LT1576 Linear Technology, LT1576 Datasheet - Page 15

• Manufacturer Part Number: LT1576
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
Threshold voltage for lockout is about 2.44V. A 3.5 A 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
current is an issue, R
due to initial bias current and changes with temperature
should be considered.
V
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
the output node. Resistor values can be calculated from:
IN
R
R
= Minimum input voltage
LO
HI
2 44
10
R
k
LO IN
INPUT
to 100k 25k suggested
V R
V
U
C1
LO
LO
2 44
can be raised to 100k, but the error
3 5
INFORMATION
R
R
U
LO
HI
V
A
LO
10k or less. If shutdown
IN
SHDN
W
LT1576
FB
can be added to
Figure 4. Undervoltage Lockout
3.5 A
U
2.44V
0.4V
GND
R
FB
–
+
+
–
25k suggested for R
V
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
IN
IN
V = Hysteresis in input voltage level
R
R
R
R
= Input voltage at which switching stops as input
= 12V. Let R
HI
FB
HI
FB
voltage descends to trip level
R
R
25 12 2 44 1 5 5 1 1 5
25 10 33
110 5 1 5
LO IN
STANDBY
HI
TOTAL
SHUTDOWN
k
k
2 35
V
k
V
OUT
LO
2 44 25 3 5
2 44
2 44
= 25k.
V
SW
LO
V
110
R
366
LO
k
V V
LT1576/LT1576-5
k
3 5
k
OUT
V is therefore 1.5V and
+
A
A
1
OUTPUT
1576 F04
V
15