lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 15

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The V
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:
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
V
lockout is disabled and the supply turns on.
Example: Select RB = 49.9kΩ, V
(based on a 15V minimum input voltage)
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(ON)
ΔV
RA RB
RA
FB
OUT(BIAS)
=
=
= 486.1kΩ (499kΩ resistor is selected)
pin input bias current is typically 25nA, so use of
49 9
Figure 2. Output Voltage Feedback Divider
V
.
FB
•
is the input voltage at which the undervoltage
k
PIN
⎛
⎜
⎝
SUPPLY(OFF)
Ω
= 25nA • R2
V
SUPPLY ON
•
1 35
⎛
⎜
⎝
U
.
14 5
1 35
L1
.
.
V
(
V
V
U
would be 13.2V.
)
–
R2
R1
–
1
⎞
⎟
⎠
1
⎞
⎟
⎠
W
C
SUPPLY(ON)
3724 F02
OUT
V
OUT
U
= 14.5V
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
This pin contains a low impedance clamp at 6V, so the
SHDN pin will sink current from the pull-up resistor(R
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 recom-
mended: 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
regulation is achieved, the soft-start circuit is disabled.
The soft-start circuit will re-enable when the V
below 70% of regulation, which corresponds to 300mV of
control hysteresis on the V
trolled recovery from a “brown-out” condition.
I
SHDN
=
V
Figure 3. Undervoltage Lockout Circuit
IN
V
R
OUT
– 6
PU
SHDN PIN
V
Figure 4.Soft-Start Circuit
SUPPLY
IN
V
through a large value pull-up resistor.
C
SS1
A
FB
R
SS
pin. This allows for a con-
RA
RB
3724 F03
C
LT3724
SS
FB
3724 F04
pin. Once 95%
LT3724
FB
pin drops
15
3724fb
PU
):