LT1375HV Linear Technology, LT1375HV Datasheet - Page 15

• Manufacturer Part Number: LT1375HV
• Description: 1.5A/ 500kHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
Threshold voltage for lockout is about 2.38V, slightly less
than the internal 2.42V reference voltage. 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 38
10
R
k
LO
to 100k 25k suggested
V R
INPUT
V
IN
U
LO
LO
C1
2 38
3 5
can be raised to 100k, but the error
INFORMATION
R
V
R
U
LO
HI
A
LO
10k or less. If shutdown
IN
SHDN
LT1375/LT1376
W
FB
can be added to
Figure 4. Undervoltage Lockout
3.5 A
U
2.38V
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 is therefore 1.5V and V
12V. Let R
SWITCH NODE CONSIDERATIONS
For maximum efficiency, switch rise and fall times are
made as short as possible. To prevent radiation and high
frequency resonance problems, proper layout of the com-
ponents connected to the switch node is essential. B field
IN
V = Hysteresis in input voltage level
R
R
R
R
= Input voltage at which switching stops as input
HI
FB
HI
FB
voltage descends to trip level
114 5 1 5
R
25 12 2 38 1 5 5 1 1 5
25 10 41
R
LO IN
LO
HI
STANDBY
k
TOTAL
SHUTDOWN
k
2 29
k
= 25k.
V
V
2 38 25 3 5
OUT
2 38
2 38
V
LO
SW
V
114
380
R
k
V V
2 3 5
k
k
OUT
LT1375/LT1376
+
A
A
1
1375/76 F04
OUTPUT
V
15
IN
=