sc4510 Semtech Corporation, sc4510 Datasheet - Page 14

• Manufacturer Part Number: sc4510
• Description: Sc4510 High-performance Synchronous Buck Controller With Reference Tracking
• Manufacturer: Semtech Corporation

For simplicity R
be estimated from
In the application circuit the current feedback signal had
to be halved to account for variations explained above and
ensure that the current limit is at least 20% above nominal
at minimum input.
b) When the required current limit value I
I
for R
to set the current limit.
Similar steps and equations apply to the current limit setting
and scaling for current sinking mode.
OVERCURRENT PROTECTION AND HICCUP MODE
In addition to the current limit capabilty, SC4510 provides
overcurrent protection in case the convertor output is
shorted to ground. If this fault condition happens, the
controller senses the output voltage via voltage feedback
pin IN-. When the sensed voltage is below 70% of the
normal feedback voltage, the controller shuts down both
top and bottom MOSFETs. At the same time, a current sink
of 1 A discharges the soft start capacitor C
to the SS/EN pin.
When the capacitor is discharged until its voltage reaches
0.4V, the controller initiates the soft start process. If the
short circuit fault persists, the controller shuts down the
convertor again when the voltage across the soft start
capacitor reaches 3.4V. This hiccup process repeats until
the fault condition is removed. Under this situation, it is
important to make sure that the convertor does not fail.
One important parameter is the convertor thermal condition
which is directly related to the effective inductor and
MOSFETs current. The effective currents in inductor and
MOSFETs can be estimated using the following equations.
POWER MANAGEMENT
Application Information (Contd.)
LMcp
2004 Semtech Corp.
remove R
s
and solve
I
LM
I
LM
s1
R
s2
solve
eq
R
can be omitted and the current limit can
R
eq
s
C
75
s
R
R
mV
s
s
3
V
R
o
L
eq
R
s
75
R
s
R
1
mV
s
1
LM
SS
is less than
connected
14
a) The time it takes to discharge the capacitor from 3.4V to
0.4V
In the application circuit, C
as 140 mS.
b) The time interval of the soft start process from 0.4V to
3.4V
When C
Notice that during the soft start process, the convertor only
starts switching when the voltage at SS/EN exceeds 1.3V.
Then,
c) The effective operation time interval
This is the interval where the gate drive outputs are active
and current builds up in the inductor.
The effective inductor current is then
It turns out that
capacitor value and is determined as 0.3I
lead to a reasonable thermal condition in the convertor
hiccup operation. Note that
average value of the inductor current since the convertor
is still in the soft start mode during the tsso interval.
SS
I
Leff
= 47 nF, t
t
t
sso
ssr
t
ssf
ILeff
C
C
I
ssr
SS
SS
LMcp
C
is calculated as 70 mS.
SS
is independent of the soft start
3 (
3 (
SS
ILeff
3 (
= 47 nF and t
4 .
4 .
t
4 .
2
ssf
2
1
calculated is not the true
. 1
t
0
. 0
A
A
A
sso
) 4 .
) 3
) 4
V
V
t
V
ssr
.
LMcp
www.semtech.com
SC4510
ssf
.
.
is calculated
. This should
.