SS6611GOTR datasheet(12/16 Pages) SSC

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SS6610/11G

4/21/2006 Rev.3.01

Shutdown

The whole circuit is shutdown when

SHDN

is low. In

shutdown mode, the current can flow from the battery

to the output due to the body diode of the P-MOSFET.

VOUTfalls to approximately (Vin - 0.6V) and LX remains

high impedance. The capacitance and load at OUT de-

termine the rate at which VOUT decays. Shutdown

can be pulled as high as 6V. Regardless of the volt-

age at OUT.

Selecting the Output Voltage

VOUT can be simply set to 3.3V/5.0V by connecting the

FB pin to OUT/GND due to the use of an internal resis-

tor divider in the IC (Fig.32 and Fig.33). In order to

adjust output voltage, a resistor divider is connected

to VOUT, FB, GND (Fig.34). Vout can be calculated

by the following equation:

R5=R6 [(VOUT / VREF )-1] .....................................(1)

where V REF =1.23V and VOUT ranges from 1.8V to

5.5V. The recommended R6 is 240k

Ω.

Low-Battery Detection

The SS6610 series contains an on-chip comparator with

50mV internal hysteresis (REF, REF+50mV) for low

battery detection. If the voltage at LBI falls below the

internal reference voltage, LBO ( an open-drain out-

put) sinks current to GND.

Component Selection

1. Inductor Selection

An inductor value of 22

µH performs well in most

applications. The SS6610 series also work with

inductors in the 10

µH to 47µH range. An inductor

with higher peak inductor current creates a higher

output voltage ripple (IPEAK X output filter capaci-

tor ESR). The inductor’s DC resistance signifi-

cantly affects efficiency. We can calculate the

maximum output current as follows:

























−

OUT

OFF

LIM

OUT

)

MAX

(

OUT

……………………………………………………(2)

where IOUT(MAX)=maximum output current in

amps

VIN=input voltage

L=inductor value in

µH

η =efficiency (typically 0.9)

tOFF=LX switch’ off-time in µs

ILIM=1.0A or 0.65A

2. Capacitor Selection

The output ripple voltage is related to the peak

inductor current and the output capacitor ESR.

Besides output ripple voltage, the output ripple

current may also be of concern. A filter capacitor

with low ESR is helpful to the efficiency and the

steady state output current of the SS6610 series.

Therefore a NIPPON MCM Series tantalum

capacitor of 100

µF/6V is recommended. A smaller

capacitor (down to 47μF with higher ESR) is ac-

ceptable for light loads or in applications that can

tolerate higher output ripple.

3. PCB Layout and Grounding

Since the SS6610/11’s switching frequency can

range up to 500kHz, the SS6610/11 can be very

sensitive. Careful printed circuit layout is im-

portant for minimizing ground bounce and noise.

The OUT pin should be as clear as possible,

and the GND pin should be placed close to the

ground plane. Keep the IC’s GND pin and the

ground leads of the input and output filter capaci-

tors less than 0.2in (5mm) apart. In addition,

keep all connections to the FB and LX pins as

short as possible. In particular, when using ex-

ternal feedback resistors, locate them as close

to the FB pin as possible. To maximize output power

and efficiency, and minimize output ripple voltage,

use a ground plane and solder the IC’s GND di-

rectly to the ground plane. Fig. 35 to 37 are the

recommended layout diagrams.

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