LTC1235 Datasheet, PDF (8/16 Pages) Linear Technology – Microprocessor Supervisory Circuit

LTC1235
APPLICATI S I FOR ATIO
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 ac-
counts 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 ac-
tive 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.
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
connection 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 satisfied, 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).
V2
V_CC
RESET
t1
LOW LINE
V_CC = 5V
PB RST
RESET
RESET
V2
V1
V1 = RESET VOLTAGE THRESHOLD
V1
V2 = RESET VOLTAGE THRESHOLD +
RESET THRESHOLD HYSTERESIS
t1
t1 = RESET ACTIVE TIME
Figure 1. Reset Active Time
LTC1235 F01
t1
LOGIC LOW
LOGIC
HIGH
t2
LOGIC HIGH
LOGIC LOW
t1 = PUSH-BUTTON RESET LOW TIME
t2 = RESET ACTIVE TIME
Figure 2. Push-Button Reset
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