MP1522 – 25V STEP-UP CONVERTER IN A SOT23-5 PACKAGE

MP1522 Rev. 1.9

www.MonolithicPower.com

4

12/9/2005

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© 2005 MPS. All Rights Reserved.

Variable Frequency

Constant-Peak-Current Operation

When the power MOSFET is turned on the

inductor current increases until the 0.45A

current limit is reached. The Power MOSFET is

then turned off for a minimum of 550ns. At the

end of this 550ns transition if the feedback pin

is still lower than the 1.23V internal reference

the power MOSFET will again be turned on,

otherwise the MP1522 waits until the voltage

drops below the threshold before turning on the

MOSFET again. This process allows for optimal

use of the inductor, while minimizing the output

ripple and size of the output capacitor and

maintaining low operating current.

Setting the Output Voltage

The output voltage is set by a resistive voltage

divider connecting the output voltage to FB. The

voltage divider reduces the output voltage down

by the ratio:

1

R

2

R

1

R

V

V

FB

OUT

+

=

⎟

⎠

⎞

⎜

⎝

⎛

+

=

1

R

2

R

1

R

V

V

FB

OUT

1.23V feedback threshold. Substituting the

feedback threshold voltage:

⎟

⎠

⎞

⎜

⎝

⎛

+

=

1

R

2

R

1

R

V

23

.

1

R1 can range from 10kΩ to 200kΩ. R2 is

determined the equation:

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

1

V

V

1

R

2

R

FB

OUT

For example for an output voltage of 20V and

R1 = 130kΩ the formula yields:

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

Ω

=

1

V

23

.

1

20

k

130

2

R

R2 = 2MΩ the closest standard 1% resistor

value.

The feedback divider requires a feed-forward

capacitor connected in parallel with R2. This

capacitor is typically in the range of 1nF-10nF.

On/Off Control

The EN input is used to turn the converter on

and off. A high input at EN turns the converter

on and a low level turns the converter off. When

EN is not being used, connect to the input

source for automatic startup. For inputs greater

resistor.

Input Capacitor

To reduce noise, use a low ESR/ESL type input

capacitor. A 1µF surface mount ceramic

capacitor close to the input supply side of the

inductor minimizes EMI and ripple current at the

input source. See the section on printed circuit

board layout for reducing noise and emissions.

A bypass capacitor is typically required to

prevent noise at the MP1522. This capacitor

should also be a low ESR/ESL type capacitor.

Place a 0.1µF surface mount ceramic capacitor

between BIAS and GND as close to the

MP1522 as possible.

Output Capacitor

To minimize output voltage ripple and noise use

a low ESR/ESL type output capacitor. Typically

a 1µF surface mount ceramic capacitor is

sufficient. When using aluminum electrolytics,

tantalum or aluminum polymer capacitors the

ESR/ESL dominate the output ripple. See the

section on printed circuit board layout.

Inductor

The inductor stores energy during the power

MOSFET switch on-time. This energy is

transferred to the output during the switch off-

time. Select an inductor whose peak-current

rating is greater than the 450mA peak current

limit of the MP1522. Lower value inductors yield

higher switching frequencies and thus lower

output ripple voltage. However, using too low a

value inductor will cause an increase in peak

current limit due to the delay of the current-limit

comparator. Choose an inductor such that the

peak current limit is reached in 550ns or longer.

The minimum value inductor for this criterion is:

()

H

V

10

31

.

1

L

IN

6

µ

×

×

≥

−

use an inductor value greater than 6.8µH.