SS6621 datasheet(9/12 Pages) SSC

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SS6620/21/2

Synchronous Rectification

Using the internal synchronous rectifier eliminates the

need for an external Schottky diode. Therefore, the

cost and board space is reduced. During the cycle of

off-time, the P-MOSFET turns on and shunts the N-

MOSFET. Due to the low turn-on resistance of the

MOSFET, the synchronous rectifier significantly im-

proves efficiency without the addition of an external

component. Thus, the conversion efficiency can be as

high as 93%.

Reference Voltage

The reference voltage (REF) is nominally 1.23V for

excellent T.C. performance. In addition, the REF pin

can source up to 100

µA to an external circuit with

good load regulation (<10mV). A bypass capacitor of

0.1

µF is required for proper operation and good per-

formance.

Shutdown

The whole circuit is shutdown when

SHDN

is low.

During shutdown mode, current can flow from the bat-

tery to the output through the body diode of the P-

MOSFET. VOUT falls to approximately (Vin - 0.6V) and

LX remains high impedance. The capacitance and

load at OUT determine the rate at which VOUT decays.

Shutdown can be pulled as high as 6V, regardless of

the voltage at OUT.

Current Limit Select Pin

The SS6622 series allows a selectable inductor

current limit of either 0.45A or 0.8A. This allows flexi-

bility in designing for higher current or smaller appli-

cations. CLSEL draws 1.4

µA when connected to OUT.

BATT/Damping Switch

The SS6622 is designed with an internal damping

switch (Fig. 15) to reduce ringing at LX. The damping

switch supplies a path to quickly dissipate the energy

stored in the inductor and reduces the ringing at LX.

Damping LX ringing does not reduce VOUT ripple, but

does reduce EMI. A value of R1=200

Ω works well for

most application while reducing efficiency by only 1%.

Larger R1 values provide less damping, but less im-

pact on efficiency. In principle, lower values of R1 are

needed to fully damp LX when the VOUT /VIN ratio is

high.

Selecting the Output Voltage

VOUT can be simply set to 3.3V by connecting the FB

pin to OUT due to the internal resistor divider (Fig.

16). In order to adjust the output voltage, a resistor

divider is connected to VOUT, FB, GND (Fig. 17). Use

the following equation to calculate:

R5=R6 [(VOUT / VREF )-1] where VREF =1.23V and

VOUT may range from 1.8V to 4V.

Low-Battery Detection

The SS6620 series contain 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

output) sinks current to GND.

Rev.2.01 6/06/2003

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