LTC1235CS datasheet(8/16 Pages) LINER

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LTC1235

APPLICATI

I FOR ATIO

To help prevent mistriggering due to transient loads, VCC

pin should be bypassed with a 0.1

µF capacitor with the

leads trimmed as short as possible.

LOW LINE is the output of the precision voltage compara-

tor C1. When VCC falls below the reset voltage threshold,

LOW LINE goes low. LOW LINE returns high as soon as

VCC 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).

Power Monitoring

The LTC1235 uses a bandgap voltage reference and a

precision voltage comparator C1 to monitor the 5V supply

input on VCC (see BLOCK DIAGRAM). When VCC 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 VCC, so the reset outputs

become active when VCC 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 VCC 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 VCC pin do not

activate the reset outputs. Response time is typically 10

µs.

Figure 1. Reset Active Time

Figure 2. Push-Button Reset

VCC

t1 = RESET ACTIVE TIME

V1 = RESET VOLTAGE THRESHOLD

V2 = RESET VOLTAGE THRESHOLD +

RESET THRESHOLD HYSTERESIS

LTC1235 F01

RESET

LOW LINE

LOGIC LOW

LOGIC

HIGH

LOGIC HIGH

LOGIC LOW

VCC = 5V

RESET

PB RST

t1 = PUSH-BUTTON RESET LOW TIME

t2 = RESET ACTIVE TIME

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