LTC3805
16
3805fg
Figure 7. Circuit to Increase the Overcurrent
Threshold for Small Switch Currents
LTC3805
OC
GATE
I
SENSE
R
SENSE1
R
SENSE2
R
SLOPE
GND
3805 F07
I
OC
= 10µA
I
SLOPE
be slowed by using a larger value of SSFLT capacitor. It is
also possible to trip overcurrent protection during a load
step especially if the trip threshold is lowered by making
R
OC
> R
OC(CRIT)
.
Another overcurrent protection strategy is keep the
converter running as current limiting reduces the duty
cycle and the output voltage sags. In this case, the goal
is often keep the converter in normal operation over as
wide a range as possible, including current limiting, and
to trigger the overcurrent trip only to prevent damage. To
implement this strategy use a value of R
OC
smaller than
R
OC(CRIT)
. This also reduces sensitivity to overcurrent trips
caused by transient operation. In the limit, set R
OC
= 0
and connect the OC pin directly to R
SENSE
. This causes an
overcurrent trip near minimum duty cycle or around 6%.
In some cases it may be desirable to increase the trip
threshold even further. In this strategy, the converter is
allowed to operate all the way down to minimum duty
cycle at which point the cycle-by-cycle current limit of
the I
SENSE
pin is lost and switch current goes up propor-
tionally to the output current. Figures 7 and 8 show two
ways
to do this. Figure 7 is for relatively low currents with
relatively large values of R
SENSE
. Using this circuit the
overcurrent trip threshold is increased from 100mV to:
V
OC
=
R
SENSE1
+ R
SENSE2
R
100mV
where it is assumed that the values of R
SENSE1
and
R
SENSE2
are so small that the I
OC
= 10µA threshold adjust-
ment current produces a negligible change in V
OC
.
For larger currents, values of the current sense resistors
must be very small and the circuit of Figure 7 becomes
impractical. The circuit of Figure 8 can be substituted and
the current sense threshold is increased from 100mV to:
V
OC
=
R1+ R2
R1
100mV
where the values of R1 and R2 should be kept below 10Ω
to prevent the I
OC
= 10µA threshold adjustment current
from producing a shift in V
OC
.
External Soft-Start Fault Timeout
The external soft-start is programmed by a capacitor C
SS
from the SSFLT pin to GND. At the initiation of soft-start
the voltage on the SSFLT pin is quickly charged to 0.7V
at which point GATE begins switching. From that point,
a 6µA current charges the voltage on the SSFLT pin until
the voltage reaches about 2.25V at which point soft-start
is over and the converter enters closed-loop regulation.
The soft-start time t
SS(EXT)
as a function of the soft-start
capacitor, C
SS
, is therefore:
t
SS(EXT)
= C
SS
2.25 − 0.7V
6µA
After soft-start is complete, the voltage on the SSFLT
pin continues to charge to about a final value of 4.75V.
Note that choosing a value of C
SS
less than 5.8nF has
no effect since it would attempt to program an external
soft-start time t
SS(EXT)
less than the mandatory minimum
Figure 8. Circuit to Increase the Overcurrent
Threshold for Large Switch Currents
LTC3805
OC
GATE
I
SENSE
R1
R2
R
SLOPE
GND
3805 F08
R
SENSE
I
OC
= 10µA
I
SLOPE
applications inForMation
