MP21148 – 1A, SYNCHRONOUS, STEP-DOWN CONVERTER WITH CCM

MP21148 Rev. 1.1

www.MonolithicPower.com

10

2/16/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP21148 uses constant-on-time control

with an input voltage feed-forward to stabilize

the switching frequency over the full input range.

It achieves 1A of continuous output current from

a 2.3V-to-5.5V input voltage with excellent load

and line regulation. The output voltage can be

regulated as low as 0.6V.

Constant-on-Time Control

Compared to fixed-frequency PWM control,

constant-on-time control offers a simpler control

loop and a faster transient response. By using

an input voltage feed forward, the MP21148

maintains a nearly constant switching frequency

across the input and output voltage ranges. The

switching pulse on time can be estimated with

Equation (1):

OUT

ON

IN

V

T

0.417 s

V







(1)

To prevent inductor current runaway during the

load transient, the MP21148 uses a fixed

minimum off time of 60ns.

Enable (EN)

When the input voltage is greater than the

under-voltage

lockout

(UVLO)

threshold

(typically 2V) the MP21148 can be enabled by

pulling EN higher than 1.2V. Leave EN floating

or pull EN down to ground to disable the

MP21148. There is an internal 1M

Ω resistor

from EN to ground.

When the device is disabled, the MP21148

goes into output discharge mode automatically.

Its internal discharge MOSFET provides a

resistive discharge path for the output capacitor.

Soft Start (SS)

The MP21148 has a built-in soft start that

ramps up the output voltage at a controlled slew

rate to avoid overshooting at start-up. The soft

start time is about 0.5ms, typically.

Current Limit

The MP21148 has a 2A high-side switch

current limit. When the high-side switch reaches

its current limit, the MP21148 remains in hiccup

mode until the current drops. This prevents the

inductor current from continuing to rise and

damaging components.

Short Circuit and Recovery

The

MP21148

also

enters

short-circuit

protection mode when it reaches the current

limit and attempts to recover with hiccup mode.

In this process, the MP21148 disables the

output power stage, discharges the soft-start

capacitor and attempts to soft start again

automatically.

If

the

short-circuit

condition

remains after the soft start ends, the MP21148

repeats

this

cycle

until

the

short

circuit

disappears and the output rises back to

regulation levels.