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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

For best results, use ceramic, tantalum or low

ESR electrolytic capacitors to reduce the output

voltage ripple.

Feedback Resistors

The resistor divider determines the output

voltage. Choose appropriate values for R1 and

R2 to maintain VFB at 2.5V. Avoid large R2 value

(typically 5kΩ to 10kΩ).

Feedback Capacitor

The feedback capacitor provides a sample and

hold function. Small capacitors result in poor

regulation at light loads, and large capacitors

affect the circuit operation. Roughly estimate an

optimal capacitor value using the following

equation:

1 Vo

2 R1 + R2

⋅ Co

Io

≤ CFB

≤

Vo

R1 + R2

⋅ Co

Io

Choose the nearest appropriate value.

Dummy Load

A dummy load is required to maintain the load

regulation. This ensures sufficient inductor

energy to charge the sample and hold capacitor

to detect the output voltage. Normally a 3mA

dummy load is needed and can be adjusted to

the regulated voltage. Increasing the dummy load

reduces the efficiency and no-load consumption.

Use a zener diode if no-load regulation is not a

concern.

Auxiliary VCC Supply

R3 D3

VCC

D2

C3

R1

MP157

FB

SOURCE

R2

SOURCE

C4

L1

VOUT

Figure 9: Auxiliary VCC Supply Circuit

The MP156 drops below the 30mW no-load

power requirement. This chip requires a diode

(D3) to reduce overall power consumption.

In addition, the MP156 requires an external VCC

supply. This supply is derived from the resistor

connected between C3 and C4. For values

above VO=7V, determine R3 as per the formula

below.

R ≈ Vo − 5.8V

180μA

Surge Performance

Select an appropriate input capacitor value to

obtain a good surge performance. Figure 10

shows the half-wave rectifier. Table 2 shows the

capacitance required under normal condition for

different surge voltages.

UL

L1

+

C1

C2

UN

-

Figure 10: Half-Wave Rectifier

Table 2: Recommended Capacitance

Surge

voltage

500V

1000V

2000V

C1

1μF

10μF

22μF

C2

1μF

4.7μF

10μF

Layout Guide

PCB layout is very important for reliable

operation, and good EMI and thermal

performance. Please follow these guidelines to

optimize performance.

1) Minimize the loop area formed by the input

capacitor, IC, freewheeling diode, inductor

and output capacitor.

2) Place the power inductor far away from the

input filter.

3) Place a capacitor valued at several hundred

pF between the FB pin and source as close

the IC as possible.

4) Connect the exposed pad with the DRAIN pin

to a larger copper area to improve thermal

performance.

MP156 Rev. 1.15

www.MonolithicPower.com

14

4/21/2016

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© 2016 MPS. All Rights Reserved.

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