MP158 –SMALL, ENERGY-EFFICIENT, OFF-LINE REGULATOR

MP158 Rev. 1.0

www.MonolithicPower.com

9

10/17/2014

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© 2014 MPS. All Rights Reserved.

OPERATION

MP158 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

minimum

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 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 operation when 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

3.5mA

−

−

τ=

×

+

×

μ

Figure 2 shows the typical waveforms with VCC

under-voltage lockout.

Figure 2: VCC Under-Voltage Lockout

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.

MP158 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 72μs to 22.5μs and

finally to the 9μs normal operation off-time limit

(see Figure 3).

Figure 3:

min off

τ

at Start-Up

Constant Voltage Operation

MP158 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 4 shows this operation

under CCM in detail.