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

MP157 Rev. 1.04

www.MonolithicPower.com

16

6/27/2016

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Output Capacitor

The output capacitor is required to maintain the

DC output voltage. The output voltage ripple

can be estimated by:

CCM _ ripple

ESR

so

i

Vi R

8f C



  

, for CCM

2

pk

o

o

DCM _ ripple

pk

ESR

so

pk

II

I

VI

R

fC

I

















, for

DCM

Ceramic, tantalum or low ESR electrolytic

capacitors are used to reduce the output

voltage ripple.

Feedback Resistors

The resistor divider determines the output

voltage. Choose appropriate values for R1 and

value (typically 4kΩ 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:

oo

oo

FB

12

o

1

2

o

VC

VC

1

C

2R

R

I

R

R

I









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.

MP157

VCC

FB

SOURCE

SOURCE

C3

R1

R2

D2

C4

L1

VOUT

R3 D3

If output voltage is higher than the voltage of

Vcc, an auxiliary Vcc supply by connecting a

diode (D3) and a resistor (R3) between C3 and

C4 can be implemented to reduce overall power

consumption. Then the voltage of Vcc can be

clamped to 5V, and the internal regulator will be

turned off all the times. For values above







o

V5V

R

165 A

Surge Performance

Select an appropriate input capacitor value to

obtain a good surge performance. With the

input capacitors C4 (10μF) and C5 (10μF), the

board can pass 1000V surge test. Table 2

shows the capacitance required under normal

condition for different surge voltage.

Table 2: Recommended Capacitance

Surge

voltage

500V

1000V

2000V

C4

C5

Shown in

Figure 9

The board can pass 2kV surge test by using the

circuit below.

1) Change the fuse resistor F1 (10ohm/1W) to

SS-5-2A

2) Add a MOV RV1 (TVR14431)

3) Add a resistor R8 (39Ω)