Datasheet

LT3724
15
3724fd
APPLICATIONS INFORMATION
The V
FB
pin input bias current is typically 25nA, so use
of extremely high value feedback resistors could cause a
converter output that is slightly higher than expected. Bias
current error at the output can be estimated as:
∆V
OUT(BIAS)
= 25nA • R2
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 LT3724 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 RB. Then:
RA = RB •
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 RB = 49.9kΩ, V
SUPPLY(ON)
= 14.5V (based
on a 15V minimum input voltage)
RA = 49.9kΩ •
14.5V
1.35V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
= 486.1kΩ (499kΩ resistor is selected)
If low supply current in standby mode is required, select
a higher value of RB.
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 LT3724 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 soft-start function forces the programmed slew rate
while the converter output rises to 95% of regulation,
which corresponds to 1.185V on the V
FB
pin. Once 95%
regulation is achieved, the soft-start circuit is disabled.
The soft-start circuit will re-enable when the V
FB
pin drops
below 70% of regulation, which corresponds to 300mV
of control hysteresis on the V
FB
pin. This allows for a
controlled recovery from a “brown-out” condition.
Figure 2. Output Voltage Feedback Divider Figure 3. Undervoltage Lockout Circuit
L1
V
FB
PIN
R2
R1
V
OUT
C
OUT
3724 F02
SHDN PIN
RA
RB
V
SUPPLY
3724 F03
Figure 4.Soft-Start Circuit
R
SS
LT3724
V
OUT
C
SS1
C
SS
3724 F04
A