LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 16

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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LTC3805-5
APPLICATIONS INFORMATION
the I
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
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
For larger currents, values of the current sense resistors
must be very small and the circuit of Figure 6 becomes im-
practical. The circuit of Figure 7 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,
16
SENSE2
V
V
SENSE
OC
OC
=
=
Figure 6. Circuit to Increase the Overcurrent
Threshold for Small Switch Currents
are so small that the I
R
R
pin is lost and switch current goes up propor-
1
SENSE
R
LTC3805-5
+
1
R
OC
R
I
2
SENSE
GATE
GND
SENSE
1
100
OC
= 10μA threshold adjustment current
+
R
mV
R
SENSE
SLOPE
1
OC
SENSE
2
I
OC
.
OC
I
100
SLOPE
= 10μA
= 10μA threshold adjust-
. Using this circuit the
38055 F06
mV
R
R
SENSE2
SENSE1
SENSE1
OC
.
and
SS
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
since it would attempt to program an external soft-start
time t
soft-start time t
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
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
the converter successfully restarts.
t
t
SS EXT
FTO EXT
SS(EXT)
(
(
Figure 7. Circuit to Increase the Overcurrent
Threshold for Large Switch Currents
)
LTC3805-5
SS
)
=
=
less than the mandatory minimum internal
is therefore
I
C
SENSE
GATE
C
GND
SS(IN)
SS
OC
SS
2 25 0 7
SS(EXT)
4 75
SS
R
.
= 1.8ms.
SLOPE
.
. If the fault is eventually removed
6μ
−
SS
2μ
V
A
I
as a function of the soft-start
OC
−
less than 5.8nF has no effect
.
A
I
SLOPE
= 10μA
0 7
FTO(EXT)
V
.
V
R2
R1
38055 F07
is therefore
R
SENSE
38055fa