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

MP158 Rev. 1.0

www.MonolithicPower.com

10

10/17/2014

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MOS

Diode

2.5V

Use the following equation to determine the

output voltage:

R1 R2

Vo 2.55V

R2

+

=×

Frequency

Foldback

and

Peak

Current

Compression

The MP158 remains highly efficient under light-

load

condition

by reducing

the

switching

frequency and peak current automatically.

Under light-load or no-load conditions, the output

voltage drops very slowly, which increases the

MOSFET off time. Thus the switching frequency

decreases as the load decreases.

Determine the switching frequency as:

in

o

o

s

peak

o

in

(V

V )

V

f

2L(I

I ) V

−

=⋅

−

, for CCM

−

=⋅

in

O

o

o

s

2

peak

in

2(V

V ) I V

f

LI

V

, for DCM

At the same time, the peak current limit

decreases as the off-time increases. In standby

mode, the frequency and the peak current are

both minimized, allowing for a smaller dummy

load. As a result, peak-current compression

helps further reduce no-load consumption. Peak

current limit can be estimated from the following

equation (

Peak

off

I

101mA (1mA / s) (

9 s)

=−

μ × τ − μ

EA Compensation

MP158

has

internal

error

amplifier

and

compensation loop. It samples the feedback

voltage 6µs after the MOSFET turns off, and

regulates the output based on the 2.55V

reference voltage.

Figure 5: EA and Ramp Compensation

Ramp Compensation

An internal ramp compensation circuit improves

the load regulation. As shown in Figure 5, an

exponential signal added to pull down the

reference voltage of the feedback comparator.

The ramp compensation is a function of the load

conditions: the compensation is about the

1mV/µs under full-load conditions compensation

increases exponentially as the peak current

decreases.

Over-Load Protection (OLP)

Maximum output power of MP158 is limited by

maximum switching frequency and peak current

limit. If the load current is too large, output

voltage drops, so that the FB voltage drops.

When the FB voltage drops below 1.7V, it is

considered as an error flag and timer starts. If the

This timer duration avoids triggering OLP when

the power supply starts up or load transition. The

power supply should start up in less than 120ms

per the following equation:

Delay

50kHz

120ms

fs

τ≈

×

Short-Circuit Protection (SCP)

The MP158 monitors the peak current, and shuts

down when the peak current rises above SCP

threshold through short-circuit protection. The