MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER

MP2108 Rev 1.1

www.MonolithicPower.com

7

10/2/2006

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© 2006 MPS. All Rights Reserved.

The MP2108 measures the output voltage

through an external resistive voltage divider and

compares it to the internal 0.9V reference to

generate the error voltage at COMP. The

current-mode regulator uses the voltage at

COMP and compares it to the inductor current

to regulate the output voltage. The use of

current-mode regulation improves transient

response and control loop stability.

At the beginning of each cycle, the high-side

N-Channel MOSFET is turned on, forcing the

inductor current to rise. The current at the drain

of

the

high-side

MOSFET

is

internally

measured and converted to a voltage by the

current sense amplifier.

That voltage is compared to the error voltage at

COMP. When the inductor current rises

sufficiently, the PWM comparator turns off the

high-side switch and turns on the low-side

switch; forcing the inductor current to decrease.

The average inductor current is controlled by

the voltage at COMP, which in turn, is

controlled by the output voltage. Thus the

output voltage controls the inductor current to

satisfy the load.

Since the high-side N-Channel MOSFET

bootstrap capacitor from LX to BST is required

to drive the high-side MOSFET gate. When LX

is driven low (through the low-side MOSFET),

the BST capacitor is internally charged. The

voltage at BST is applied to the high-side

MOSFET gate to turn it on. Voltage is

maintained until the high-side MOSFET is

turned off and the low-side MOSFET is turned

on, and the cycle repeats. Connect a 10nF or

greater capacitor from BST to SW to drive the

high-side MOSFET gate.

APPLICATION INFORMATION

MP2108

LX

VIN

BST

FB

COMP

SS

PGND

SGND

VREF

RUN

OUTPUT

2.5V / 2A

INPUT

2.6V to 6V

C5

10nF

C6

10nF

C3

2.2nF

C7

10nF

C4

OPEN

1

3

7

4

5

9

8

6

10

2

Figure 2—Typical Application Circuit

Internal Low-Dropout Regulator

The internal power to the MP2108 is supplied

from the input voltage (VIN) through an internal

2.4V low-dropout linear regulator, whose output

is VREF. Bypass VREF to SGND with a 10nF

or greater capacitor for proper operation. The

internal regulator can not supply more current

than is required to operate the MP2108.

Therefore, do not apply any external load to

VREF.

Soft-Start

The MP2108 includes a soft-start timer that

slowly ramps the output voltage at startup to

prevent excessive current at the input.

When power is applied to the MP2108, and

RUN is asserted. A 2µA internal current source

charges the external capacitor at SS. As the

capacitor charges, the voltage at SS rises. The

MP2108 internally limits the feedback threshold

voltage at FB to that of the voltage at SS. This

forces the output voltage to rise at the same