ltc2903-1 Linear Technology Corporation, ltc2903-1 Datasheet - Page 12

• Manufacturer Part Number: ltc2903-1
• Description: Precision Quad Supply Monitor In 6-lead Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC2903-1
In any supervisory application, supply noise riding on the
monitored DC voltage can cause spurious resets, particu-
larly when the monitored voltage approaches the reset
threshold. A less than desirable but commonly used
technique used to mitigate this problem adds hysteresis to
the input comparator. The amount of added hysteresis,
usually specified as a percentage of the trip threshold,
effectively degrades the advertised accuracy of the part.
To maintain high accuracy, the LTC2903-1 does not use
hysteresis.
To minimize spurious resets while maintaining threshold
accuracy, the LTC2903-1 employs two forms of noise
filtering. The first line of defense incorporates proprietary
tailoring of the comparator transient response. Transient
events receive electronic integration in the comparator
and must exceed a certain magnitude and duration to
cause the comparator to switch. Figure 5 illustrates the
typical transient duration versus comparator overdrive
(as a percentage of the trip threshold V
the comparators. Once any comparator is switched, the
reset line pulls low. The reset time-out counter starts once
all inputs return above threshold. The nominal reset delay
time is 200ms. The counter clears whenever any input
12
Figure 5. Typical Transient Duration vs Overdrive
Required to Trip Comparator
400
350
300
250
200
150
100
50
0
0.1
RESET COMPARATOR OVERDRIVE (% OF V
T
A
= 25 C
U
1
U
RESET OCCURS
ABOVE CURVE
10
W
RT
29031 F05
) required to trip
RTX
100
)
U
drops back below threshold. This reset delay time effec-
tively provides further filtering of the voltage inputs. A
noisy input with frequency components of sufficient mag-
nitude above f = 1/t
preventing oscillatory behavior on the reset line.
Although all four comparators have built-in glitch filtering,
use bypass capacitors on the V1 and V2 inputs because
the greater of V1 or V2 supplies the V
0.1 F ceramic capacitor satisfies most applications). Ap-
ply filter capacitors on the V3 and V4 inputs in extremely
noisy situations.
Reset Output Rise and Fall Time Estimation
The reset output line contains a weak pull-up current
source to the V2 supply. Use an external pull-up resistor
when the output needs to pull to another voltage and/or
when the reset output needs a faster rise time. The open-
drain output allows for wired-OR connections when more
than one signal needs to pull down on the reset line.
Estimate output rise time for the open-drain output with-
out an external pull-up using:
where R
and C
room temperature, the average R
50k . When externally pulling up to voltages higher than
V2, an internal network automatically protects the weak
pull-up circuitry from reverse currents.
The reset output has very strong pull-down capability.
Estimate the output fall time using:
where R
and C
room temperature, the average R
With a 150pF load capacitance the reset line can pull down
in about 13ns.
t
t
RISE
FALL
LOAD
LOAD
PD
PU
2.2 • R
2.2 • R
is the on-resistance of the pull-down transistor
is the external load capacitance on the pin. At
is the external load capacitance on the pin. At
is the on-resistance of the pull-up transistor
PU
PD
• C
• C
RST
LOAD
LOAD
= 5Hz holds the reset line low,
PD
is approximately 40 .
PU
CC
is approximately
for the part (a
29031f