DESCRIPTION GÉNÉRALE

The ADM705/ADM706/ADM707/ADM708 microprocessor supervisory circuits are suitable for monitoring 5 V power supplies/batteries and microprocessor activity.

The ADM705/ADM706 provide power-supply monitoring circuitry that generate a reset output during power-up, powerdown, and brownout conditions. The reset output remains operational with VCC as low as 1 V. Independent watchdog monitoring circuitry is also provided. This is activated if the watchdog input has not been toggled within 1.60 sec.

In addition, there is a 1.25 V threshold detector to warn of power failures, to detect low battery conditions, or to monitor an additional power supply. An active low, debounced manual reset input (MR) is also included.

The ADM707 and ADM708 provide a similar functionality to the ADM705 and ADM706 and only differ in that a watchdog timer function is not available. Instead, an active high reset output (RESET) is available as well as the active low reset output (RESET). The ADM707 and ADM708 are identical except for the reset threshold monitor levels, which are 4.65 V and 4.40 V, respectively.

All devices are available in narrow 8-lead PDIP and 8-lead SOIC packages.

 

CARACTÉRISTIQUES

Guaranteed RESET valid with VCC = 1 V

190 μA quiescent current

Precision supply voltage monitor

4.65 V (ADM705/ADM707)

200 ms reset pulse width

Debounced TTL/CMOS manual reset input (MR)

Active high reset output (ADM707/ADM708)

Voltage monitor for power fail or low battery warning

 

CANDIDATURES

Microprocessor systems

Ordinateurs

Contrôleurs

Intelligent instruments

Critical microprocessor supply monitoring

 

CIRCUIT INFORMATION

POWER FAIL RESET OUTPUT

RESET is an active low output that provides a reset signal to  the microprocessor whenever the VCC input is below the reset threshold. An internal timer holds RESET low for 200 ms after the voltage on VCC rises above the threshold. This functions as a power-on reset signal for the microprocessor. It allows time for both the power supply and the microprocessor to stabilize after power-up. The RESET output is guaranteed to remain valid (low) with VCC as low as 1 V. This ensures that the microprocessor is held in a stable shutdown condition as the power supply voltage ramps up.

In addition to RESET, an active high RESET output is also available on the ADM707/ADM708. This is the complement of RESET and is useful for processors requiring an active high reset signal.

POWER FAIL COMPARATOR

The power fail comparator is an independent comparator that can monitor the input power supply. The comparator inverting input is internally connected to a 1.25 V reference voltage. The noninverting input is available at the PFI input. This input can monitor the input power supply via a resistive divider network.

When the voltage on the PFI input drops below 1.25 V, the comparator output (PFO) goes low, indicating a power failure. For early warning of power failure, the comparator monitors the preregulator input by choosing an appropriate resistive divider network. The PFO output can interrupt the processor so a shutdown procedure is implemented before power is lost.

VALID RESET BELOW 1 V VCC

The ADM705/ADM706/ADM707/ADM708 are guaranteed to provide a valid reset level with VCC as low as 1 V. As VCC drops below 1 V, the internal transistor does not have sufficient drive to hold the voltage RESET at 0 V. A pull-down resistor can connect externally, to hold the line low if required.

MICROPROCESSOR WITH BIDIRECTIONAL RESET

To prevent contention for microprocessors with a bidirectional reset line, a current limiting resistor must be inserted between the ADM705/ADM706/ADM707/ADM708 RESET output pin and the microprocessor RESET pin. This limits the current to a safe level if there are conflicting output reset levels. A suitable resistor value is 4.7 kΩ. If the reset output is required for other uses, it must be buffered.