MP158 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR

MP158 Rev. 1.0

www.MonolithicPower.com

13

10/17/2014

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Topology Options

MP158 can be used in common topologies, such

as Buck, Buck-Boost, Boost and Flyback, as

illustrated in table 1.

Component Selection

Input Capacitor

The input capacitor supplies the DC input voltage

for the converter. Figure 7 shows the typical DC

bus voltage waveform of a half-wave rectifier.

Vin

DC input voltage

t

AC input voltage

Figure 7: Input Voltage Waveform

Typically, a half-wave rectifier requires an input

capacitor rated at 3uF/W for the universal input

condition. When using a full-wave rectifier, input

capacitor is chosen as 1.5~2uF/W for the

universal input condition. A minimum DC voltage

below 70V should be usually avoided, since it

may cause thermal issue.

Inductor

The MP158 has a minimum off-time limit that

determines the maximum power output. The

maximum power increases as the inductor

increases. Using a very small inductor may

cause failure at full load, but a larger inductor

means a higher OLP load. It is recommended to

select an inductor with the minimum value that

can supply the rated power. Estimate the

maximum power with:

τ

o max

o peak

V

PV (I

)

2L

, for CCM

=⋅

τ

2

omax

peak

min off

11

PLI

2

, for DCM

For

mass

production,

tolerance

on

the

parameters, such as peak current limitation,

minimal

off

time,

should

be

taken

into

consideration.

To reduce costs, use a standard off-the-shelf

inductor no less than the calculated value.

Freewheeling Diode

The diode should be selected based on

maximum input voltage and peak current.

The freewheeling diode’s reverse recovery can

affect efficiency and circuit operation under CCM,

so use an ultra fast diode such as the EGC10JH.

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

Δ

=+ Δ ⋅

, 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 2mA

dummy load is needed and can be adjusted

according to the regulated voltage. The dummy

load reduces the efficiency and increases the no-

load consumption.