MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR

MP155 Rev. 1.12

www.MonolithicPower.com

14

10/28/2013

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Output Capacitor

The output capacitor maintains the DC output

voltage. Estimate the output voltage ripple as:

CCM _ ripple

ESR

so

i

Vi R

8f C

Δ

=+ Δ ⋅

, for CCM

2

pk

o

o

DCM _ ripple

pk

ESR

so

pk

II

I

VI

R

fC

I

⎛⎞

−

=⋅

+

⋅

⎜⎟

⎜⎟

⎝⎠

, for DCM

Use ceramic, tantalum, or low-ESR electrolytic

capacitors to lower the output voltage ripple.

Feedback Resistors

The resistor divider determines the output

voltage. Choose appropriate R1 and R2 values to

maintain the FB voltage at 2.5V. Avoid very large

values for R2 (typical values between 5kΩ

 

to10kΩ.

Feedback Capacitor

The feedback capacitor provides a sample-and-

hold function. Small capacitors result in poor

regulation at light load condition, and large

capacitors can impact circuit operation. Estimate

the capacitor range as per the following equation:

oo

oo

FB

12

o

1

2

o

VC

VC

1

C

2R

R

I

R

R

I

⋅≤

≤

⋅

++

Choose an appropriate value given practical

considerations.

Dummy Load

A dummy load maintains the load regulation. This

ensures sufficient inductor energy to charge the

sample-and-hold capacitor to detect the output

voltage. Start with a 3mA dummy load and adjust

as necessary.

VCC Supply

The MP155 obtains a low no-load power

consumption by external VCC supply. This

a diode and resistor between C3 and C4 as per

the values listed in Table 2.

Table 2: Recommended VCC Supply Resistor

Values

Vout

Resistor value

3.3V

NC

（

5V

NC

12V

24.8k

16V

40.8k

24V

72.8k

Note:

5) NC= no connection

Surge Performance

To obtain a good surge performance, select an

appropriate input capacitor that meets different

surge tests. Figure 10 shows the half-wave

rectifier. Table 3 shows the required capacitance

under normal conditions for different surge

voltages.

Figure 10: Half-Wave Rectifier

Table 3: Recommended Capacitor Values

Surge

voltage

500V

1000V

2000V

C1

1μF

10μF

22μF

C2

1μF

4.7μF

10μF