RT8032 datasheet(8/11 Pages) RICHTEK

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RT8032

DS8032-02

March 2011

www.richtek.com

Application Information

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation :

where VREF equals to 0.8V typical.

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

Figure 1. Setting the Output Voltage

⎟

⎠

⎞

⎜

⎝

⎛ +

REF

OUT

RT8032

GND

VOUT

Input Average Current Limit Setting

The input current limit circuit is programmed by an external

resistor on ISET. This allows the user to program a

maximum average input current. For applications such

as USB that the current from the bus must be limited, the

value of RLIM and CC1 can be calculated as following

equation.

RLIM = 0.8 / (70 x 10

-6 x IIN (A))

CC1 = 16 x 10

-6 / RLIM

,, RC1 = 1kΩ

Soft-Start

The soft-start function is combined with shutdown. When

the SHDN/SS pin is brought above 1V (typ.), the IC will

be enabled. The components of RSS and CSS provide a

slow ramping voltage on the SHDN/SS pin to provide a

soft-start function.

Input Over Voltage Protection

The RT8032 equips input over voltage protection function.

When the input voltage exceeds 6V, the next switching

cycle of the IC will be terminated. Once the input voltage

is lower than 6V, the IC will enter normal operation again.

100% Duty Cycle Operation

When the input supply voltage decreases toward the output

voltage, the duty cycle increases toward the maximum

on-time. Further reduction of the supply voltage forces

the main switch to remain on for more than one cycle and

eventually reaching 100% duty cycle. The output voltage

will then be determined by the input voltage minus the

voltage drop across the internal P-MOSFET and the

inductor.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

ΔIL increases with higher VIN and decreases

with higher inductance.

⎥⎦

⎤

⎢⎣

⎡ −

⎥⎦

⎤

⎢⎣

⎡

OUT

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite or mollypermalloy

cores. Actual core loss is independent of core size for a

fixed inductor value but it is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core losses and are preferred

at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates

“hard”, which means that

inductance collapses abruptly when the peak design

current is exceeded.

This result in an abrupt increase in inductor ripple current

and consequent output voltage ripple.

Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don't radiate energy but generally cost more

than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price vs. size requirements and

any radiated field/EMI requirements.

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