MP2125 – DUAL 1.5A, 1.2MHZ SYNCHRONOUS STEP-DOWN CONVERTER

MP2125 Rev. 1.0

www.MonolithicPower.com

7

12/24/2013

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OPERATION

The MP2125 is a dual channel, fixed frequency,

current mode step-down converter, optimized for

low voltage, Li-Ion battery powered applications

where high efficiency and small size are critical.

MP2125 integrates two high side PFET main

switches and two low side synchronous rectifiers.

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 switches and synchronous rectifiers. The

MP2125 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 for each channel, 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. The

synchronous rectifier will stay on until the next

oscillator cycle.

Dropout Operation

The MP2125 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 MP2125 can operate down to 2.7V 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 the output is shorted to ground, 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.