MP173– NON-ISOLATED OFFLINE REGULATOR
Ramp Compensation
Frequency Foldback and Peak Current
Compression
An internal ramp compensation circuit improves
the load regulation. An exponential voltage signal
is added to pull down the reference voltage of the
feedback comparator (see Figure 4). The ramp
compensation is a function of the load conditions:
the compensation is about the 1 mV/µs under
full-load conditions. The compensation increases
exponentially as the peak current decreases.
The MP173 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.
Over-Load Protection (OLP)
The switching frequency is determined with
Equation (3) and Equation (4):
The maximum output power of the MP173 is
limited by the maximum switching frequency and
peak current limit. If the load current is too large,
the output voltage drops, causing the FB voltage
to drop.
(V Vo ) Vo
in
fs
fs
, for CCM
, for DCM
(3)
(4)
2L(Ipeak Io ) V
in
2(V VO ) IoVo
in
When the FB voltage drops below VFB_OLP, it is
considered an error flag, and the timer starts. If
the timer reaches 220 ms (fS = 28 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 220 ms (fS = 28 kHz). The OLP delay
time is calculated using Equation (6):
LI2peak
V
in
As the peak current limit decreases from 420 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:
28kHz
Delay 220ms
(6)
fs
IPeak 420mA (1.6mA /s)(off 12s) (5)
Short-Circuit Protection (SCP)
EA Compensation
The MP173 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.
FB
Comparator
-
+
EA
Thermal Shutdown (TSD)
-
VFB
+
To prevent thermal induced damage, the MP173
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.
MP173 recovers when the junction temperature
drops below 120°C.
M
Vramp
+
+
Vramp
+
-
V
ref
2.55V
Ipeak
Figure 4—EA and ramp compensation
Open-Loop Detection
MP173 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.
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.
MP173 Rev. 1.0
8/19/2015
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