Charging Control Switch Functional Description

The charging switch transistor V304 controls the charging current from the charger input to the battery. During charging the transistor is forced in saturation and the voltage drop over the transistor is 0.2-0.4V depending upon the current delivered by the charger. Transistor V304 is controlled by the PWM output from N301, pin 23 via resistors R309, R308 and transistor V303. The output from N301 is of open drain type. When transistor V304 is conducting the output from N301 pin is low. In this case resistors R305 and R306 are connected in parallel with R304. This arrangement increases the base current through V304 to put it into saturation.

Transistors V304, V302, V303 and V312 forms a simple voltage regulator circuitry. The reference voltage for this circuitry is taken from zener diode V301. The feedback for the regulator is taken from the collector of V304. When the PWM output from N301 is active, low, the feedback voltage is determined by resistors R308 and R309. This arrangement makes the charger control switch circuitry to act as a programmable voltage regulator with two output voltages depending upon the state of the PWM output from N301. When the PWM is inactive, in high impedance the feedback voltage is almost the same as on the collector of V304. Due to the connection the voltage on V303 and V302 emitters are the same. The influence of the current thru R305 and R306 can be neglected in this case.

The charging switch circuit diagram is shown in following figure. The figure is for reference only.

Technical Documentation System Module

This feedback means that the system regulates the output voltage from V304 in such a way that the base of V303 and V302 are at the same voltage. The voltage on V302 is determined by the V301 zener voltage. The darlington connection of V312 and V302 service two purposes 1 the load on the voltage reference V301 is decreased, 2 the output voltage on V304 is decreased by the VBE voltage on V312 which is a wanted feature. The voltage reduction allows a relative temperature stable zener diode to be used and the output voltage from V304 is at a suitable level when the PWM output from N301 is not active.

The circuitry is self starting which means that an empty battery is initially charged by the regulator circuitry around the charging switch transistor. The battery is charged to a voltage of maximum 4.8V. This charging switch circuitry allows for both NiCd, NiMH and Lithium type of batteries to be used.

When the PWM output from N301 is active the feedback voltage is changed due to the presence of R308 and R309. When the PWM is active the charging switch regulator voltage is set to 10.5V maximum. This means that even if the voltage on the charger input exceeds 11.5V the battery voltage will not exceed 10.5 V. This protects N301 from over voltage even if the battery was to be detached while charging.

V305 is a schottky diode that prevents the battery voltage from reverse bias V304 when the charger is not connected. The leakage current for V305 is increasing with increasing temperature and the leakage current is passed to ground via R308, V303 and R304. This arrangement prevents V304 from being reversed biased as the leakage current increases at high temperatures.

V300 is a 16V transient suppressor. V300 protects the charger input and in particular V304 for over voltage. The cut off voltage is 16V with a maximum surge voltage up to 25V. V300 also protects the input for wrong polarity since the transient suppressor is bipolar.

System Module Technical Documentation

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