MP171 –NON-ISOLATED OFFLINE REGULATOR
OPERATION
48us
18us
10us
MP171 is a green-mode-operation regulator: The
peak current and the switching frequency both
decrease with a decreasing load. As a result, it
offers excellent light-load efficiency and improves
overall average efficiency. Also, the regulator
incorporates multiple features and operates with
a minimal number of external components.
Driver
128 Switching cycle 128 Switching cycle
Figure 2—
at start-up
minoff
Constant Voltage (CV) Operation
The MP171 acts as a fully integrated regulator
when used in buck topology (see Typical
Application on page 1).
The MP171 regulates the output voltage by
monitoring the sampling capacitor.
At the beginning of each cycle, the integrated
MOSFET turns on while the feedback voltage
drops below the 2.55 V 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 sampling capacitor (C3) voltage is
charged to the output voltage when the
freewheeling diode (D1) turns on. In his way, the
sampling capacitor (C3) samples and holds the
output voltage for output regulation. The
sampling capacitor (C3) voltage decreases when
the L1 inductor current falls below the output
current. When the feedback voltage falls below
the 2.55 V reference voltage, a new switching
cycle begins. Figure 3 shows this operation in
continuous conduction mode (CCM).
Start-Up and Under-Voltage Lockout
The internal high-voltage regulator self-supplies
the IC from DRAIN. When VCC voltage reaches
VCCOFF, the IC starts switching, and the internal
high-voltage regulator turns off. The internal high-
voltage regulator turns on to charge the external
VCC capacitor when the VCC voltage falls below
VCCON. A small capacitor (in the low μF range)
maintains the VCC voltage and thus lowers the
capacitor cost.
The IC stops switching when the VCC voltage
drops blow VCCstop
.
Under fault conditions—such as OLP, SCP, and
TSD—the IC stops switching, and an internal
current source (~16 μA) discharges the VCC
capacitor. The internal high-voltage regulator will
not charge the VCC capacitor until the VCC
voltage drops below VCCpro. The re-start time
can be estimated using Equation (1):
MOSFET
Diode
VCC VCCpro VCCOFF VCCpro
(1)
restart CVCC
ICCLATCH
Iregulator
IL
Ipeak
Io
Soft Start (SS)
Vo
The IC stops operation when the VCC voltage
drops below VCCstop; the IC begins operation
when VCC charges to VCCOFF. Every time the
chip starts operation there is a soft-start period.
The soft start prevents the inductor current from
overshooting by limiting the minimum off time.
VFB
2.55V
Figure 3—VFB vs. VO
Equation (2) determines the output voltage:
MP171 adopts a 2 phase minimum off time limit
soft start. Each soft-start phase retains 128
switching cycles. During the soft start, the off
time limit gradually shortens from 48 μs to 18 μs
and finally reaches the normal operation off time
limit (see Figure 2).
R1R2
(2)
Vo 2.55V
R2
MP171 Rev. 1.0
9/30/2015
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