MPQ2128 – 3MHz, 1A, SYNCHRONOUS, STEP-DOWN CONVERTER

MPQ2128 Rev. 1.1

www.MonolithicPower.com

6

4/1/2013

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© 2013 MPS. All Rights Reserved.

FUNCTIONAL DESCRIPTION

MPQ2128

is

a

3MHz,

constant

frequency,

current-mode, PWM, step-down converter. The

device integrates both a main switch and a

synchronous

rectifier,

which

provides

high

efficiency and eliminates the need for an external

Schottky diode. It is ideal for powering portable

equipment that runs from a single Li-lon or Li-

Polymer battery.

The MPQ2128 can achieve 100% duty cycle.

The duty cycle, D, of the step-down converter is

defined as:

OUT

ON

OSC

IN

V

D

t

f

100%

100%

V

=

××

≈

×

Where,

τ

Peak-Current –Mode PWM Control

Slope-compensated, current-mode, PWM control

provides stable switching and cycle-by-cycle

current

limiting

for

superior

load

and

line

response, and protects the internal main switch

and

synchronous

rectifier.

The

MPQ2128

switches at a constant frequency (3MHz) and

regulates the output voltage. During each cycle

the PWM comparator modulates the power

transferred to the load by changing the inductor

peak current based on the feedback error

voltage.

During normal operation, the main switch turnS

on to ramp-up the inductor current with each

rising edge of the internal oscillator, and switches

off when the peak inductor current rises above

the error voltage.

When the main switch is off, the synchronous

rectifier turns on immediately and remains on

until the next cycle starts.

Dropout Operation

The MPQ2128 allows the main switch to remain

on for more than one switching cycle and

increases the duty cycle while the input voltage

drops down to the output voltage. When the duty

cycle reaches 100%, the main switch is held ON

to deliver current to the output up to the PFET

current limit. The output voltage is then the

difference between the input voltage and the

voltage drop across the main switch and the

inductor.

Short-Circuit Protection

The MPQ2128 has short-circuit protection. When

the output is shorted to ground, the oscillator

frequency reduces to prevent the inductor current

from increasing beyond the PFET current limit.

The PFET current limit also drops to lower the

short-circuit current. The frequency and current

limit return to their normal values once the short

circuit condition is removed and the feedback

voltage reaches 0.6V.

Maximum Load Current

The MPQ2128 can operate at an input voltage as

low as 2.5V. However the maximum load current

decreases with the lower input due to a large

voltage

drop

across

the

main

switch

and

synchronous rectifier. The slope compensation

signal reduces the peak inductor current as a

function of the duty cycle to prevent sub-

harmonic oscillations at duty cycles greater than

50%. Conversely the current limit increases as

the duty cycle decreases.