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DS2710G Datasheet(PDF) 10 Page - Maxim Integrated Products |
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DS2710G Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page 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 by an external resistor on the TMR pin to VSS and pro- 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: t(minutes) = 1.5 x RTMR( 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 CTEST to VSS. The detection threshold can be set from 32mV to 400mV. The following formula approximates the setting for the detection threshold: VTEST = 8000/RCTEST (Value in Volts) Since the charge rate is controlled by the external sense resistor (RSENSE) between VN1 and VN0, the test threshold can be expressed as impedance as follows: Impedance Threshold = ( 8000/RCTEST )/( 0.113/RSENSE ) = 70796 x ( RSENSE/RCTEST ) For example, an application charging at 1.13A (RSENSE = 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- referenced sense threshold level of VIREF - 1/2 VHYS- COMP sets the average charge current in the example 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 VDD pin of the IC. 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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