

=×

R1 R2

Vo 2.5V

R2

Frequency Foldback

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

drops very slowly, thus increasing the time when

the MOSFET turn-on time. The frequency

decreases as the load decreases. The MP156

remains

highly

efficiency

under

light-load

condition by reducing the switching frequency

automatically.

Determine the switching frequency 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 smaller

dummy

load.

As

a

result,

peak-current

compression

helps

to

reduce

no-load

consumption. Estimate the peak current limit

from the following equation (

module’s off time):

Peak

off

I

290mA (1mA / s) (

18 s)

=−

μ × τ − μ

Minimum Off-Time Limit

The MP156 implements a minimum off-time limit.

During normal operation, the minimum off-time

limit is 18μs, and this limit gradually shortens

during the start-up period from 72μs, to 36μs, to

18μs (see Figure 4). Each minimum off-time

retains 128 switching cycles. This soft-start

function provides a safe start-up.

128 Switching cycle

72us

≥

128 Switching cycle

36us

≥

18us

≥

Driver

Figure 4:

min off

τ

at Start-Up

EA Compensation

-

+

-

+

2.5V

EA

+

+

-

+

FB

Comparator

Figure 5: EA and Ramp Compensation

The

MP156

features

error

amplifier

(EA)

compensation function (Shown in Figure 5) to

improve load regulation. 6µs after the MOSFET

turns off, the MP156 samples the feedback

voltage. Thus EA compensation regulates the

2.5V reference voltage with the load to improve

overall power regulation.

Ramp Compensation

An internal ramp compensation circuit precisely

maintains the output voltage. Figure 6 shows an

exponential voltage sinking source added to pull

down the reference voltage of the feedback

comparator. The ramp compensation is a

function of the load conditions: the compensation

is about the 1mV/µs under full-load conditions

compensation increases exponentially as the

load decreases.