RT6208 datasheet(13/17 Pages) RICHTEK

13 / 17 page

RT6208

is a registered trademark of Richtek Technology Corporation.

DS6208-00

May

2015

www.richtek.com

Input Under Voltage Lockout

The RT6208 implements a protection feature which

disables switching when the input voltage is too low. If

VIN falls below 3.9V typical, an under voltage detector

disables switching. Switching is enabled when the input

voltage exceeds 4.2V typical (4.5V maximum).

Enable Operation

The EN pin can be used to shutdown or activate the

chip. Pulling the EN pin low (<1V) will shutdown the

device. During shutdown mode, the RT6208 quiescent

current drops to lower than 3

A. Driving the EN pin

high (>1.4V) will turn on the device again. Leaving the

EN pin floating will pull the EN pin up to 2V internally

and enable RT6208.

Soft-Start

The RT6208 provides an internal soft-start function to

prevent large inrush current and output voltage

overshoot when the converter starts up. The soft-start

automatically begins once the chip is enabled. During

soft-start, it clamps the ramp of internal reference

voltage which is compared with FB signal. The typical

soft-start duration is 1ms.

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the

triangular current at the Source of the high-side

MOSFET. To prevent large ripple current, a low ESR

input capacitor sized for the maximum RMS current

should be used. The approximate RMS current

equation is given :

OUT

RMS

OUT(MAX)

OUT

= I



This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT / 2. This simple worst case condition is

commonly used for design because even significant

deviations do not offer much relief. Choose a capacitor

rated at a higher temperature than required. Several

capacitors may also be paralleled to meet size or

height requirements in the design.

The output capacitor, COUT, filters the inductor’s ripple

current and stores energy to satisfy the load current

when the RT6208 is in sleep mode. The value of the

output capacitor must be large enough to accept the

energy stored in the inductor without a large change in

output

voltage.

achieve

output

voltage

peak-peak ripple less than 1% of the output voltage,

the output capacitor must be :

PEAK

OUT







 



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 :

PD(MAX) = (TJ(MAX)  TA) / JA

where TJ(MAX) is the maximum junction temperature,

TA is the ambient temperature, and JA is the junction to

ambient thermal resistance.

For recommended operating condition specifications,

the maximum junction temperature is 125

C. The

junction to ambient thermal resistance,

JA, is layout

dependent. For SOP-8 (Exposed Pad) package, the

thermal resistance,

JA, is 30.6C/W on a standard

JEDEC 51-7 four-layer

thermal

test

board. For

SOT-23-6 package, the thermal resistance,

JA, is

208.2

C/W on a standard JEDEC 51-7 four-layer

thermal test board. For SOT-23-8 package, the thermal

resistance,

JA, is 186.2C/W on a standard JEDEC

51-7 four-layer thermal test board. The maximum

power dissipation at TA = 25C can be calculated by

the following formula :

PD(MAX) = (125C  25C) / (30.6C/W) = 3.26W for

SOP-8 (Exposed Pad) package

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