SS6620 データシートの表示（PDF） - Silicon Standard Corp.
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SS6620 Datasheet PDF : 12 Pages
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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 100µA to an external circuit with
good load regulation (<10mV). A bypass capacitor of
0.1µF is required for proper operation and good per-
formance.
Shutdown
The whole circuit is shutdown when VSHDN 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 1.4µA 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=200Ω 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
www.SiliconStandard.com
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