MP157 –SMALLEST ENERGY EFFICIENT OFF-LINE REGULATOR WITH EXTENDED POWER RANGE

MP157 Rev. 1.04

www.MonolithicPower.com

11

6/27/2016

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OPERATION

MP157 is a green mode operation regulator. With

the load decreasing, the peak current and the

switching frequency will both decreasing with the

load. As a result, it still offers excellent efficiency

performance at light load, thus better average

efficiency is achieved. As shown in the typical

application diagram, the regulator is designed to

operate with a minimum number of external

components.

It

incorporates

the

following

features as described in the following sections.

Start-up and Under Voltage Lock-out

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

as soon as the voltage on pin VCC reaches

regulator turns on to charge the external Vcc

capacitor When the Vcc voltage decreases below

such as several uF capacitor is enough to hold

on the voltage of Vcc and this can lower the cost

by decreasing the value of the capacitor.

(3.3V, typical), the IC stops working, then the

internal high voltage regulator charges the Vcc

capacitor again.

When fault conditions happen, such as OLP,

SCP, and OTP, the IC stops working and an

internal current source, around 16uA, will

discharge the Vcc capacitor, before the Vcc

high voltage regulator will not start to charge the

Vcc capacitor again. So when the fault conditions

happen, the restart time can be calculated by the

following equation,











CC

restart

VCC

VCC

V2.4V

4.6V 2.4V

tC

C

16uA

3.5mA

Figure 2 shows the typical waveform with VCC

under voltage lock out.

VCC

Internal

Current

Source

Driving

Signal

ON

OFF

Figure 2: VCC Under-Voltage Lock Out

Constant Voltage Operation

MP157 is a fully integrated regulator when used

in the Buck solution, as shown in the typical

application on page2.

At the beginning of each cycle, the integrated

MOSFET is turned ON when the feedback

voltage is below the reference voltage — 2.5V,

which indicates insufficient output voltage. The

ON period is determined by the peak current limit.

After the ON period elapses, the integrated

MOSFET is turned OFF. The Freewheeling diode

(D1) will not be turned ON until inductor (L1)

charges the voltage of the sampling capacitor

(C3) to equal the output voltage. The sampling

capacitor voltage changes along with the output

voltage. The sampling capacitor can sample and

hold the output voltage to keep the output voltage

regulated. The voltage of the sampling capacitor

will decrease when the current of inductor is

smaller than the output current. When the

feedback

voltage

falls

below

the

internal

reference voltage — 2.5V, another switching

cycle begins. The detail operation in CCM is

shown as Figure 3.