Timeout Selection

FAST-CHARGE state normally operates until -

ΔV termi-

nation. In the event that termination does not occur cor-

rectly, a safety timeout is required. This timeout is set

vides secondary protection against significant over-

charging. The value of the TMR resistor should be

chosen so that the timeout is greater than the FAST-

CHARGE time expected in the application, but not so

much greater that its protection is compromised. If the

timer expires during FAST-CHARGE, the timer count is

reset and charging proceeds to the TOP-OFF charge

state. The TMR resistor also sets the timed charge

duration of TOP-OFF state. The TOP-OFF timeout peri-

od is fixed at half the FAST-CHARGE timeout period.

When the timer expires in TOP-OFF, the DS2710 enters

the MAINTENANCE state.

Resistors can be selected to support FAST-CHARGE

timeout periods of 0.5hr to 5hr and TOP-OFF timeout

periods of 0.25hr to 2.5hr. The programmed FAST-

CHARGE time approximately follows the equation:

Ω)/1000

Impedance-Test Threshold

Selection

The DS2710 tests the cell impedance every 31s while

in FAST-CHARGE state. Impedance is measured by

comparing the cell voltage during normal charging to

the cell voltage with no charge current (CS output held

high). The resulting voltage difference is compared

against the threshold set by an external resistor from

32mV to 400mV. The following formula approximates

the setting for the detection threshold:

Since the charge rate is controlled by the external

threshold can be expressed as impedance as follows:

Impedance Threshold =

= 0.100

Ω) would use a 47kΩ resistor on the CTEST pin

to set the impedance threshold to 0.150

Ω.

Application Circuit

Figure 5 shows a typical DS2710 application circuit for

charging a NiMH cell from a USB port or other 5V

charge source capable of supplying 0.5A. Q1, L1, C2,

and D2 form a switching buck-regulator circuit con-

trolled by the CS pin of the DS2710. Current is regulat-

ed through the current-sense resistor, R9, by switching

Q1 on and off as the sense resistor voltage ramps up

and down toward the preset sense voltage thresholds.

The 0.100

Ω sense resistor along with the DC ground-

to 1.13A. The sense resistor should have a proper

power rating for the chosen charge current.

The TMR resistor is set to 100k

Ω for a timeout of 2.5hr.

This is appropriate for cells with a capacity of approxi-

mately 2200mAh when charged with the 1.13A charge

current. The CTEST resistor is set to 47k

Ω for an imped-

ance-test threshold of approximately 0.150

Ω when

charging at 1.13A. Additionally, R6 protects the VP1 pin

from any stress applied to the exposed tabs of a loose

NiMH cell; R3 creates a weak pullup to offset the leak-

age through D2, which might otherwise cause a false

cell detection; and R1/C1 creates a bypass filter on the

The value of L1 in Figure 5 represents a moderate

switching speed of ~ 200kHz for FAST-CHARGE state.

L1 can be adjusted to fit specific application goals as

long as the associated change in switching speed

does not exceed the circuit’s ability to maintain proper

regulation of the sense-resistor voltage. All capacitors

should be ceramic surface-mount types of good quality

where possible. The 10µF capacitor can be of any type

that meets the application requirements. All resistors

not previously mentioned are standard surface-mount

types.

Application PCB Layout

Proper layout rules must be followed to ensure a suc-

cessful application circuit. For all modes of operation,

currents in excess of 1A can flow through the charge

and discharge paths (USB charging is specification lim-

ited to 500mA). All these paths should be properly sized

to handle the worst-case current flow, whether from

charging or from powering the load with the battery.

Switch-mode operation presents challenges with fast

voltage and current transients. Proper switch-mode

buck power-supply layout should always be observed.

Single-Cell NiMH Charger

10

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