MP156 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR
OPERATION
MP156 is a green-mode-operation regulator: the
peak current and the switching frequency both
decrease with a decreasing load. As a result, it
still offers excellent light-load efficiency, thus
improved average efficiency. The typical
application diagram shows the regulator
operating with a minimum number of external
components. It incorporates multiple features as
described in the following sections.
Start-Up and Under-Voltage Lockout
The internal high-voltage regulator self-supplies
the IC from the Drain pin. The IC starts switching
and the internal high voltage regulator turns off
when the voltage on VCC reaches 5.6V. The
internal high-voltage regulator turns on to charge
the external VCC capacitor when the VCC
voltage falls below 5.3V. A small capacitor (in the
low
μF
range) can maintain the VCC voltage and
thus lower the capacitor cost.
When the VCC voltage drops blow 3.4V, the IC
stops switching, then the internal high-voltage
regulator charges the VCC capacitor.
Under fault conditions—such as OLP, SCP, and
OTP—the IC stops switching and an internal
current source (~16μA) discharges the VCC
capacitor. The internal high-voltage regulator will
not charge the VCC capacitor until the VCC
voltage drops below 2.4V. Under fault conditions,
estimate the restart time using the following
equation,
Figure 2: VCC Under-Voltage Lockout
Constant Voltage Operation
MP156 acts as a fully-integrated regulator when
used in the Buck topography, as shown in the
typical application on page1.
At the beginning of each cycle, the integrated
MOSFET turns ON while the feedback voltage
remains below the 2.5V reference voltage, which
indicates insufficient output voltage. The peak
current limitation determines the ON period. After
the ON period elapses, the integrated MOSFET
turns off. The freewheeling diode (D1) will not
turn on until the inductor (L1) charges the
sampling capacitor (C3) voltage to equal the
output voltage. The sampling capacitor voltage
changes with the output voltage, and can sample
and hold the output voltage to regulate the output
voltage. The sampling capacitor voltage will
decrease when the L1 inductor current falls
below the output current. When the feedback
voltage falls below the 2.5V reference voltage, a
new switching cycle begins. Figure 3 shows this
operation under CCM in detail.
τ
restart
=
C
VCC
×
V
CC
−
2.4V
5.6V
−
2.4V
+
C
VCC
×
16
μ
A
3.5mA
Figure 2 shows the typical waveform with VCC
under-voltage lockout.
MP156 Rev. 1.15
4/21/2016
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