LT1777 Linear Technology, LT1777 Datasheet - Page 22

• Manufacturer Part Number: LT1777
• Description: Low Noise Step-Down Switching Regulator
• Manufacturer: Linear Technology

User Programmable Undervoltage Lockout
Figure 15 uses a resistor divider between V
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 200 A through it at the nominal SHDN pin lockout
threshold of roughly 1.25V. The somewhat arbitrary value
of 200 A 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 200 A, less 2.5 A, with the desired V
age minus 1.25V across it. (The 2.5mA factor is an allow-
ance to minimize error due to SHDN pin input current.)
LT1777
TYPICAL APPLICATIONS
22
AC COUPLED
V
OUT
2mV/DIV
NODE
(a) Before Output Filter
2 s/DIV
U
1777 F13a
Figure 15. User Programmable UVLO
IN
V
IN
IN
and ground
Figure 14. Output Node Ripple
UVLO volt-
R4
84.5k
1%
R5
6.19k
1%
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
will drop to zero, and the LT1777 will draw its V
quiescent currents from the V
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
resistive divider of R4 and R5 will continue to draw power
from V
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.)
C5
100pF
AC COUPLED
V
OUT
2mV/DIV
SHDN
LT1777
IN
NODE
. (The user should be aware that while the SHDN
1777 F15
(b) After Output Filter
2 s/DIV
IN
supply. At a lower input
1777 F14b
IN
IN
rail. The
and V
OUT
CC