MP174 – NON-ISOLATED OFF-LINE REGULATOR

MP174 Rev. 1.01

www.MonolithicPower.com

11

8/18/2016

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OPERATION

MP174 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

average

efficiency.

The

typical

application diagram shows the regulator operates

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. When VCC voltage

reaches 5.6V, 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 5.3V. A small capacitor (in the

low μF range) can maintain the VCC voltage and

thus lower the capacitor cost.

The IC stops switching when the VCC voltage

drops blow 3.4V.

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. The restart time can

be estimated using the following equation,















CC

restart

VCC

VCC

V2.4V

5.6V 2.4V

CC

16 A

4.1mA

Soft-Start

The IC stops operation when VCC voltage drops

blow 3.4V and starts operation when VCC

charges to 5.6V. Every time when 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.

MP174 adopts a 2 phase minimum off-time limit

soft-start. Each Soft-Start phase retains 128

switching cycles. During soft-start, off time limit

gradually shortens from 48μs to 24μs, and finally

to the 12μs normal operation off-time limit (see

Figure 2Error! Reference source not found.).

128 Switching cycle

48us

128 Switching cycle

24us

12us

Driver

Figure 2:

min off



at Start-Up

Constant Voltage Operation

MP174 acts as a fully-integrated regulator when

used in the Buck topology, as shown in the

typical application on page1.

It 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 2.55V 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. Sampling capacitor (C3) voltage is

charged to the output voltage, when the

freewheeling diode (D1) turns on. In this 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 2.55V reference voltage, a new switching

cycle begins. Figure 3 shows this operation

under CCM in detail.

MOSFET

Diode

I

L

2.55V

Use the following equation to determine the

output voltage: