MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR

MP155 Rev. 1.12

www.MonolithicPower.com

9

10/28/2013

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OPERATION

The MP155 is a green-mode-operation regulator.

The peak current and the switching frequency

both decrease as the load decreases to provide

excellent efficiency at light load, and thus

improve the overall average efficiency. The

typical application diagram shows that the

regulator operates using a minimal number of

external

components.

It

incorporates

the

following features:

Start-up and Under Voltage Lock-out

The internal high-voltage regulator supplies 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. When the

VCC voltage drops below 5.3V, the internal high

voltage regulator turns on again to charge the

external VCC capacitor. Use a capacitor in the

several µF range stabilize the VCC voltage and

this can lower the cost by decreasing the value of

the capacitor.

When the voltage on VCC drops blow 3.4V, the

IC stops, then the internal high-voltage regulator

charges the VCC capacitor.

When faults occur, such as overload, short circuit,

and over-heating, the IC stops working and an

internal current source (16µA) discharges the VCC

capacitor. Before the VCC voltage drops below

2.4V, the internal high-voltage regulator remains off

and the VCC capacitor remains discharged.

Estimate the restart time after a fault as:

−

−

τ=

×

+

×

CC

restart

VCC

VCC

V2.4V

5.6V 2.4V

CC

16uA

3.5mA

Figure 2 shows the typical waveform with VCC

under voltage lock out.

Figure 2: VCC Under-Voltage Lockout

Constant Voltage Operation

The MP155 is a fully-integrated regulator when

used in a Buck solution as shown in the typical

application on page 1.

The integrated MOSFET turns ON at the

beginning of each cycle when the feedback

voltage is below the reference voltage (2.5V),

which indicates insufficient output voltage. The

peak current limit determines the ON period.

After the ON period elapses, the integrated

MOSFET turns OFF. The freewheeling diode (D1)

remains OFF until the inductor current charges

the sampling capacitor (C4) voltage to the output

voltage level. Then the sampling capacitor

voltage changes with the output voltage. The

sampling capacitor can sample and hold the

output voltage to regulate the output voltage. The

sampling capacitor voltage decreases after the

inductor current drops below the output current.

When the feedback voltage falls below the

reference voltage (2.5V), a new switching cycle

begins.

Figure 3 shows the detailed operation timing

diagram under CCM.