National Instruments LM3647 Components Critical to Total Charger Performance, Minutes, Example

and the charge process will restart. This occurs only with batteries that are already fully charged, and consequently should not be recharged. If the battery voltage has not reached the Li-Ion battery qualification voltage (CEL-pin > 1.2V) within 1 minute of the Qualification Phase, the battery is considered to be defective, and the charger goes into error mode. It stays there until the battery is removed (CEL-pin < 1.0V).

The next phase is Fast Charge Constant Current. During this phase the current is constant, and the battery voltage will slowly rise (due to the charging). When the battery has reached its maximum battery voltage (CEL at 2.675V or 2.74V, depending on SEL3, it will go to the next phase which is Fast Charge Constant Voltage.

During this phase, the charger will keep the voltage constant and stay in this phase until the current has decreased to a threshold value (CS at 2.3V).

The battery is now fully charged, and the charger can behave in different modes, depending on SEL1. It can either maintenance charge the battery and restart the charge process if the battery voltage drops below the maintenance restart threshold value (CEL < 2.153V), or just maintenance charge the battery and don't restart the charge process if the battery becomes discharged. The last mode is no maintenance charge, and restarts the charge process if the battery voltage drops below the maintenance restart threshold value (CEL < 2.153V).

1.2.2.1Components Critical to Total Charger Performance

·The capacitance C2 connected to CEXT must be of a type that has low internal resistance, low loss, high stability and low dielectric absorption. The capacitance mounted on the Demo Board is a metallized polyester type from WIMA, 2220 series.

·The operational amplifiers U1 and U2 must be capable of rail-to-rail output, and have a high PSRR (PowerSup-

plyRejectionRatio), because they are both powered directly from the unregulated DC-input. U1 must also have enough current drive to control the transistor Q3. U2 should preferably have a low input offset, since this error will be amplified.

·The regulator IC2 criteria is that it has to be able to handle the input DC-voltage, and deliver enough current to drive the circuitry (all LED's, buzzer, LM3647).

·The transistor Q3 must be able to handle the charge current and (depending on charge current) must be provided with an adequate heatsink.

·The transistor Q2 must be able to handle the maximum discharge current.

·The Diode D1 must be able to handle the maximum charge current.

1.2.2.2 Clarifications Regarding Circuit Schematics

The circuitry with Q4, R26 and R27 (see section below) is used to protect the battery from excessive charge current. When the current flows through the current sense resistor R9, and is amplified by U2, the voltage at U2's output drops from 2.5V until Q4 starts conducting. It discharges the RC-network that generates the DC-voltage from the PWM-output of the LM3647.

1.2.2.3 Setting The Charge Timeout

The LM3647 uses the charge timeout value as a backup termination method if the normal termination methods fail. The charge timeout also controls the length of some of the phases like the Topping Charge phase (Ni-Cd/Ni-MH). The timeout is selectable from a charge rate of 3.2C to 0.4C. The table below shows which values will result in a certain timeout.

AN-1164

TABLE 1. Charge Timeouts

		Ni-Cd/Ni-MH	Ni-Cd/Ni-MH	Li-lon CC	Li-lon CV	Appropriate
R Value	C Value	Fast Charge
			Topping (minutes)	(minutes)	(minutes)	Charge Rates
		(minutes)
100 kΩ	0 nF	75	20	50	75	3.2C
100 kΩ	10 nF	100	25	70	100	2.4C
100 kΩ	15 nF	160	40	110	160	1.4C
100 kΩ	22 nF	190	50	130	190	1.2C
100 kΩ	33 nF	260	65	170	260	0.9C
100 kΩ	47 nF	330	80	220	330	0.7C
100 kΩ	68 nF	450	115	300	450	0.5C
100 kΩ	100 nF	540	135	360	540	0.4C

EXAMPLE 1:

AN101315-18

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