MP156 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR

MP156 Rev. 1.15

www.MonolithicPower.com

9

4/21/2016

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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,

CC

restart

VCC

VCC

V2.4V

5.6V 2.4V

CC

16 A

3.5mA

−

−

τ=

×

+

×

μ

Figure 2 shows the typical waveform with VCC

under-voltage lockout.

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.