MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR
Minimum off time limitation
MOS
The MP155 implements a minimum OFF time
limit. During the normal operation, the minimum
OFF time limit is 18µs; during start up, the
minimum OFF time limit gradually drops from
72µs, to 36µs, then to 18 µs (see Figure 4). Each
minimum OFF time has 128 switching cycles.
This soft-start function allows for safe start-up.
Diode
IL
Ipeak
Io
Vo
VFB
2.5V
≥ 72us
≥ 36us
≥18us
Driver
Figure 3: VFB vs. Vout
Monitoring the sampling capacitor regulates the
output voltage can be regulated, as per the
following equation:
128 Switching cycle 128 Switching cycle
Figure 4: τminoff at Start-Up
EA Compensation
R1+ R2
R2
Vo = 2.5V ×
FB
Comparator
Frequency Foldback
-
Under light load or no load conditions, the output
drops very slowly, which increases the time for
the MOSFET to turn ON again; i.e., frequency
decreases as the load decreases. So the MP155
can maintain a high efficiency under light load
condition by reducing the switching frequency
automatically.
+
EA
VFB
-
+
Vramp
+
+
Vramp
Vref
+
Ipeak
2.5V
-
The switching frequency can be obtained as:
Figure 5: EA and Ramp Compensation
(V − Vo ) Vo
in
fs =
fs =
⋅
, for CCM
, for DCM
To improve load regulation, the MP150
implements an error amplifier (EA) compensation
function ( Figure 5 ). The MP155 samples the
feedback voltage 6µs after the MOSFET turns off.
EA compensation regulates the 2.5V reference
voltage with the load, thus improving the power
module regulation.
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 small dummy load. As a result, the peak-
current-compression function helps to reduce no-
load consumption. Determine the peak current
limitation from the following equation where τoff is
the power module OFF time:
RAMP Compensation
An internal ramp compensation circuit precisely
maintains the output voltage. An additional
exponential voltage sinking source pulls down
the feedback comparator’s reference voltage as
shown in Figure 5. The ramp compensation is
relative to the load conditions: Under full-load
conditions, the compensation is ~1mV/µs; with a
decreasing load, the compensation increases
exponentially.
IPeak = 290mA − (1mA /μS)×(τoff −18μS)
MP155 Rev. 1.12
10/28/2013
www.MonolithicPower.com
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