MP174 – NON-ISOLATED OFF-LINE REGULATOR

MP174 Rev. 1.01

www.MonolithicPower.com

16

8/18/2016

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Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Estimate the output voltage

ripple as:

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

It is recommended to use ceramic, tantalum or

low ESR electrolytic capacitors to reduce the

output voltage ripple.

Feedback Resistors

The resistor divider determines the output

voltage. Appropriate R1 and R2 values should be

5kΩ to 10kΩ, avoid large R2 value.

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









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

according to the regulated voltage. It is a

compromise between small no load consumption

and good no load regulation, especially for

applications require 30mW no load consumption.

Use a zener to reduce no-load consumption if no-

load regulation is not a concern.

Auxiliary VCC Supply

MP174

VCC

FB

SOURCE

SOURCE

C3

R1

R2

D2

C4

L1

VOUT

R3 D3

achieve the 30mW no-load power requirement. In

order to do this, chip requires an external VCC

supply to reduce overall power consumption.

This auxiliary VCC supply is derived from the

resistor connected between C3 and C4. C4

should be set larger than recommendation above.

D3 is used in case that VCC interfere with FB, R3

is determined per the formula below.

o

S

V5.8V

R

I





 

condition. R should be adjusted to meet the

about 250uA.

Surge Performance

Appropriate input capacitor value should be

chosen to obtain a good surge performance.

Figure 9 shows the half-wave rectifier. Table 2

shows the capacitance required under normal

condition for different surge voltages. FR1 is

20Ω/2W fused resistor and L1 is 1mH for this

recommendation.

L1

C1

C2

N

L

FR1

Figure 9: Half-Wave Rectifier