MP158 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR
voltage 6µs after the MOSFET turns off, and
regulates the output based on the 2.55V
reference voltage.
MOS
Diode
IL
Ipeak
Io
Vo
VFB
2.5V
Figure 4: VFB vs. VO
Use the following equation to determine the
output voltage:
Figure 5: EA and Ramp Compensation
R1+ R2
Vo = 2.55V ×
R2
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.
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.
Over-Load Protection (OLP)
Determine the switching frequency as:
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.
(V − Vo ) Vo
in
fs =
fs =
⋅
, for CCM
, for DCM
2L(Ipeak −Io ) V
in
2(V − VO ) IoVo
in
When the FB voltage drops below 1.7V, it is
considered as an error flag and timer starts. If the
timer reaches 120ms (fs=50kHz), OLP occurs.
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
(fs=50kHz). The OLP delay time is calculated as
per the following equation:
⋅
LI2peak
V
in
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 (τoff is the MOSFET’s off time):
50kHz
τDelay ≈ 120ms×
fs
Short-Circuit Protection (SCP)
IPeak = 101mA − (1mA /μs)×(τoff − 9μs)
The MP158 monitors the peak current, and shuts
down when the peak current rises above SCP
threshold through short-circuit protection. The
EA Compensation
MP158 has internal error amplifier and
compensation loop. It samples the feedback
MP158 Rev. 1.0
10/17/2014
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