LTC2903 LINER [Linear Technology], LTC2903 Datasheet - Page 11

• Manufacturer Part Number: LTC2903
• Description: Low Voltage Supervisorwith 27 Selectable Thresholds and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

The reference input of the comparator is controlled by the
tolerance pin. The external resistive divider should make
the voltage at VM = 0.5V when the supply is at nominal
value. The actual threshold of VM accounts for the sup-
ply tolerance of ±1.5% guaranteed over the full operating
temperature range. The resulting tolerances are –6.5%,
–11.5%, –16.5% which correspond to 0.468V, 0.443V,
0.418V respectively.
Typically, the user will pick a value of R1 based on accept-
able current draw. Current used by the resistor divider will
be approximately:
Recommended range of R1 is 1k—1M. Higher values of
resistance exacerbate the degradation of threshold ac-
curacy due to leakage currents.
If the nominal value of the supply being monitored is
V
Resistor tolerances must be taken into account when
determining the overall accuracy.
Selecting the Reset Timing Capacitor
The reset timeout period can be set to one of two fi xed
internal timers or set with a capacitor in order to accom-
modate a variety of applications. Connecting a capacitor,
C
period, t
of capacitor needed for a particular timeout:
For example, using a standard capacitor value of 2.2nF
would give a 20ms timeout.
Figure 2 shows the desired reset timeout period as a
function of the value of the timer capacitor.
Leaving RT open with no external capacitor generates a
reset timeout of approximately 400μs. Shorting RT to V
generates a reset timeout of approximately 200ms.
APPLICATIONS INFORMATION
NOM
RT
R2 = R1(2V
C
R
, between the RT pin and ground sets the reset timeout
RT
1
, then
=
= t
⎛
⎝ ⎜
RST
0 5
RST
.
. The following formula approximates the value
I
V
• 110 [pF/ms]
⎞
⎠ ⎟
NOM
– 1)
CC
⎯ R ⎯ S ⎯ T Output Characteristics
The DC characteristics of the ⎯ R ⎯ S ⎯ T pull-down strength
are shown in the Typical Performance Characteristics
section. ⎯ R ⎯ S ⎯ T is an open-drain pin and thus requires an
external pull-up resistor to the logic supply. ⎯ R ⎯ S ⎯ T may be
pulled above V
are observed.
The open-drain of the ⎯ R ⎯ S ⎯ T pin allows for wired-OR con-
nection of several LTC2917/LTC2918’s.
Watchdog
LTC2917-A/LTC2918-A
A standard watchdog function is used to ensure that the
system is in a valid state by continuously monitoring
the microprocessor’s activity. The microprocessor must
toggle the logic state of the WDI pin periodically (within
upper boundary) in order to clear the watchdog timer. If
timeout occurs, the LTC2917-A/LTC2918-A asserts ⎯ R ⎯ S ⎯ T
low for the reset timeout period, issuing a system reset.
Once the reset timeout completes, ⎯ R ⎯ S ⎯ T is released to go
high and the watchdog timer starts again.
During power-up, the watchdog timer remains cleared while
⎯ R ⎯ S ⎯ T is asserted low. As soon as the reset timer times out,
⎯ R ⎯ S ⎯ T goes high and the watchdog timer is started.
Figure 2. Reset Timeout Period vs RT Capacitance
10000
1000
100
0.1
10
0.001
1
CC
, providing the voltage limits of the pin
0.01
LTC2917/LTC2918
RT PIN CAPACITANCE, C
0.1
1
10
RT
(nF)
100
29178 F02
1000
11
29178f