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EDN Access--01.01.98 Battery charger takes on two roles

-January 01, 1998



January 1, 1998


Battery charger takes on two roles

Thomas A Somerville, Motorola SPS, Tempe, AZ

The NiCd and NiMH battery charger in Figure 1 applies a battery fast-charge controller IC in a way that not only provides peak-voltage fast-charge termination--as the IC was designed to do--but also precisely regulates the charge current. The design also features maximum-temperature backup shutdown. Efficiency exceeds 90% at a VIN of 14V.

The voltage-to-current switching-regulator loop includes a duty-cycle modulator formed by the voltage ramp across the inductor's current-sense resistor, RS, and a hysteresis comparator normally used for thermistor-sensed cold-temperature detection. The IC develops the comparator threshold of 100 mV across a 10-kiloohm thermistor, which conducts the 30-µA reference that the controller IC provides. RS creates a level-shifted ramp voltage at Pin 5. When the comparator threshold crosses 100 mV, this ramp voltage causes the comparator to switch off the MTP2955E using an active-low output at Pin 3. The inductor current decreases linearly during the MOSFET off-time until the comparator hysteresis exceeds 48 mV. As the design equations in Figure 1 indicate, the proper choice of L1 and RS sets the charging current and switching frequency for a particular charger input and output voltage.

In addition to performing duty-cycle-controlled current regulation, the controller IC momentarily interrupts the charging current to perform the A/D conversion. Pin 2 is active-low for 33 msec every 1.4 sec. The first 11 msec are necessary for the battery voltage to settle; the controller IC uses the remaining 22 msec to count VFC pulses for an A/D conversion. By interrupting the charging current, the controller eliminates offsets due to charge-current flow through the charger output cable and through the battery's internal impedance during the battery-voltage sampling measurement. This interruption also eliminates noise induced in the wiring from the switching magnetic fields. The elimination of these error sources, along with the iterating nature of the converter, allows a peak-voltage-detector threshold of only -4 mV for the voltage at Pin 1. This input is a one-cell equivalent voltage developed by the one-fifth voltage divider of R3 and R4.

The threshold of the overtemperature-protection comparator is equal to the voltage at Pin 7. This voltage is equal to the 30-µA current source flowing from this pin times the value of R6. R6 should be equal to the parallel combination of R7 and the value of the thermistor at the upper temperature limit. Because the thermistor voltage sets both the temperature and the current regulation threshold, the design uses R7 to reduce the temperature sensitivity of the charging current to approximately 30% from 25 to 45°C for a typical thermistor. You can use a lower value for R7 if this variation is unacceptable, or you can eliminate the thermistor if backup protection is unnecessary.

The design includes an LED to indicate charge status. Blinking indicates regulated current fast charge; continuously on indicates fast charge has terminated or the battery pack is undervoltage (less than 1V/cell) or overvoltage (greater than 2V/cell), and continuously off indicates a disconnected input. During the state that fast charging is interrupted, a trickle current equal to (VIN-VLED-VOUT)/R9 flows into the battery.

A lower cost alternative is to replace the MTP2955E with a TIP42 pnp power transistor and replace the BS170 with a 1-kiloohm resistor to ground. This approach reduces efficiency by 5 to 10% because of the slower switching of the pnp transistor as well as the power dissipated in the base-drive resistor. (DI #2135)

Figure 1
26d21351
A fast-charge controller IC and surrounding circuitry precisely regulate the charging current, as well as implement peak-voltage fast-charge termination with maximum-temperature backup shutdown.


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Copyright c 1997 EDN Magazine, EDN Access. EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company, a unit of Reed Elsevier Inc.

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