Datasheet

22
LT1777
TYPICAL APPLICATIONS
U
Figure 14. Output Node Ripple
2µs/DIV
(b) After Output Filter
2µs/DIV
1777 F13a
(a) Before Output Filter
V
OUT
NODE
AC COUPLED
2mV/DIV
1777 F14b
User Programmable Undervoltage Lockout
Figure 15 uses a resistor divider between V
IN
and ground
to drive the SHDN node. This is a simple, cost-effective
way to add a user-programmable undervoltage lockout
(UVLO) function. Resistor R5 is chosen to have approxi-
mately 200µA through it at the nominal SHDN pin lockout
threshold of roughly 1.25V. The somewhat arbitrary value
of 200µA was chosen to be significantly above the SHDN
pin input current to minimize its error contribution, but
significantly below the typical 2.5mA the LT1777 draws in
lockout mode. Resistor R4 is then chosen to yield this
same 200µA, less 2.5µA, with the desired V
IN
UVLO volt-
age minus 1.25V across it. (The 2.5mA factor is an allow-
ance to minimize error due to SHDN pin input current.)
Behavior is as follows: Normal operation is observed at the
nominal input voltage of 24V. As the input voltage is
decreased to roughly 18V, switching action will stop, V
OUT
will drop to zero, and the LT1777 will draw its V
IN
and V
CC
quiescent currents from the V
IN
supply. At a lower input
voltage, typically 10V or so at 25°C, the voltage on the
SHDN pin will drop to the shutdown threshold, and the part
will draw its shutdown current only from the V
IN
rail. The
resistive divider of R4 and R5 will continue to draw power
from V
IN
. (The user should be aware that while the SHDN
pin lockout threshold is relatively accurate including
temperature effects, the SHDN pin shutdown threshold is
more coarse, and exhibits considerably more temperature
drift. Nevertheless the shutdown threshold will always be
well below the lockout threshold.)
Figure 15. User Programmable UVLO
R4
84.5k
1%
R5
6.19k
1%
C5
100pF
SHDN
LT1777
1777 F15
V
IN
V
OUT
NODE
AC COUPLED
2mV/DIV