MP1543 — 1.5A, 500KHZ SYNCHRONOUS RECTIFIED STEP-UP CONVERTER

MP1543 Rev. 1.0

www.MonolithicPower.com

5

1/11/2006

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© 2006 MPS. All Rights Reserved.

TM

OPERATION

FB

GND

SW

OUT

EN

UVLO

REFERENCE VOLTAGE

BIAS CURRENT

THERMAL SHUTDOWN

CONTROL

LOGIC

RS

CURRENT

SENSE

AMPLIFIER

1.2V

ERROR

AMPLIFIER

PWM

COMPARATOR

+

RAMP

OSC

500KHz

MP1543_BD01

Figure 1—Functional Block Diagram

The MP1543 uses a 500KHz fixed-frequency,

current-mode regulation architecture to regulate

the output voltage. The MP1543 measures the

output voltage through an external resistive

voltage divider and compares that to the

internal 1.2V reference to generate the error

voltage. The current-mode regulator compares

the error voltage to the inductor current to

regulate the output voltage. The use of

current-mode regulation improves transient

response and control loop stability.

When the MP1543 is disabled (EN = Low), both

power switches are off. The body of the

P-Channel MOSFET connects to SW and there

is no current path from SW to OUT. Therefore,

the output voltage discharges to ground. When

the MP1543 is enabled (EN = High), the body of

the P-Channel MOSFET connects to OUT and

forms a forward diode from SW to OUT. Thus

the output voltage rises up toward the input

voltage. When output voltage crosses 1.6V the

MP1543 starts the controller and regulates the

output voltage to the target value.

At the beginning of each cycle, the N-Channel

MOSFET switch is turned on, forcing the

inductor current to rise. The current at the

source of the switch is internally measured and

converted to a voltage by the current sense

amplifier. That voltage is compared to the error

voltage. When the inductor current rises

sufficiently, the PWM comparator turns off the

switch, forcing the inductor current to the output

capacitor through the internal P-Channel

MOSFET rectifier, which forces the inductor

current to decrease. The peak inductor current

is controlled by the error voltage, which in turn

is controlled by the output voltage. Thus the

output voltage controls the inductor current to

satisfy the load.

Soft-Start

The MP1543 includes a soft-start timer that

limits the voltage at the error amplifier output

during startup to prevent excessive current at

the input. This prevents premature termination

of the source voltage at startup due to inrush

current. This also limits the inductor current at

startup, forcing the input current to rise slowly to

the amount required to regulate the output

voltage during soft-start.