MP2012 – 1.5A SYNCHRONOUS STEP-DOWN CONVERTER

MP2012 Rev. 1.01

www.MonolithicPower.com

7

9/22/2011

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© 2011 MPS. All Rights Reserved.

OPERATION

The MP2012 is a fixed frequency 1.2MHz current

mode 1.5A step-down converter, optimized for

low voltage, Li-Ion battery powered applications

where high efficiency and small size are critical.

MP2012 integrates a high side PFET main switch

and a low side synchronous rectifier. It always

operates

in

continuous

conduction

mode,

simplifies the control scheme and eliminates the

random spectrum noise due to discontinuous

conduction mode.

The steady state duty cycle D for this mode can

be calculated as:

%

100

V

V

%

100

f

T

D

IN

OUT

OSC

ON

×

≈

×

×

=

the oscillator frequency (1.2MHz typ.).

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 as well as protection of the internal

main switch and synchronous rectifier. The

MP2012 switches at a constant frequency

(1.2MHz) and modulates the inductor peak

current to regulate the output voltage. Specifically,

for each cycle the PWM controller forces the

inductor peak current to an internal reference

level derived from the feedback error voltage. At

normal operation, the main switch is turned on at

each rise edge of the internal oscillator, and

remains on for a certain period of time to ramp up

the inductor current. As soon as the inductor

current reaches the reference level, the main

switch is turned off and immediately the

synchronous rectifier will be turned on to provide

the inductor current. In forced PWM mode, the

synchronous rectifier will stay on until the next

oscillator cycle.

Dropout Operation

The MP2012 allows the main switch to remain on

for more than one switching cycle to increase the

duty cycle when the input voltage is dropping

close to the output voltage. When the duty cycle

reaches 100%, the main switch is held on

continuously to deliver current to the output up to

the PFET current limit. In this case, the output

voltage becomes the input voltage minus the

voltage drop across the main switch and the

inductor.

Maximum Load Current

The MP2012 can operate down to 2.5V input

voltage; however the maximum load current

decreases at lower input due to a large IR drop

on 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.

Short Circuit Protection

When short circuit or over current condition

happens, and FB is lower than about 0.3V, the

MP2012 enters fold back mode. The oscillator

frequency is reduced to prevent the inductor

current from increasing beyond the PFET current

limit. The PFET current limit is also reduced to

lower the short circuit current. The frequency and

current limit will return to the normal values once

the short circuit condition is removed and the

feedback voltage approaches 0.8V.