9
LTC2900
2900f
Power-Up
The greater of V1, V2 is the internal supply voltage (V
CC
).
On power-up, V
CC
will power the drive circuits for the RST
pin. This ensures that the RST output will be low as soon
as V1 or V2 reaches 1V. The RST output will remain low until
the part is programmed. After programming, if any one of
the V
X
inputs is below its programmed threshold, RST will
be a logic low. Once all the V
X
inputs rise above their thresh-
olds, an internal timer is started and RST is released after
the programmed delay time. If V
CC
< (V3 – 1) and V
CC
< 2.4V,
the V3 input impedance will be low (1kΩ typ).
Monitor Programming
The LTC2900 input voltage combination is selected by
placing the recommended resistive divider from V
REF
to
GND and connecting the tap point to V
PG
, as shown in
Figure 4. Table 1 offers recommended 1% resistor values
for the various modes. The last column in Table 1 specifies
optimum V
PG
/V
REF
ratios (±0.01) to be used when pro-
gramming with a ratiometric DAC.
During power-up, once V1 or V2 reaches 2.4V max, the
monitor enters a programming period of approximately
150µs during which the voltage on the V
PG
pin is sampled
and the monitor is configured to the desired input combi-
nation. Do not add capacitance to the V
PG
pin. Immediately
after programming, the comparators are enabled and
supply monitoring will begin.
APPLICATIO S I FOR ATIO
WUUU
Figure 4. Monitor Programming
Table 1. Voltage Threshold Programming
V
PG
MODE V1 (V) V2 (V) V3 (V) V4 (V) R1 (kΩ) R2 (kΩ)V
REF
0 5.0 3.3 ADJ ADJ Open Short 0.000
1 5.0 3.3 ADJ –ADJ 93.1 9.53 0.094
2 3.3 2.5 ADJ ADJ 86.6 16.2 0.156
3 3.3 2.5 ADJ –ADJ 78.7 22.1 0.219
4 3.3 2.5 1.5 ADJ 71.5 28.0 0.281
5 5.0 3.3 2.5 ADJ 66.5 34.8 0.344
6 5.0 3.3 2.5 1.8 59.0 40.2 0.406
7 5.0 3.3 2.5 1.5 53.6 47.5 0.469
8 5.0 3.0 2.5 ADJ 47.5 53.6 0.531
9 5.0 3.0 ADJ ADJ 40.2 59.0 0.594
10 3.3 2.5 1.8 1.5 34.8 66.5 0.656
11 3.3 2.5 1.8 ADJ 28.0 71.5 0.719
12 3.3 2.5 1.8 –ADJ 22.1 78.7 0.781
13 5.0 3.3 1.8 –ADJ 16.2 86.6 0.844
14 5.0 3.3 1.8 ADJ 9.53 93.1 0.906
15 5.0 3.0 1.8 ADJ Short Open 1.000
8
7
6
R1
1%
R2
1%
2900 F04
V
REF
V
PG
GND
LTC2900
Supply Monitoring
The LTC2900 is a low power, high accuracy program-
mable quad supply monitoring circuit with a common
reset output and a manual reset input. Reset timing is
adjustable using an external capacitor. Single pin pro-
gramming selects 1 of 16 input voltage monitor combina-
tions. All four voltage inputs must be above predetermined
thresholds for the reset not to be invoked. The LTC2900
will assert the reset output during power-up, power-down
and brownout conditions on any one of the voltage inputs.
The inverting inputs on the V3 and/or V4 comparators are
set to 0.5V when the positive adjustable modes are se-
lected (Figure 5). The tap point on an external resistive
divider, connected between the positive voltage being
