Datasheet

LT3845A
18
3845afa
Supply UVLO and Shutdown
The SHDN pin has a precision voltage threshold with
hysteresis which can be used as an undervoltage lockout
threshold (UVLO) for the power supply. Undervoltage
lockout keeps the LT3845A in shutdown until the supply
input voltage is above a certain voltage programmed by
the user. The hysteresis voltage prevents noise from falsely
tripping UVLO.
Resistors are chosen by first selecting R
B
. Then
R
A
= R
B
•
V
SUPPLY(ON)
1.35V
– 1
V
SUPPLY(ON)
is the input voltage at which the undervoltage
lockout is disabled and the supply turns on.
Example: Select R
B
= 49.9k, V
SUPPLY(ON)
= 14.5V (based
on a 15V minimum input voltage)
R
A
= 49.9k •
14.5V
1.35V
– 1






= 486.1k (499k resistor is selected)
If low supply current in standby mode is required, select
a higher value of R
B
.
The supply turn off voltage is 9% below turn on. In the
example the V
SUPPLY(OFF)
would be 13.2V.
If additional hysteresis is desired for the enable function,
an external positive feedback resistor can be used from
the LT3845A regulator output.
The shutdown function can be disabled by connecting the
SHDN pin to the V
IN
through a large value pull-up resistor.
This pin contains a low impedance clamp at 6V, so the SHDN
pin will sink current from the pull-up resistor(R
PU
):
I
SHDN
=
V
IN
– 6V
R
PU
Because this arrangement will clamp the SHDN pin to the
6V, it will violate the 5V absolute maximum voltage rating of
the pin. This is permitted, however, as long as the absolute
maximum input current rating of 1mA is not exceeded.
Input SHDN pin currents of <100µA are recommended: a
1MΩ or greater pull-up resistor is typically used for this
configuration.
Soft-Start
The desired soft-start time (t
SS
) is programmed via the
C
SS
capacitor as follows:
C
SS
=
2µA • t
SS
1.231V
The amount of time in which the power supply can withstand
a V
IN
, V
CC
or V
SHDN
UVLO fault condition (t
FAULT
) before
the C
SS
pin voltage enters its active region is approximated
by the following formula:
t
FAULT
=
C
SS
• 0.65V
50µA
APPLICATIONS INFORMATION
L1
V
FB
PIN
R2
R1
V
OUT
C
OUT
3845A F03
SHDN PIN
R
A
R
B
V
SUPPLY
3845A F04
Figure 3. Output Voltage Feedback Divider
Figure 4. Undervoltage Feedback Divider