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MC33340EVK

Battery Fast Charge Controllers

ON Semiconductor 

MC33340 MC33342

Figure 8. Typical Charge Characteristics for NiCd and NiMh Batteries

1.6

Vmax

70

–∆V

dV

1.5

dt

60

Tmax

1.4

50

1.3

40

Voltage

1.2

30

Temperature

1.1

20

Relative Pressure

1.0

10

0

40

80

120

160

CHARGE INPUT PERCENT OF CAPACITY

OPERATING DESCRIPTION

The MC33340/342 starts up in the fast charge mode when

power is applied to VCC. A change to the trickle mode can

occur as a result of three possible conditions. The first is if the

Vsen input voltage is above 2.0 V or below 1.0 V. Above 2.0 V

indicates that the battery pack is open or disconnected, while

below 1.0 V indicates the possibility of a shorted or defective

cell. The second condition is when the MC33340/342 detects

a fully charged battery by measuring a negative slope in

battery voltage. The MC33340/342 recognize a negative

voltage slope after the preset holdoff time (thold) has elapsed

during a fast charge cycle. This indicates that the battery

pack is fully charged. The third condition is either due to the

battery pack being out of a programmed temperature range,

or that the preset timer period has been exceeded.

There are three conditions that will cause the controller to

return from trickle to fast charge mode. The first is if the Vsen

input voltage moved to within the 1.0 to 2.0 V range from

initially being either too high or too low. The second is if the

battery pack temperature moved to within the programmed

temperature range, but only from initially being too cold. Third

is by cycling VCC off and then back on causing the internal

logic to reset. A concise description of the major circuit blocks

is given below.

Negative Slope Voltage Detection

A representative block diagram of the negative slope

voltage detector is shown in Figure 9. It includes a

Synchronous Voltage to Frequency Converter, a Sample

Timer, and a Ratchet Counter. The Vsen pin is the input for the

Voltage to Frequency Converter (VFC), and it connects to the

rechargeable battery pack terminals through a resistive

voltage divider. The input has an impedance of

approximately 6.0 MΩ and a maximum voltage range of

–1.0 V to VCC + 0.6 V or 0 V to 10 V, whichever is lower. The

10 V upper limit is set by an internal zener clamp that

provides protection in the event of an electrostatic discharge.

The VFC is a charge–balanced synchronous type which

generates output pulses at a rate of FV = Vsen (24 kHz).

The Sample Timer circuit provides a 95 kHz system clock

signal (SCK) to the VFC. This signal synchronizes the FV

output to the other Sample Timer outputs used within the

detector. At 1.38 second intervals the Vsen Gate output goes

low for a 33 ms period. This output is used to momentarily

interrupt the external charging power source so that a precise

voltage measurement can be taken. As the Vsen Gate goes

low, the internal Preset control line is driven high for 11 ms.

During this time, the battery voltage at the Vsen input is

allowed to stabilize and the previous FV count is preloaded.

At the Preset high–to–low transition, the Convert line goes

high for 22 ms. This gates the FV pulses into the ratchet

counter for a comparison to the preloaded count. Since the

Convert time is derived from the same clock that controls the

VFC, the number of FV pulses is independent of the clock

frequency. If the new sample has more counts than were

preloaded, it becomes the new peak count and the cycle is

repeated 1.38 seconds later. If the new sample has two fewer

counts, a less than peak voltage event has occurred, and a

register is initialized. If two successive less than peak voltage

events occur, the –∆V ‘AND’ gate output goes high and the

Fast/Trickle output is latched in a low state, signifying that the

battery pack has reached full charge status.

Negative slope voltage detection starts after 60 ms have

elapsed in the fast charge mode. This does not affect the

Fast/Trickle output until the holdoff time (thold) has elapsed

during the fast charge mode. Two scenarios then exist.

Trickle mode holdoff is implemented to ignore any initial drop

in voltage that may occur when charging batteries that have

been stored for an extended time period. If the negative slope

voltage detector senses that initial drop during the holdoff

time, and the input voltage rises as the battery charges, the

Fast/Trickle output will remain open. However, if the negative

slope voltage detector senses a negative drop in voltage

during the holdoff time and the input voltage never rises

above that last detected level, the Fast/Trickle output will

latch into a low state. The negative slope voltage detector

has a maximum resolution of 2.0 V divided by 1023, or 1.955

mV per count with an uncertainty of ±1.0 count. This yields a

detection range of 1.955 mV to 5.865 mV. In order to obtain

maximum sensing accuracy, the R2/R1 voltage divider must

be adjusted so that the Vsen input voltage is slightly less than

2.0 V when the battery pack is fully charged. Voltage

variations due to temperature and cell manufacturing must

be considered.

MOTOROLA ANALOG IC DEVICE DATA

5

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