LTC3805 Linear Technology, LTC3805 Datasheet - Page 16

• Manufacturer Part Number: LTC3805
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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LTC3805
APPLICATIONS INFORMATION
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
Another overcurrent protection strategy is keep the con-
verter 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
This also reduces sensitivity to overcurrent trips caused by
transient operation. In the limit, set R
the OC pin directly to R
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
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
overcurrent trip threshold is increased from 100mV to:
where it is assumed that the values of R
R
ment current produces a negligible change in V
16
OC
SENSE2
V
SENSE
OC
> R
=
OC(CRIT)
are so small that the I
Figure 7. Circuit to Increase the Overcurrent
Threshold for Small Switch Currents
R
pin is lost and switch current goes up propor-
SENSE1
LTC3805
.
R
SENSE1
I
SENSE
GATE
+ R
GND
OC
SENSE
SENSE2
R
SLOPE
SENSE
. This causes an overcurrent
OC
OC
100mV
I
OC
= 10μA threshold adjust-
I
SLOPE
smaller than R
. Using this circuit the
= 10μA
3805 F07
OC
= 0 and connect
R
R
SENSE2
SENSE1
SENSE1
OC
OC(CRIT)
.
and
.
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:
where the values of R1 and R2 should be kept below 10Ω
to prevent the I
from producing a shift in V
External Soft-Start Fault Timeout
The external soft-start is programmed by a capacitor C
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
capacitor C
After soft-start is complete, the voltage on the SSFLT
pin continues to charge to about a fi nal value of 4.75V.
Note that choosing a value of C
no effect since it would attempt to program an external
soft-start time t
V
t
SS(EXT)
OC
=
Figure 8. Circuit to Increase the Overcurrent
Threshold for Large Switch Currents
R
SS
1
= C
R
+ R
LTC3805
is therefore
1
SS(EXT)
OC
SS
I
2
SENSE
GATE
GND
= 10μA threshold adjustment current
OC
100mV
2.25 − 0.7V
SS(EXT)
less than the mandatory minimum
R
6µA
SLOPE
OC
as a function of the soft-start
.
I
OC
I
SLOPE
= 10μA
SS
less than 5.8nF has
R
R
3805 F08
2
1
R
SENSE
3805fb
SS