15
LT1576/LT1576-5
APPLICATIONS INFORMATION
WUU
U
Threshold voltage for lockout is about 2.44V. A 3.5µA bias
current flows
out
of the pin at threshold. This internally
generated current is used to force a default high state on
the shutdown pin if the pin is left open. When low shut-
down current is not an issue, the error due to this current
can be minimized by making R
LO
10k or less. If shutdown
current is an issue, R
LO
can be raised to 100k, but the error
due to initial bias current and changes with temperature
should be considered.
Rk
R
RV V
VR A
LO
HI
LO IN
LO
=
()
=
−
()
−
()
10
244
244 35
to 100k 25k suggested
.
..µ
V
IN
= Minimum input voltage
Keep the connections from the resistors to the shutdown
pin short and make sure that interplane or surface capaci-
tance to the switching nodes are minimized. If high resis-
tor values are used, the shutdown pin should be bypassed
with a 1000pF capacitor to prevent coupling problems
from the switch node. If hysteresis is desired in the
undervoltage lockout point, a resistor R
FB
can be added to
the output node. Resistor values can be calculated from:
–
+
–
+
2.44V
0.4V
GND
V
SW
LT1576
INPUT
R
FB
R
HI
1576 F04
OUTPUT
SHDN
STANDBY
IN
TOTAL
SHUTDOWN
3.5µA
R
LO
C1
+
Figure 4. Undervoltage Lockout
R
RV VV V
R
A
RRV V
HI
LO IN OUT
LO
FB HI OUT
=
−+
()
+
[]
−
()
=
()( )
2
44
1
244 35
./
..
/
∆∆
∆
µ
25k suggested for R
LO
V
IN
=
Input voltage at which switching stops as input
voltage descends to trip level
∆V = Hysteresis in input voltage level
Example: output voltage is 5V, switching is to stop if input
voltage drops below 12V and should not restart unless
input rises back to 13.5V. ∆V is therefore 1.5V and
V
IN
= 12V. Let R
LO
= 25k.
R
k
kA
k
k
Rk k
HI
FB
=
−+
()
+
[]
−
()
=
()
=
=
()
=
25 12 2
44
15 5 1 15
244 25 35
25 10 33
235
110
110 5 1 5 366
../ .
..
.
.
/.
µ