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LTC1235CN-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC1235CN-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC1235 8 1235fa APPLICATIONS INFORMATION 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 accounts 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 active 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 10μs. Figure 1. Reset Active Time Figure 2. Push-Button Reset To help prevent mistriggering due to transient loads, VCC 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 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 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 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). VCC t1 t1 = RESET ACTIVE TIME V1 = RESET VOLTAGE THRESHOLD V2 = RESET VOLTAGE THRESHOLD + RESET THRESHOLD HYSTERESIS t1 V2 V2 V1 V1 1235 F01 RESET LOW LINE LOGIC LOW t1 LOGIC HIGH t2 LOGIC HIGH LOGIC LOW VCC = 5V RESET RESET PB RST t1 = PUSH-BUTTON RESET LOW TIME t2 = RESET ACTIVE TIME 1235 F02 |
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