SC46208
enough attenuation time for inductor, thus prevent it from running away. When the output voltage increases, the
frequency increases to 1.25MHz gradually.
Over Voltage Detection
When the over-voltage detect circuit detects the output voltage is more than 6% of the set value, it will shutdown
the PMOS switch to prevent the output voltage from over-shoot until the voltage drops to set value.
Current Limit and Thermal Shutdown
Current limit is implemented through internal comparator, it limits the PMOS current in the 1.5A in one cycle,
thus to protect the SC46208 in over load condition.
When the die temperature is over 150°C due to the long time over load operation, SC46208 is shutdown
automatically until the temperature lower to normal state, the hysteresis temperature during this period is about
20°C.
APPLICATION NOTES
Refering to typical application circuit of fig 1, the components selection begins with inductor L, then C
IN
and
C
OUT
:
Inductor Selection
The SC46208 can utilize small surface mount and chip inductors due to its fast 1.25MHz switching frequency.
For most applications the SC46208 operates with inductors of 1μH to 4.7μH. The large inductance will decrease
the ripple current, but high inductance values will also increase the PCB size, and the low inductance value result
in high ripple currents. The ripple also relative with the VIN and V
OUT
:
I
ripple
=
1 V
IN
−
V
OUT
V
OUT
×
×
fs
L
V
IN
The rating current of inductor is equal to half of the max. load current adds half the max. ripple current ,to handle
the peak inductor current without saturating. Choose inductor ripple current
I
ripple
approximately 40% of the
maximum load current
I
LOAD(max)
, so the rating current
I
sat
satisfied with following formula:
1
I
sat
>
I
LOAD(max)
+
I
ripple
=
(1
+
20%)I
LOAD(max)
2
The max. load current is 600mA, for most applications, the inductor of 720mA rating value is enough; for higher
efficiency, please choose an inductor with low DC series resistance (DCR). To reduce the noise, use a shielded
bobbin inductor.
Capacitor Selection
The low ESR capacitor is required to keep the output voltage ripple small. There, the multilayer ceramic
capacitor is recommended due to their low ESR and small size. In the most applications, 10-22μF output
capacitors are enough, higher capacitor can be used to reduce the output ripple and improve transient
response.
The low ESR input capacitor reduces the surge current drawn from the input and switching noise from the
device. Ceramic capacitor is the best selection for input capacitor, and the capacitor should be as near as
possible to chip. A 4.7μF ceramic capacitor is sufficient for most applications.
Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small
temperature coefficients.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.1
2008.09.10
Page 5 of 11
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