MP174A – NON-ISOLATED OFF-LINE REGULATOR

MP174A Rev. 1.0

www.MonolithicPower.com

12

2/27/2019

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2019 MPS. All Rights Reserved.

OPERATION

The

MP174A

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.

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

lower the capacitor cost.

The IC stops switching when the VCC voltage

drops below 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 Equation (1):















CC

restart

VCC

VCC

V2.4V

5.6V 2.4V

CC

16 A

4.1mA

(1)

Soft Start

The IC stops operation when the VCC voltage

drops below 3.4V, and it starts operation when

VCC is charged to 5.6V. 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.

The MP174A adopts a 2 phase minimum off time

limit soft start. Each soft-start phase retains 256

switching cycles. During soft start, the off time

limit gradually shortens from 48μs to 24μs, and

finally to the 12μs normal operation off time limit

(see Figure 2).

Figure 2:



Min_ off at Start-Up

Constant Voltage Operation

The MP174A acts as a fully-integrated regulator

when used in the buck topology (see the Typical

Application on page 1).

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. The sampling capacitor (C3) voltage is

charged to the output voltage when the

freewheeling diode (D1) turns on. This allows the

sampling capacitor (C3) to sample and hold the

output voltage for the 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

Ipeak

Io

Vo

VFB

2.55V

Figure 3: VFB vs. VO