RT8036 datasheet(13/17 Pages) RICHTEK

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RT8036

DS8036-02 April 2011

www.richtek.com

Application Information

The RT8036 is an integrated power management IC

including one Buck converter and four linear regulators.

The RT8036 features a fixed output voltage to eliminate

the need of external feedback resistors and simplify the

PCB layout. The RT8036 fix the output voltage by internal

resistor and keep the output voltage between -3% to 3%.

Please refer to the ordering information for detailed output

voltage setting.

Buck Enable Control

Pull the ENB pin (>1.5V) to turn on the buck converter

and to pull low the ENB pin (<0.4V) to turn off the buck

converter.

Soft-Start

The RT8036 has a soft-start to control the output voltage

rise time and limit the current surge at the startup. The

soft-start will begin while EN rises above high threshold.

Buck Current Limiting

A current limit feature allows the RT8036 to protect itself

and external components during overload conditions. In

operating mode, the inductor peak current under 600mA

is normally used. The current limit prevents the loss of

current control seen in some products when the output

voltage is pulled low in serious overload conditions.

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 VINB and

decreases with higher inductance.

OUT

⎡⎤

Δ=

× −

⎢⎥

⎣⎦ ⎣⎦

Having a lower ripple current reduces the ESR losses in

the output capacitors and the output voltage ripple. Highest

efficiency operation is achieved at low frequency with small

ripple current. This, however, requires a large inductor. A

reasonable starting point for selecting the ripple current

ΔIL = 0.4 (IMAX). The largest ripple current occurs at the

highest VINB. To guarantee that the ripple current stays

below a specified maximum, the inductor value should be

chosen according to the following equation :

OUT

L(MAX)

IN(MAX)

⎡⎤ ⎡

⎤

=× −

⎢⎥ ⎢

⎥

×Δ

⎢⎥ ⎢

⎥

⎣⎦ ⎣

⎦

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 results 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.

CINB and COUTB Selection

The input capacitance, CINB, is needed to filter the

trapezoidal current at the source of the top MOSFET. To

prevent large ripple voltage, a low ESR input capacitor

sized for the maximum RMS current should be used. RMS

current is given by :

OUTB

INB

RMS

OUTB(MAX)

INB

OUTB

=−

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