SC2446 Semtech Corporation, SC2446 Datasheet - Page 21

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

for R
R
If the current limit is to be set to I
power circuit parameter and C
R
Similar steps and equations apply to the current limit
setting and scaling for current sinking mode.
Remark 4: When the current limit I
the designer has the freedom of selecting higher Rds(ON)
MOSFETs to reduce cost. As a result, R
and I
MOSFET’s is always preferred, the current-limit setting
technique described above allows quick adjustment on a
well-tested prototype without the need to replace the
power MOSFETs.
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 (75% for Ch1, 72%
for Ch2) of the reference voltage, the controller will shut
off both the top and the bottom MOSFET’s. Meanwhile
an internal 1.
capacitor C
When the capacitor is discharged to 0.5V, a 2 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
POWER MANAGEMENT
Application Information (Cont.)
s2
s
=4.12 k , R
2004 Semtech Corp.
is then obtained from
s
LMcp
and R
is reduced. Although the use of low-cost
32
S3
(C
.
s3
33
A current source discharges the soft-start
=190 k
) connected to the SS/EN pin.
I
LM
R
eq
R
R
s
2
s
R
C
R
s
s
and R
s
R
3
R
s
V
3
s
R
O
3
L
R
eq
R
s
s2
LM
s
,
75
s
, it is calculated that
=4.22 k .
.
= 5A with the existing
mV
LM
is lower than I
,
eg
is increased
LMcp
,
21
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.
a) The time taken to discharge the capacitor from 3.2V
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
Leff
32
= 0.1
32
0.30 I
= 0.1
µ
F, t
LMcp
µ
ssf
F, t
is calculated as 193ms.
t
t
t
and is independent of the soft start
sso
ssf
ssr
I
ssr
Leff
R
is calculated as 135ms. Note that
C
C
o
o
C
2
I
is related to the divider resistors
32
32
32
LMcp
3 (
3 (
3 (
1
2 .
2 .
2 .
t
h
1
ssf
h
2
2
4 .
t
R
sso
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0
0
1
A
A
o
A
5 .
5 .
2 .
t
. 1
ssr
V )
V )
V )
.
o1
.
.
.
at top and R
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