Datasheet LT1510, LT1510-5 (Analog Devices) - 10

LT1510/LT1510-5
U U W U APPLICATIONS INFORMATION
where RPROG is the total resistance from PROG pin to even this low current drain. A 47k resistor from adapter ground. output to ground should be added if Q3 is used to ensure that the gate is pulled to ground. For example, 1A charging current is needed. With divider current set at 25µA, R4 = 2.465/25µA = 100k 2. V ( 465 )( ) 2000 and, R = = 4 9 . k PROG 3 A 1 R4 V 2.465 k ( )( − ) ( . − . ) BAT 100 8 4 2 465 = Charging current can also be programmed by pulse width R3 2.465 + R4 0.05 A µ 2.465 + 100k 0.05 A ( )= ( µ ) modulating IPROG with a switch Q1 to RPROG at a frequency higher than a few kHz (Figure 4). Charging current will be = k 240 proportional to the duty cycle of the switch with full current Lithium-ion batteries typically require float voltage accu- at 100% duty cycle. racy of 1% to 2%. Accuracy of the LT1510 OVP voltage is When a microprocessor DAC output is used to control ±0.5% at 25°C and ±1% over full temperature. This leads charging current, it must be capable of sinking current to the possibility that very accurate (0.1%) resistors might at a compliance up to 2.5V if connected directly to the be needed for R3 and R4. Actually, the temperature of the PROG pin. LT1510 will rarely exceed 50°C in float mode because charging currents have tapered off to a low level, so 0.25% LT1510 resistors will normally provide the required level of overall PROG accuracy. 300Ω
External Shutdown
RPROG CPROG 4.64k 1µF The LT1510 can be externally shut down by pulling the VC 5V Q1 pin low with an open drain MOSFET, such as VN2222. The VN2222 0V PWM VC pin should be pulled below 0.8V at room temperature IBAT = (DC)(1A) 1510 F04 to ensure shutdown. This threshold decreases at about 2mV/°C. A diode connected between the MOSFET drain
Figure 4. PWM Current Programming
and the VC pin will still ensure the shutdown state over all temperatures, but it results in slightly different conditions
Lithium-Ion Charging
as outlined below. The circuit in Figure 2 uses the 16-pin LT1510 to charge If the VC pin is held below threshold, but above ≈ 0.4V, the lithium-ion batteries at a constant 1.3A until battery volt- current flowing into the BAT pin will remain at about age reaches a limit set by R3 and R4. The charger will then 700µA. Pulling the VC pin below 0.4V will cause the current automatically go into a constant-voltage mode with cur- to drop to ≈ 200µA and reverse, flowing out of the BAT pin. rent decreasing to zero over time as the battery reaches full Although these currents are low, the long term effect may charge. This is the normal regimen for lithium-ion charg- need to be considered if the charger is held in a shutdown ing, with the charger holding the battery at “float” voltage state for very long periods of time, with the charger input indefinitely. In this case no external sensing of full charge voltage remaining. Removing the charger input voltage is needed. causes all currents to drop to near zero. Current through the R3/R4 divider is set at a compromise If it is acceptable to have 200µA flowing into the battery value of 25µA to minimize battery drain when the charger while the charger is in shutdown, simply pull the VC pin is off and to avoid large errors due to the 50nA bias current directly to ground with the external MOSFET. The resistor of the OVP pin. Q3 can be added if it is desired to eliminate divider used to sense battery voltage will pull current out 10