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 N300, pin 34 via resistors R309, R308 and transistor V311. The output from N300 is of open drain type. When transistor V304 is conducting the output from N300 pin is low. In this case resistors R305 and R306 are connected in parallel with R304. This arrangement increases the base current thru V304 to put it into saturation.
Transistors V304, V302, V303 and V311 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 N300 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 N300. 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 V311 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.
This feedback means that the system regulates the output voltage from V304 in such a way that the base of V303 and V311 are at the same voltage. The voltage on V302 is determined by the V301 zener voltage. The darlington connection of V303 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 V302 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 N300 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 N300 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 N300 from over voltage even if the battery was to be detached while charging.
The RC network C304, R308 and R309 also acts as a delay circuitry when switching from one output voltage to an other. This happens when the PWM output from N300 is pulsing. The reason for the delay is to reduce the surge current that will occur when V304 is put into conducting state. Before V304 is put in conducting state there is a significant voltage drop over V304. The energy is stored in capacitors in the charger and these capacitors must first be drained in order to put the charger in constant current mode. This is done by discharging the capacitors into the battery. The delay caused by C304 will reduce the surge current thru V304 to an acceptable value.
R301 and R326 are used to regulate the zener current. During charging with empty battery the zener voltage might drop due to low zener current but this is no problem since the regulator is operating in constant current mode while
Technical Documentation charging. The zener voltage is more important when the charger voltage is high or in case that the PWM output from n300 is inactive. In this case the charger idle voltage is present at the charger supply pins.
R300 and R327 together with V304 forms a constant current source. The surge current limitation behavior is frequency dependent since L107 is an inductor. The purpose of these circuits is to reduce the surge current through V304 when it is put in conducting state. Due to the low resistance value required in L107 this arrangement is not very effective and the RC network R308, R309 and C304 contributes more to the surge current reduction.
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, V311 and R304. This arrangement prevents V304 from being reversed biased as the leakage current increases at high temperatures.
Components L107, C300, C301, C302 and L108 forms a filter for EMC attenuation. The circuitry reduces the conductive EMC part from entering the charger cable causing an increase in emission as the cable will act as an antenna.
V100 is a 18V transient suppressor. V100 protects the charger input and in particular V304 for over voltage. The cut off voltage is 18V with a maximum surge voltage up to 25V. V100 also protects the input for wrong polarity since the transient suppressor is bipolar.
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