MP156 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR
Minimum Off-Time Limit
MOS
The MP156 implements a minimum off-time limit.
During normal operation, the minimum off-time
limit is 18μs, and this limit gradually shortens
during the start-up period from 72μs, to 36μs, to
18μs (see Figure 4). Each minimum off-time
retains 128 switching cycles. This soft-start
function provides a safe start-up.
Diode
IL
Ipeak
Io
Vo
VFB
2.5V
≥ 72us
≥ 36us
≥18us
Driver
Figure 3: VFB vs. VO
Thus monitoring the sampling capacitor regulates
the output voltage. Use the following equation to
determine the output voltage:
128 Switching cycle 128 Switching cycle
Figure 4:
at Start-Up
τminoff
R1+ R2
EA Compensation
Vo = 2.5V ×
R2
FB
Comparator
Frequency Foldback
-
Under light-load or no-load conditions, the output
drops very slowly, thus increasing the time when
the MOSFET turn-on time. The frequency
decreases as the load decreases. The MP156
remains highly efficiency under light-load
condition by reducing the switching frequency
automatically.
+
EA
VFB
-
+
Vramp
+
+
Vramp
Vref
+
Ipeak
2.5V
-
Determine the switching frequency as:
(V − Vo ) Vo
Figure 5: EA and Ramp Compensation
The MP156 features error amplifier (EA)
compensation function (Shown in Figure 5) to
improve load regulation. 6µs after the MOSFET
turns off, the MP156 samples the feedback
voltage. Thus EA compensation regulates the
2.5V reference voltage with the load to improve
overall power regulation.
in
fs =
fs =
⋅
, for CCM
, for DCM
2L(Ipeak −Io ) V
in
2(V − VO ) IoVo
in
⋅
LI2peak
V
in
At the same time, the peak current limit
decreases from 290mA as the off-time increases.
In standby mode, the frequency and the peak
current are both minimized, allowing for a smaller
Ramp Compensation
An internal ramp compensation circuit precisely
maintains the output voltage. Figure 6 shows an
exponential voltage sinking source 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
load decreases.
dummy load. As
compression helps
a
result, peak-current
reduce no-load
to
consumption. Estimate the peak current limit
from the following equation (τoff is the power
module’s off time):
IPeak = 290mA − (1mA /μs)×(τoff −18μs)
MP156 Rev. 1.15
4/21/2016
www.MonolithicPower.com
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