Datasheet

8
LT3436
3436fa
APPLICATIONS INFORMATION
WUU
U
shutdown pin can be used. The threshold voltage of the
shutdown pin comparator is 1.35V. A 3µA internal current
source defaults the open pin condition to be operating (see
Typical Performance Graphs). Current hysteresis is added
above the SHDN threshold. This can be used to set voltage
hysteresis of the UVLO using the following:
R
VV
A
R
V
VV
R
A
HL
H
1
7
2
135
135
1
3
=
−
µ
=
−
()
+µ
.
.
V
H
– Turn-on threshold
V
L
– Turn-off threshold
Example: switching should not start until the input is
above 4.75V and is to stop if the input falls below 3.75V.
V
H
= 4.75V
V
L
= 3.75V
R
VV
A
k
R
V
VV
k
A
k
1
475 375
7
143
2
135
475 135
143
3
50 4
=
−
µ
=
=
−
()
+µ
=
..
.
..
.
Keep the connections from the resistors to the SHDN pin
short and make sure that the interplane or surface capaci-
tance to the switching nodes are minimized. If high resis-
tor values are used, the SHDN pin should be bypassed with
a 1nF capacitor to prevent coupling problems from the
switch node.
CATCH DIODE
The suggested catch diode (D1) is a B220A Schottky. It is
rated at 2A average forward current and 20V reverse
voltage. Typical forward voltage is 0.5V at 2A. The diode
conducts current only during switch off time. Peak reverse
voltage is equal to regulator output voltage. Average
forward current in normal operation is equal to output
current.
SHUTDOWN AND UNDERVOLTAGE LOCKOUT
Figure 4 shows how to add undervoltage lockout (UVLO)
to the LT3436. Typically, UVLO is used in situations where
the input supply is current limited , or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
Figure 4. Undervoltage Lockout
1.35V
GND
INPUT
R1
3436 F04
SHDN
V
CC
IN
LT3436
3µA
R2
C1
7µA
An internal comparator will force the part into shutdown
below the minimum V
IN
of 2.6V. This feature can be used
to prevent excessive discharge of battery-operated sys-
tems. If an adjustable UVLO threshold is required, the