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MP156

Small, Energy-Efficient, Off-line Regulator 30mW No-Load Power Consumption

Monolithic Power Systems 

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

MOS

Diode

IL

Vo

VFB

2.5V

I peak

Io

Figure 3: VFB vs. VO

Thus monitoring the sampling capacitor regulates

the output voltage. Use the following equation to

determine the output voltage:

Vo = 2.5V × R1+ R2

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:

fs

=

(Vin − Vo ) ⋅

2L(Ipeak − Io )

Vo

Vin

,

for

CCM

fs

=

2(

Vin − VO

LI2peak

)

⋅

Io Vo

Vin

,

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 (τoff is the power

module’s off time):

IPeak = 290mA − (1mA / μs) × (τoff − 18μs)

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.

Driver ≥ 72us

≥ 36us

≥ 18us

128 Switching cycle 128 Switching cycle

Figure 4: τminoff at Start-Up

EA Compensation

FB

Comparator

-

EA

VFB

-

+

+

+

+

Vramp

Vramp

+ Vref

- 2.5V

Ipeak

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.

MP156 Rev. 1.15

www.MonolithicPower.com

10

4/21/2016

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© 2016 MPS. All Rights Reserved.

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