Datasheet

LTC3805-5
15
38055fe
APPLICATIONS INFORMATION
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 6 and 7 show two
ways to do this. Figure 6 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
SENSE1
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 6 becomes
impractical. The circuit of Figure 7 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 10W
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 internal
soft-start time t
SS(IN)
= 1.8ms.
If there is an overcurrent fault detected on the OC pin, the
LTC3805-5 enters a shutdown mode while a 2µA current
discharges the voltage on the SSFLT pin from 4.75V to
about 0.7V. The fault timeout t
FTO(EXT)
is therefore:
t
FTO(EXT)
= C
SS
4.75V − 0.7V
2µA
At this point, the LTC3805-5 attempts a restart.
In the event of a persistent fault, such as a short-circuit
on the converter output, the converter enters a “hiccup”
mode where it continues to try and restart at repetition
rate determined by C
SS
. If the fault is eventually removed
the converter successfully restarts.
Figure 6. Circuit to Increase the Overcurrent
Threshold for Small Switch Currents
Figure 7. Circuit to Increase the Overcurrent
Threshold for Large Switch Currents
LTC3805-5
OC
GATE
I
SENSE
R
SENSE1
R
SENSE2
R
SLOPE
GND
38055 F06
I
OC
= 10µA
I
SLOPE
LTC3805-5
OC
GATE
I
SENSE
R1
R2
R
SLOPE
GND
38055 F07
R
SENSE
I
OC
= 10µA
I
SLOPE