EM78P458AP datasheet(49/76 Pages) EMC

49 / 76 page

EM78P458/459

OTP ROM

applications are involved, extra devices are required to assist in solving the power-up problems.

1. External Power on Reset Circuit

The circuit shown in Fig. 22 implements an external RC to produce a reset pulse. The pulse width

(time constant) should be kept long enough to allow Vdd to reach minimum operation voltage. This

circuit is used when the power supply has a slow rise time. Because the current leakage from the

/RESET pin is about

±5µA, it is recommended that R should not be great than 40 K. In this way, the

voltage at Pin /RESET is held below 0.2V. The diode (D) acts as a short circuit at power-down. The

capacitor, C, is discharged rapidly and fully. Rin, the current-limited resistor, prevents high current

discharge or ESD (electrostatic discharge) from flowing into Pin /RESET.

EM78P458

EM78P459

/RESET

VDD

Rin

Fig. 22 External Power on Reset Circuit

2. Residue-Voltage Protection

When battery is replaced, device power (Vdd) is taken off but residue-voltage remains. The

residue-voltage may trips below Vdd minimum, but not to zero. This condition may cause a poor

power on reset. Fig. 23 and Fig. 24 show how to build a residue-voltage protection circuit

EM78P458

EM78P459

/RESET

VDD

100K

1N4684

10K

33K

VDD

Fig. 23

Circuit 1 for the Residue Voltage Protection

This specification is subject to change without prior notice.

06.25.2004 (V1.4)

EM78P458AP Datasheet(PDF) 49 Page - ELAN Microelectronics Corp