Datasheet

LTC690/LTC691
LTC694/LTC695
15
690ff
For more information www.linear.com/690
applicaTions inForMaTion
Table 2. LTC691 and LTC695 Reset Active Time and Watchdog Timeout Selections
OSC SEL OSC IN
WATCHDOG TIME-OUT PERIOD RESET ACTIVE TIME
NORMAL
(Short Period)
IMMEDIATELY
AFTER RESET
(Long Period) LTC691 LTC695
Low External Clock Input 1024 clks 4096 clks 512 clks 2048 clks
Low External Capacitor*
400ms
70pF
• C
1.6sec
70pF
• C
200ms
70pF
• C
800ms
70pF
• C
Floating or High
Floating or High
Low
Floating or High
100ms
1.6 sec
1.6 sec
1.6 sec
50ms
50ms
200ms
200ms
*The nominal internal frequency is 10.24kHz. The nominal oscillator frequency with external capacitor is f
OSC
(Hz) =
184,000
C(pF) • 1025
Pushbutton Reset
The LTC690 family does not provide a logic input for direct
connection to a pushbutton. However, a pushbutton in
series with a 100Ω resistor connected to the RESET output
pin (Figure 13) provides an alternative for manual reset.
Connecting a 0.1µF capacitor to the RESET pin debounces
the pushbutton input.
The 100Ω resistor in series with the pushbutton is required
to prevent the ringing, due to the capacitance and lead
inductance, from pulling the RESET pins of the MPU and
LTC69X below ground.
If a dedicated pushbutton reset input is desired, the
LTC1235 is a good choice (Figure 14). It has all the func-
tions of the LTC695 and provides pushbutton reset as an
extra feature. Its pushbutton is internally debounced and
invokes the normal 200ms reset sequence. This eliminates
the need for the 100Ω resistor and 0.1µF capacitor. It also
provides a more consistent reset pulse.
5V
100Ω
V
CC
LTC690/LTC691
LTC694/LTC695
GND
690 F13
RESET
0.1µF
MPU
(e.g. 6805)
RESET
5V
V
CC
LTC1235
GND
690 F14
RESET
MPU
(e.g. 6805)
RESET
PBRST
Figure 13. The External Pushbutton Reset
Figure 14. The External Pushbutton Reset with the LTC1235