Datasheet

LT3507
21
3507fb
For more information www.linear.com/LT3507
applications inForMation
FB Resistor Network
The output voltage of the LDO regulator is programmed
with a resistor divider (Refer to Block Diagram) between the
emitter of the external NPN pass resistor and the feedback
pin, FB4. Choose the resistors according to
R1=R2
V
OUT4
800mV
−1
⎛
⎝
⎜
⎞
⎠
⎟
The parallel combination of R1 and R2 should be 10k or
less to avoid bias current errors.
PROGRAMMABLE OVERVOLTAGE AND
UNDERVOLTAGE LOCKOUT
The LT3507 provides two input pins that allow user-
programmable overvoltage and undervoltage lockout. Both
the trip levels and hysteresis can be set by resistor values.
V
INSW
provides a switched V
IN1
to minimize power con-
sumption in shutdown. V
INSW
is connected to V
IN1
when
the LT3507 is operating, with a saturation voltage of about
0.3V. It is high impedance when the LT3507 is in shutdown
(all three RUN pins low).
The programmable lockout is a pair of comparators with
the trip level set at 1.2V. The OVLO comparator trips when
the OVLO pin exceeds 1.2V while the UVLO compara
-
tor trips when the UVLO pin drops below 1.2V. These
comparators shut down all four regulators until the input
voltage recovers.
The comparators also activate current sources that gener
-
ate hysteresis to eliminate chatter
. The UVLO comparator
activates a 10µA
current sink on the UVLO pin. The OVLO
comparator activates a 10µA current source on the OVLO
pin. These currents generate hysteresis voltage through
the resistance of the divider string.
Figure 11. Undervoltage and Overvoltage Lockout Circuit
–
+
–
+
1.2V
UVLO
UVLO
V
INSW
R3
R4
R1
R2
OVLO
10µA
10µA
OVLO
3507 F11
Figure 11 shows a typical connection. The threshold
voltages are:
V
OVTH
=0.3V+1.2V • 1+
R3
R4
⎛
⎝
⎜
⎞
⎠
⎟
V
UVTH
=0.3V+1.2V • 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
The hysteresis voltages are:
V
OVHYST
= 10µA • R3
V
UVHYST
= 10µA • R1
If the overvoltage lockout is not used, the OVLO pin must
be tied to ground. If the undervoltage lockout is not used,
the UVLO pin must be tied to V
INSW
.
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