Datasheet

LTC1235
8
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APPLICATIONS INFORMATION
Power Monitoring
The LTC1235 uses a bandgap voltage reference and a
precision voltage comparator C1 to monitor the 5V supply
input on V
CC
(see Block Diagram). When V
CC
falls below
the reset voltage threshold, the reset outputs are forced
to active states. The reset voltage threshold accounts for
a 5% variation on V
CC
, so the reset outputs become active
when V
CC
falls below 4.75V (4.65V typical). On power-up,
the reset signals are held active states for a minimum
of 140ms after the reset voltage threshold is reached to
allow the power supply and microprocessor to stabilize.
On power-down, the RESET signal remains active low
even with V
CC
as low as 1V. This capability helps hold
the microprocessor in stable shutdown condition. Figure
1 shows the timing diagram of the RESET signal.
The precision voltage comparator, C1, typically has 40mV
of hysteresis which ensures that glitches at V
CC
pin do not
activate the reset outputs. Response time is typically 10μs.
Figure 1. Reset Active Time
Figure 2. Push-Button Reset
To help prevent mistriggering due to transient loads, V
CC
pin should be bypassed with a 0.1μF capacitor with the
leads trimmed as short as possible.
LOW LINE is the output of the precision voltage compara-
tor C1. When V
CC
falls below the reset voltage threshold,
LOW LINE goes low. LOW LINE returns high as soon as
V
CC
rises above the reset voltage threshold.
Push-Button Reset
The LTC1235 provides an logic input pin for direct con-
nection to a push-button. The push-button reset input,
PB RST, requires an active low signal. Internally, this input
signal is debounced and timed for a minimum of 40ms.
When this condition is satisfi ed, the reset pulse generator
forces the reset outputs to active states. The reset signals
will remain in active states for a minimum of 140ms from
the moment the push-button reset input is released from
logic low level (Figure 2).
V
CC
t1
t1 = RESET ACTIVE TIME
V1 = RESET VOLTAGE THRESHOLD
V2 = RESET VOLTAGE THRESHOLD +
RESET THRESHOLD HYSTERESIS
t1
V2
V2
V1
V1
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RESET
LOW LINE
LOGIC LOW
t1
LOGIC
HIGH
t2
LOGIC HIGH
LOGIC LOW
V
CC
= 5V
RESET
RESET
PB RST
t1 = PUSH-BUTTON RESET LOW TIME
t2 = RESET ACTIVE TIME
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