SC2447 Semtech Corporation, SC2447 Datasheet - Page 21

• Manufacturer Part Number: SC2447
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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 9.5 A current source
charging the soft start capacitor C
the SS/EN pin. The soft-start voltage V
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 V
3.2V, an internal net 37 A current source will discharge
the soft-start capacitor C
tripping the over-current comparator. If V
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 7.5 A current
source discharges C
When the capacitor is discharged below 0.5V, the overload
hiccup latch is reset, the 9.5 A 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:
to 2.85V is
If C
b) The time taken to discharge the capacitor from 2.85V
to 0.5V is
If C
c) The soft start time from 0.5V to 3.2V is
POWER MANAGEMENT
a) The time taken to discharge the capacitor from 3.2V
Application Information (Cont.)
2006 Semtech Corp.
32
32
= 0.1
= 0.1
µ
µ
F, t
F, t
ssf1
ssf2
t
t
ssf
ssf
t
is calculated as 31.3ms.
ssr
is calculated as 0.945ms.
1
2
=
=
32
=
C
(C
C
C
32
32
32
33
32
) .
3 (
(C
(
3 (
. 2
2 .
33
2 .
85
9
7
37
) during the off time after
−
5 .
−
5 .
−
. 2
µ
. 0
µ
µ
. 0
A
A
85
A
) 5
) 5
32
V
)
V
V
SS/EN
(C
.
SS/EN
.
.
33
), connected to
is higher than
will reach its
SS/EN
falls to
21
When C
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
The average inductor current is then
I
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
voltage. The voltage feedback gain h=0.5/V
to the divider resistors as follows:
Once either R
calculated for the desired output voltage V
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.
The following table lists a few standard resistor
combinations for realizing some commonly used output
voltages.
Leff
o1
at top and R
R
R
0.34 I
32
1 o
2 o
V
(
- 1
o
= 0.1
O (
O (
) h
(
) V
h /
h
h
) m
) m
LMcp
o1
µ
I
2
0
0
1
F, t
o2
0 0
Leff
K
t
6 .
2 .
or R
sso
and is independent of the soft start
at bottom) sets the converter output
=
ssr
=
R
8
0
0
1
6 0
9 .
8 .
K
I
o
C
o2
LMcp
is calculated as 28.4ms. Note
2
32
=
1
is chosen, the other can be
1
1
1
4 .
3 (
K
2 .
4 .
t
1
K
ssf
2 .
−
h
1
9
2
+
−
h
1
5 .
1
0 .
2
K
5 .
. 1
t
t
K
www.DataSheet4U.com
R
µ
ssf
sso
25
A
o
2
1
. 2
+
.
1
2
1
)
1 6
V
6 .
8 .
K
t
K
ssr
.
.
. 4
2
1
2 0
www.semtech.com
4
5 .
K
SC2447
K
o
. Since the
o
. 5
3
5
1
2 6
is related
3 .
6 .
K
K