MPS MP150 - Page 10

MP150 – OFFLINE PRIMARY-SIDE REGULATOR
MP150 Rev. 1.14 www.MonolithicPower.com 10
9/25/2018 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2018 MPS. All Rights Reserved.
Figure 3: V
FB
vs V
OUT
Monitoring the sampling capacitor regulates the
output voltage, as per the following equation:


R1 R2
Vo 2.5V
R2
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 MP150
can maintain a high efficiency under light load
condition by reducing the switching frequency
automatically.
The switching frequency can be obtained 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 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 τ
off
is
the power module OFF time:
Peak off
I 290mA (1mA / s) ( 18 s)
Minimum OFF Time Limit
The MP150 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.
72us
36us
18us
Figure 4: t
minoff
at Start-Up
EA Compensation
-
+
V
FB
-
+
V
ref
2.5V
EA
+
+
-
+
FB
Comparator
V
ramp
V
ramp
I
peak
Figure 5: EA and Ramp Compensation
To improve load regulation, the MP150
implements an error amplifier (EA) compensation
function (Figure 5). The MP150 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.
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