Datasheet

LTC6995-1/LTC6995-2
15
699512fa
For more information www.linear.com/LTC6995-1
applicaTions inForMaTion
Power-On Reset (POR) Function
When power is applied to the LTC6995 the output is held
low for t
START
, then takes on the value of the POL bit as the
clock cycle begins. If POL = 0 (DIVCODE < 8) the output
will remain low for a programmable interval of t
START
+
1/2 t
OUT
, assuming the RST pin is inactive. This makes the
LTC6995 useful as a programmable long-time power-on
reset (POR), with the low output used to hold a system
in reset for a fixed period after power is applied. Timing
begins when the V
+
supply exceeds approximately 1.4V.
To prevent additional output transitions after the initial POR
time, the oscillator can be disabled by removing the SET
pin current. This prevents the internal master oscillator
output from clocking the frequency dividers or output,
while keeping it biased so it can resume operation quickly.
The easiest way to implement this feature is to connect
R
SET
between the SET and OUT pins.
Figure 12 shows the basic power-on reset function. When
the half cycle times out, the output goes high, eliminates
the SET pin current, and stops additional OUT pin transi
-
tions. The output remains high until the device is reset by
driving
the RST input or power is cycled off then back on.
The POR interval is only one half of an oscillator period so
component selection is slightly different. Table 2 provides
the component values required for one half cycle time
intervals. Timing starts after a short startup delay time
following the application of the V
+
supply.
LTC6995-1
t
POR
= 1 SECOND FOR VALUES SHOWN
POL = 0
DIVCODE = 3
NDIV = 512
RST = V
+
FOR LTC6995-2
RST
GND
SET
OUT
V
+
DIV
R
SET
191k
R2
280k
R1
1M
0.1µF
2.25V TO 5.5V
POR
OUT
POL = 0
699512 F12
t
DELAY
(1/2 t
OUT
)
t
START
TIMER STOPPED
~1.4V STARTS TIMER
POWER-ON RESET
V
+
Figure 12. Active Low Power-On Reset
(1 Second Interval Example)
Table 2. Power-On Reset (POR). One Shot, One Half Cycle Delay Programming
Output Low During Time Interval, POL = 0
DIVCODE t
DELAY
TIME INTERVAL (1/2 t
OUT
) R1 (kΩ) R2 (kΩ) ~R
SET
(kΩ)
0 512µs to 8.2ms Open Short t
DELAY(MS)
• 97.6
1 4.1ms to 65.5ms 976 102 t
DELAY(MS)
• 12.2
2 32.8ms to 524.3ms 976 182 t
DELAY(MS)
• 1.5
3 262.1ms to 4.2sec 1000 280 t
DELAY(SEC)
• 190.7
4 2.1sec to 33.6sec 1000 392 t
DELAY(SEC)
• 23.8
5 16.8sec to 4.5min 1000 523 t
DELAY(MIN)
• 178.6
6 2.2min to 35.8min 1000 681 t
DELAY(MIN)
• 22.7
7 17.9min to 4.8hrs 1000 887 t
DELAY(HR)
• 167.6
Note: Power-On Reset Time = t
DELAY
+ t
START