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EV1542DK-00A

700KHz/1.3MHz Boost Converter with a 2A Switch

Monolithic Power Systems 

MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH

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 at COMP. The voltage at the output of

the error amplifier is an amplified version of the

difference between the 1.25V reference voltage

and the feedback voltage. When these two

voltages are equal, the PWM comparator turns

off the switch forcing the inductor current to the

output capacitor through the external rectifier.

This causes the inductor current to decrease.

The peak 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.

The use of current mode regulation improves

transient response and control loop stability.

APPLICATION INFORMATION

Components referenced below apply to Typical Application Circuit on page 1.

Selecting the Soft-Start Capacitor

The MP1542 includes a soft-start timer that

limits the voltage at COMP during startup to

prevent excessive current at the input. This

prevents premature termination of the source

voltage at startup due to input current

overshoot. When power is applied to the

MP1542, and enable is asserted, a 6µA internal

current source charges the external capacitor at

SS. As the SS capacitor is charged, the voltage

at SS rises. The MP1542 internally clamps the

voltage at COMP to 700mV above the voltage

at SS. The soft-start ends when the voltage at

SS reaches 0.45V. This limits the inductor

current at start-up, forcing the input current to

rise slowly to the current required to regulate

the output voltage.

The soft-start period is determined by the

equation:

t SS = 75 × CSS

Where CSS (in nF) is the soft-start capacitor

from SS to GND, and tSS (in µs) is the soft-start

period.

Determine the capacitor required for a given

soft-start period by the equation:

CSS = 0.0133 × t SS

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 10kΩ for the low-side

resistor R2 of the voltage divider. Determine the

high-side resistor R1 by the equation:

where VOUT is the output voltage.

For R2 = 10kΩ and VFB = 1.25V, then

R1 (kΩ) = 8kΩ (VOUT – 1.25V).

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise at the IC to a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice.

Use an input capacitor value greater than

4.7µF. The capacitor can be electrolytic,

tantalum or ceramic. However since it absorbs

the input switching current it requires an

adequate ripple current rating. Use a capacitor

with RMS current rating greater than the

inductor ripple current (see Selecting The

Inductor to determine the inductor ripple

current).

To insure stable operation place the input

capacitor as close to the IC as possible.

Alternately a smaller high quality ceramic 0.1µF

capacitor may be placed closer to the IC with

the larger capacitor placed further away. If

using this technique, it is recommended that the

larger capacitor be a tantalum or electrolytic

type. All ceramic capacitors should be placed

close to the MP1542.

R1 = R2(VOUT − VFB )

VFB

MP1542 Rev. 1.2

www.MonolithicPower.com

6

12/14/2005

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© 2005 MPS. All Rights Reserved.

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