Keywords: switching speed, frequency, calculation, inductor, inductor selection, battery charger ics, battery chargers, nimh charge controllers, smart charger, nimh, chargers, batteries, packs, ds2715, ics, circuits, low cost
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APPLICATION NOTE 4214

Abstract: The switching speed of the DS2715 battery charger varies based on charge mode, inductor selection, and battery voltage. In some cases, the speed under various conditions should be known to account for noise sensitivity, peak-to-peak charge current, and inductor selection constraints. This application note provides an easy-to-use spreadsheet for estimating a circuit's switching speed with different component values, battery parameters, and charge modes.

The various charge states are entered depending on the state of the battery stack. When the battery voltage is less than 1.0V, precharge mode is used to slowly charge depleted batteries. Fast-charge mode is only used when the battery voltage is above 1.0V and not more than 1.65V, but may also terminate when other fast-charge conditions are met. Topoff mode follows fast-charge mode for a preset period of time. R

The control circuit for regulating the sense resistor voltage consists of a comparator with hysteresis and a reference voltage, which drives an open-drain output pin. The hysteresis of the precharge and topoff modes is approximately 25% of the fast-charge value due to the smaller reference voltage used in those modes. Switching speed is determined by the amount of time needed for I

V = L × di/dt and dt = L × di/V

Hence, the amount of time for the on and off states of Q

In order to simplify the switching-speed approximation, let us make the assumption that the voltage across the inductor for each Q

V

V

Since the battery voltage may vary over a 1.0V to 1.65V range in fast-charge mode, it is useful to calculate the inductor voltage and then the switching speed at several operating points. A spreadsheet is available for download that will perform the calculations described in this application note for three different battery voltages in fast-charge mode, and at one battery stack voltage in precharge and topoff modes.

Once the inductor voltages for each state and each operating point are calculated, any delays in the control circuit may be factored into the on and off times for Q

di(on delay) = dt × V(on)/L, where dt = t

di(off delay) = dt × V(off)/L, where dt = t

This extra change in current can be thought of as I

di(Q

di(Q

This adjusted delta I

The equations described in this application note have been implemented in a spreadsheet for easily estimating the switching speed of a DS2715 switch-mode charger circuit. The spreadsheet describes each of the parameters that must be entered by the user, or that may be optionally modified by the user (

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The first set of parameters relates to the component properties of the reference schematic. The second section contains the various operating points of the battery stack used in the calculations. The third section describes the internal set points of the current-sense comparator; the default values in this section should be verified from the data sheet of the DS2715. Finally, the last section of parameters is for the delays inherent to the DS2715 and any extra delays induced by the switching circuit that must be considered for accurate estimation.

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