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HA1835P

Watchdog Timer

Hitachi -> Renesas Electronics 

HA1835P/HA1848P

Functional Description

Voltage Regulator

The HA1835P, and HA1848P supplies 5 V regulated output from a wide VCC input range (VCC = 6 V to

17.5 V). The external PNP transistor should be selected according to the supply current demand of the

system. Connect a more than 100 µF capacitor between Vout and GND to realize the 5 V regulated output.

The resistor connected from the Vadj terminal to the Vout or GND terminal fine tunes the output voltage.

A resistor between Vadj and the Vout decreases the output voltage, and a resistor between Vadj and GND

increases the output voltage.

R (Vout − Vadj) ≈

5 ⋅ Vout − 7.14

5 − Vout

(kΩ)

R (Vadj − GND) ≈

7.14

Vout − 5

(kΩ)

Current Limiter

To protect the external transistor from overcurrent, connect a current-sense resistor between the CS

terminal and the VCC terminal. The value of this resistor should be:

RSC ≈

0.3 V

Iout (Limit)

(at Ta = 25°C)

Power-On Reset

The HA1835P, and HA1848P can output a reset pulse to start the microcomputer at power on (figure 6).

The external resistor RR and capacitor CR determine the power-on reset timing.

Note that RES and RES are open-collector terminals. They should therefore be connected to a pull-up

resistor of about 5 kΩ.

Watchdog Timer

The watchdog timer is a fail-safe function. It can reset the microcomputer system if the system runs out of

control. It does this be monitoring a pulse output by the system's software. It uses a bandpass filter to

determine whether the pulse frequency is within the system's normal frequency band (figure 7, 8). External

resistor and capacitor Rf and Cf determine the frequency range of the bandpass filter. If the pulse

frequency is not within the frequency band, the HA1835P, and HA1848P outputs a reset pulse.

7

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