8

RT8525

www.richtek.com

DS8525-01 March 2012

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

the ambient temperature, and

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

SOP-14 packages, the thermal resistance,

W on a standard JEDEC 51-3 single-layer thermal test

°C can

be calculated by the following formula :

°C − 25°C) / (100°C/W) = 1.000W for

SOP-14 package

The maximum power dissipation depends on the operating

resistance,

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

×

××

+

××

η

OUT

OUT

LPK

IN

VI

VIN D T

I

=

V2 L

where

η is the efficiency of the power converter.

⎡⎤

⎛⎞ ⎛

⎞

×

+ Δ−

+

− Δ−

×

⎜⎟ ⎜

⎟

⎢⎥

⎝⎠ ⎝

⎠

⎣⎦

×× Δ

IN

IN

L

OUT

IN

L

OUT

OUT

OUT

OUT1

SW

V

11

1

Q =

I

I

I

I

I

I

22

2

V

1 = C

V

f

estimate the value of

×

Δ

××

η

OUT

OUT1

OUT

SW

DI

V

=

Cf

Finally, by taking ESR into consideration, the overall output

ripple voltage can be determined as the following

equation :

×

Δ×

+

××

OUT

OUT

IN

OUT

SW

DI

V

= I

ESR

η Cf

Figure 3. The Output Ripple Voltage without the

Contribution of ESR

Diode Selection

Schottky diodes are recommended for most applications

because of their fast recovery time and low forward voltage.

The power dissipation, reverse voltage rating and pulsating

peak current are the important parameters for Schottky

diode selection. Make sure that the diode's peak current

the maximum output voltage.

Capacitor Selection

Output ripple voltage is an important index for estimating

the performance. This portion consists of two parts, one

is the product of input current and ESR of output capacitor,

another part is formed by charging and discharging

process of output capacitor. Refer to figure 3, evaluate

definition of Q, the Q value can be calculated as following

equation :

Time

Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

Δ

Δ

Input Current

selected with a saturated current rating greater than the

peak current provided by the following equation :