MP173– NON-ISOLATED OFFLINE REGULATOR

MP173 Rev. 1.0

www.MonolithicPower.com

9

8/19/2015

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© 2015 MPS. All Rights Reserved.

OPERATION

MP173 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

overall average efficiency. Also, the regulator

incorporates multiple features and operates with

a minimum number of external components.

The MP173 acts as a fully integrated regulator

when used in buck topology (see Typical

Application on page 1).

Start-Up and Under-Voltage Lockout

The internal high-voltage regulator self-supplies

the IC from DRAIN. When VCC voltage reaches

high-voltage regulator turns off. The internal high-

voltage regulator turns on to charge the external

VCC capacitor when the VCC voltage falls below

maintains the VCC voltage and thus lowers the

capacitor cost.

The IC stops switching when the VCC voltage

Under fault conditions—such as OLP, SCP, and

TSD—the IC stops switching, and an internal

current source (~16 μA) discharges the VCC

capacitor. The internal high-voltage regulator will

not charge the VCC capacitor until the VCC

can be estimated using Equation (1):

pro

OFF

pro

restart

VCC

CCLATCH

regulator

VCC VCC

VCC

VCC

C

II

















(1)

Soft Start (SS)

The IC stops operation when the VCC voltage

starts operation there is a soft-start period. The

soft start prevents the inductor current from

overshooting by limiting the minimum off time.

MP173 adopts a 2 phase minimum off time limit

soft start. Each soft-start phase retains 128

switching cycles. During a soft start, the off time

limit gradually shortens from 48 μs to 24 μs and

finally reaches the normal operation off-time limit

(see Figure 2).

128 Switching cycle

48us



128 Switching cycle

24us



12us



Driver

Figure 2—

min off



at start-up

Constant Voltage (CV) Operation

The MP173 regulates the output voltage by

monitoring the sampling capacitor (C3).

At the beginning of each cycle, the integrated

MOSFET turns on while the feedback voltage

drops below the 2.55 V 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. 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 2.55 V reference voltage, a new switching

cycle begins. Figure 3 shows this operation in

continuous

conduction

mode

(CCM).

MOSFET

Diode

I

L

2.55V

Equation (2) determines the output voltage:





R1 R2

Vo 2.55V

R2

(2)