SC2447 查看數據表（PDF） - Semtech Corporation
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SC2447 Datasheet PDF : 26 Pages
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SC2447
POWER MANAGEMENT
Application Information (Cont.)
Overload Protection and Hiccup
During start-up, the capacitor from the SS/EN pin to
ground functions as a soft-start capacitor. After the
converter starts and enters regulation, the same
capacitor operates as an overload shutoff timing
capacitor. There is an internal net 9.5µA current source
charging the soft start capacitor C32(C33), connected to
the SS/EN pin. The soft-start voltage VSS/EN will reach its
final value if no current limit occurs. As the load current
increases, the cycle-by-cycle current-limit comparator will
first limit the inductor current. If VSS/EN is higher than
3.2V, an internal net 37µA current source will discharge
the soft-start capacitor C32(C33) during the off time after
tripping the over-current comparator. If V falls to
SS/EN
2.85V, the controller will shut off both the top and the
bottom MOSFETs by pulling down the GDL and tri-stating
the GDH output (PWM). An internal net 7.5µA current
source discharges C32(C33) .
When the capacitor is discharged below 0.5V, the overload
hiccup latch is reset, the 9.5µA current source recharges
the SS/EN capacitor and converter restarts. The overload
hiccup function is enabled when the soft start capacitor
voltage exceeds 3.2V. The converter will repeatedly start
and shut off until it is no longer overloaded when the
soft-start voltage exceeds 3.2V. This hiccup mode of
overload protection is a form of foldback current limiting.
The following calculations estimate the average inductor
current when the converter output is shorted to the
ground:
a) The time taken to discharge the capacitor from 3.2V
to 2.85V is
tssf 1
=
C32
(3.2 − 2.85)V
37µA
.
If C32 = 0.1µF, tssf1 is calculated as 0.945ms.
When C32 = 0.1µF, tssr is calculated as 28.4ms. Note
that during soft start, the converter only starts switching
when the voltage at SS/EN exceeds 1.25V.
d) The effective start-up time is
tsso
= C32
(3.2 −1.25)V
9.5µA
.
The average inductor current is then
I Leff
=
I LMcp
tssf 1
tsso
+ tssf 2
+ tssr
.
ILeff ≈ 0.34 ILMcp and is independent of the soft start
capacitor value. The converter will not overheat in
hiccup.
Setting the Output Voltage
The non-inverting inputs of channel 1 and channel 2 error
amplifiers are brought out as device pins (Pin 10 and Pin
8). These pins can be tied to the precision 0.5V reference
output (Pin 9) of the SC2447. A simple voltage divider
(R at top and R at bottom) sets the converter output
o1
o2
voltage. The voltage feedback gain h=0.5/V is related
o
to the divider resistors as follows:
Ro 2
=
h
1− h
Ro1 .
Once either R or R is chosen, the other can be
o1
o2
calculated for the desired output voltage V . Since the
o
number of standard resistance values is limited, the
calculated resistance may not be available as a standard
value resistor. As a result, there will be a set error in the
converter output voltage. This non-random error is
caused by the feedback voltage divider ratio. It cannot
be corrected by the feedback loop.
b) The time taken to discharge the capacitor from 2.85V
to 0.5V is
tssf 2
=
C32
(2.85 − 0.5)V
7.5µA
.
If C32 = 0.1µF, tssf2 is calculated as 31.3ms.
c) The soft start time from 0.5V to 3.2V is
tssr
= C32
(3.2 − 0.5)V
9.5µA
.
The following table lists a few standard resistor
combinations for realizing some commonly used output
voltages.
Vo (V) 0.6 0.9 1.2 1.5 1.8 2.5 3.3
(1-h)/h 0.2 0.8 1.4 2 2.6
4
5.6
Ro1 (Ohm) 200 806 1.4K 2.0K 2.61K 4.02K 5.62K
Ro2 (Ohm) 1K 1K 1K 1K 1K 1K 1K
© 2006 Semtech Corp.
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