LT1576CS8-SYNC Просмотр технического описания (PDF) - Linear Technology
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LT1576CS8-SYNC Datasheet PDF : 28 Pages
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APPLICATIONS INFORMATION
RFB
INPUT
RHI
C1
RLO
LT1576
IN
SHDN
2.44V
–
3.5µA
+
+
0.4V
–
GND
LT1576/LT1576-5
VSW
STANDBY
TOTAL
SHUTDOWN
OUTPUT
+
1576 F04
Figure 4. Undervoltage Lockout
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 RLO 10k or less. If shutdown
current is an issue, RLO can be raised to 100k, but the error
due to initial bias current and changes with temperature
should be considered.
( ) RLO = 10k to 100k 25k suggested
( ( ) ) RHI
=
RLO VIN − 2.44V
2.44V − RLO 3.5µA
VIN = 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 RFB can be added to
the output node. Resistor values can be calculated from:
[ ( ) ] RLO VIN − 2.44 ∆V/ VOUT + 1 + ∆V
RHI =
( ) 2.44 −RLO 3.5µA
( )( ) RFB = RHI VOUT /∆V
25k suggested for RLO
VIN = 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
VIN = 12V. Let RLO = 25k.
[ ( ) ] 25k 12 − 2.44 1.5/5 + 1 + 1.5
RHI =
( ) 2.44 − 25k 3.5µA
( ) 25k 10.33
=
=110k
2.35
( ) RFB = 110k 5/1.5 = 366k
15
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