LTC2938/LTC2939
10
293839ff
Supply Monitoring
The LTC2938 and LTC2939 are low power, high accuracy
confi gurable four (LTC2938) and six (LTC2939) supply
monitoring circuits with reset output and watchdog func-
tions. Both watchdog and reset timeouts are adjustable
using external capacitors. Single-pin confi guration selects
one of sixteen input voltage monitor combinations. All four
(LTC2938) or six (LTC2939) voltage inputs must be above
predetermined thresholds for the reset not to be invoked.
The LTC2938/LTC2939 assert the reset during power-up,
power-down and brownout conditions on any one of the
voltage inputs.
Power-Up
The greater of V1 or V2 serves as the internal supply
voltage (V
CC
). On power-up, V
CC
powers the drive circuits
for the RST output. This ensures that the RST output will
be low as soon as V1 or V2 reaches 1V. The RST output
remains low until the part is confi gured. After confi gura-
tion, if any one of the supply monitor inputs is below its
confi gured threshold, RST will be at logic low. Once all
the monitor inputs rise above their thresholds, an internal
timer is started and RST is released after the confi gured
delay time.
APPLICATIONS INFORMATION
Figure 1. 1.5% Threshold Accuracy Improves System Reliability
Threshold Accuracy
Consider a 5V system with ±5% tolerance. The 5V supply
may vary between 4.75V to 5.25V. System ICs powered by
this supply must operate reliably within this band (and a
little more as subsequently explained). A perfectly accurate
supervisor for this supply generates a reset at exactly
4.75V. However, no supervisor is perfect. The actual reset
threshold of a supervisor varies over a specifi ed band.
The LTC2938/LTC2939 varies ±1.5% around its nominal
threshold voltage (see Figure 1) over temperature.
The reset threshold band and the power supply tolerance
bands should not overlap. This prevents false or nuisance
resets when the power supply is actually within its speci-
fi ed tolerance band.
The LTC2938 and LTC2939 have ±1.5% reset threshold
accuracy, so a 5% threshold is typically set to 6.5% below
the nominal input voltage. Therefore, a typical 5V, 5%
threshold is 4.675V.
The threshold is guaranteed to lie in the band between
4.750V and 4.600V over temperature. The powered system
must work reliably down to the low end of the threshold
band, or risk malfunction before a reset signal is properly
issued.
5V
4.75V
4.675V
±1.5%
THRESHOLD
BAND
4.6V
NOMINAL
SUPPLY
VOLTAGE
SUPPLY TOLERANCE
MINIMUM
RELIABLE
SYSTEM
VOLTAGE
IDEAL
SUPERVISOR
THRESHOLD
REGION OF POTENTIAL MALFUNCTION
–5%
–6.5%
–8%
293839 F01
