SC2446A Semtech Corporation, SC2446A Datasheet - Page 16

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

Note that R
the bias current effect of the current amplifier in
SC2446A.
b) When the required current limit I
one just needs to remove R
for R
R
Similar steps and equations apply to the current limit
setting and scaling for current sinking mode.
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. As the load current increases, the cycle-by-
cycle current-limit comparator will first limit the inductor
current. Further increase in loading will cause the output
voltage (hence the feedback voltage) to fall. If the
feedback voltage falls to less than (50% for Ch1, 50%
for Ch2) of the reference voltage, the controller will shut
off both the top and the bottom MOSFETs. Meanwhile
an internal net 1.2 A current source discharges the soft-
start capacitor C
When the capacitor is discharged to 0.5V, a 1.8 A current
source recharges the SS/EN capacitor and converter
restarts. If overload persists, the controller will shut down
the converter when the soft start capacitor voltage
exceeds 3.2V. The converter will repeatedly start and shut
off until it is no longer overloaded. 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.
POWER MANAGEMENT
Application Information (Cont.)
s2
2005 Semtech Corp.
is then obtained from
s
and R
S3
S2
.
is selected as R
32
I
LM
(C
R
33
R
L
) connected to the SS/EN pin.
s
R
2
R
s
R
C
s
s
s
3
R
R
V
s1
s
3
O
s
R
3
and solve
L
S
R
//R
L
R
s
,
50
s
.
S1
mV
LM
in order to reduce
,
is less than I
LMcp
,
16
to 0.5V
If C
b) The soft start time from 0.5V to 3.2V
When C
during soft start, the converter only starts switching when
the voltage at SS/EN exceeds 1.2V.
c) 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 input of the channel-one error amplifier
is internally tied the 0.5V voltage reference output (Pin
8). The non-inverting input of the channel-two error
amplifier is brought out as a device pin (Pin 10) to which
the user can connect Pin 8 or an external voltage
reference. A simple voltage divider (R
bottom) sets the converter output voltage. The voltage
feedback gain h=0.5/V
value as
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
Leff
a) The time taken to discharge the capacitor from 3.2V
32
= 0.1
32
0.30 I
= 0.1
µ
F, t
LMcp
µ
ssf
o1
F, t
is calculated as 225ms.
or R
t
t
t
and is independent of the soft start
sso
ssf
ssr
I
ssr
Leff
R
is calculated as 150ms. Note that
o2
o
C
C
o
C
2
is related to the divider resistors
I
32
32
32
LMcp
is chosen, the other can be
3 (
3 (
3 (
1
2 .
2 .
2 .
t
h
1
1
1
ssf
h
2 .
8 .
8 .
t
R
sso
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0
0
1
o
A
A
A
5 .
5 .
2 .
t
. 1
ssr
V )
V )
V )
.
o1
.
.
.
SC2446A
at top and R
www.semtech.com
o
. Since the
o2
at