MP171 –NON-ISOLATED OFFLINE REGULATOR
Ramp Compensation
Frequency Foldback and Peak Current
Compression
An internal ramp compensation circuit improves
the load regulation. As shown in Figure 4, a
voltage sinking source is added to pull down the
reference voltage of the feedback comparator.
The ramp compensation is relative to the
MOSFET off time, and increases exponentially
as the off time increases. The compensation is
about 1mV/µs under min off time switching
condition.
The MP171 remains highly efficient at light-load
conditions by reducing the switching frequency
automatically.
Under light-load or no-load conditions, the output
voltage drops very slowly, which increases the
MOSFET off time. Thus, the frequency
decreases along with the load.
The switching frequency is determined with
Equation (3) and Equation (4):
Over-Load Protection (OLP)
The maximum output power of the MP171 is
limited by the maximum switching frequency and
the peak current limit. If the load current is too
large, the output voltage drops, causing the FB
voltage to drop.
(V Vo ) Vo
in
, for CCM
, for DCM
(3)
(4)
fs
fs
2L(Ipeak Io ) V
in
2(V VO ) IoVo
in
LI2peak
V
in
When the FB voltage drops below VFB_OLP, it is
considered an error flag, and the timer starts. If
the timer reaches 170 ms (fS = 36 kHz), OLP
occurs. This timer duration avoids triggering OLP
when the power supply starts up or the load
transitions. The power supply should start up in
less than 170 ms (fS = 36 kHz). The OLP delay
time is calculated using Equation (6):
As the peak current limit decreases from 105 mA,
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. The peak current
limit can be estimated from Equation (5) where
τoff is the off time of the power module:
36kHz
(6)
Delay 170ms
IPeak 105mA (0.4mA /s)(off 10s) (5)
fs
Short-Circuit Protection (SCP)
EA Compensation
FB
The MP171 monitors the peak current and shuts
down when the peak current rises above the
SCP threshold through short-circuit protection.
The power supply resumes operation with the
removal of the fault.
Comparator
-
+
EA
-
VFB
Thermal Shutdown (TSD)
+
M
Vramp
+
To prevent thermal induced damage, the MP171
stops switching when the junction temperature
exceeds 150°C. During thermal shutdown (TSD),
the VCC capacitor is discharged to VCCpro, and
the the internal high-voltage regulator re-charges.
MP171 recovers when the junction temperature
drops below 120°C.
+
Vramp
+
-
V
ref
2.55V
Ipeak
Figure 4—EA and ramp compensation
MP171 has an internal error amplifier (EA)
compensation loop. It samples the feedback
voltage 6 µs after the MOSFET turns off and
regulates the output based on the 2.55 V
reference voltage.
Open-Loop Detection
If VFB is less than 0.5 V, the IC stops switching,
and a re-start cycle begins. During a soft start,
the open-loop detection is blanked.
MP171 Rev. 1.0
9/30/2015
www.MonolithicPower.com
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10
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