DS2770

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RECHARGEABLE LITHIUM PULSE CHARGING Figure 3

NiMH CHARGER OPERATION

During NiMH charging, charge current from a constant current or current-limited supply is gated to the

battery under control of UV and CC pins. Since the transition from trickle charge to fast charge occurs

when the battery reaches 3.0V, a three-cell NiMH battery will begin fast charge at 1.0V per cell. A

voltage divider can be used to adjust the transition point to a higher per cell voltage, though values in the

Voltage Measurement Register would be affected by the divider ratio.

At the start of fast charge, the DS2770 uses the most recent temperature measurement to determine the

appropriate dT/dt termination rate for the ambient conditions from Table 2 below. During the first 5

the blanking period, dT/dt detection begins using one of the initial rates from Table 2 and requires that the

rate of temperature rise continue at or above the selected rate for 2.8 minutes. Internally, values from the

Temperature Measurement Register are averaged in determining the sustained rate of temperature rise.

The effective rate shown in Table 2 is based on NiMH charge data and represents the predicted

instantaneous dT/dt rate at termination.

NiMH CHARGE TERMINATION THRESHOLDS Table 2

dT/dt THRESHOLD

AMBIENT

TEMPERATURE (

°C)

INITIAL

(

°C/MIN)

EFFECTIVE

(

°C/MIN)

0 to +5

0.30

1.0

+5 to +15

0.45

1.0

+15 to +40

0.60

1.0

t

CD

Complete

Charge

Pulse Charge

Charge

Trickle

VIN

t

VCV

CC

UV

LB

CV

V

V

Pulse

Region

Bulk

Region