MP2109 – DUAL 1.2MHz 800mA SYNCHRONOUS STEP-DOWN CONVERTER

MP2109 Rev.1.3

www.MonolithicPower.com

5

2/21/2006

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© 2006 MPS. All Rights Reserved.

TM

OPERATION

IN1/

IN2

EN1/

EN2

FB1/

FB2

GND1/

GND2

SW1/

SW2

+

+

--

PWMCMP

PWM

OSC

+

--

EAMP

ICS

SLOPE COMP

FEEDBACK

ERROR\

AMP

0.6V

EAO

CC

17pF

BIAS

&

VOLTAGE

REFERENCE

PWM

CONTROL

LOGIC

1.0MHz

OSCILLATOR

IAMP

10X

CURRENT

SENSE

AMP

DH

DL

+

--

MAIN

SWITCH

(PCH)

SYNCHRONOUS

RECTIFIER

(NCH)

MP2109_BD02

Figure 1—Functional Block Diagram (Diagram represents ½ of the MP2109)

The MP2109 has dual independent constant

frequency current mode PWM step-down

converters. The MP2109 is optimized for low

voltage, Li-Ion battery powered applications

where high efficiency and small size are critical.

The MP2109 uses external resistor dividers to

set two output voltages independently from

0.6V to 6V. The device integrates both main

switches and synchronous rectifiers, which

provides high efficiency and eliminates the

need for an external Schottky diode. The

MP2109 can achieve 100% duty cycle. The

duty cycle D of each step-down converter is

defined as:

%

100

V

V

%

100

f

T

D

IN

OUT

OSC

ON

×

=

×

×

=

Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle

current limit for superior load and line response

and protection of the internal main switch and

synchronous rectifier. Each channel switches at

a constant frequency (1.2MHz) 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 is turned on

for a certain time to ramp the inductor current at

each rising edge of the internal oscillator, and

switched off when the peak inductor current is

above the error voltage. When the main switch

is off, the synchronous rectifier will be turned on

immediately and stay on until the next cycle

starts.