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TA8002

5V VOLTAGE REGULATOR WITH RESET TIMER

Toshiba 

TA8002S/AS

FUNCTIONAL DESCRIPTION

The TA8002S / AS incorporates a constant-voltage 5V power supply function to feed the 5V logic circuits and a

reset function to ensure stable operation of the system. These functions are explained below.

(1) Constant-voltage 5V power supply function

This constant-voltage function has the reference voltage Vref in the IC that is insusceptible to

temperature changes and input voltage fluctuations. The power supply circuit is designed in such a way

that this voltage is stepped up to 5V by using an OP amp and a voltage-dividing resistor. These OP amp

and dividing resistor and an output transistor connected to the OP amp output together configure a

closed loop.

The output voltage of 5V is obtained by shorting the ADJ pin to GND. The output voltage is reduced to

3.5V by leaving this pin open, allowing for a further reduction in the amount of current consumed by the

entire system during standby.

(2) System reset function (See Timing Chart)

· Voltage monitoring function

When powered on, the power-on reset timer starts counting the moment the voltage VCC applied to

the CPU exceeds 4.6V. When powered off, this voltage monitoring function outputs a reset signal

immediately when VCC drops below 4.6V. A reset signal also is output immediately when VCC drops

for some reason during normal operation. Then, when VCC is restored to the normal voltage and

exceeds 4.6V, the power-on reset timer starts counting.

· Power-on reset timer function

To allow the 5V constant voltage to stabilize at power-on, as well as prevent the circuit system from

operating erratically at power-on, the device remains reset for a predetermined time before being

released from the reset state. The duration of this time can be set as desired by choosing appropriate

values for the external resistor and capacitor connected to the TC pin.

The system starts charging the capacitor when the VCC voltage exceeds 4.6V. When this charging

voltage exceeds 1.7V, the reset signal is inverted to deactivate the reset.

TIMING CHART

Note: TRESET : Reset timer time

3

2002-03-12

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