Datasheet LT1571 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | Constant-Current/Constant-Voltage Battery Charger with Preset Voltage and Termination Flag |
| Seiten / Seite | 16 / 9 — APPLICATIO S I FOR ATIO. Charge Current Programming. Figure 2. … |
| Dateiformat / Größe | PDF / 220 Kb |
| Dokumentensprache | Englisch |
APPLICATIO S I FOR ATIO. Charge Current Programming. Figure 2. Undervoltage Lockout. Figure 3. PWM Current Programming
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LT1571 Series
U U W U APPLICATIO S I FOR ATIO
is achieved with VC at 1.1V. With a 0.33µF capacitor, the The lockout voltage will be VIN = VZ + 1V. time to reach full charge current is about 9ms and it is For example, for a 24V adapter to start charging at 22V assumed that input voltage to the charger will reach full IN, choose V value in less than 3ms. Capacitance can be increased up to Z = 21V. When VIN is less than 22V, D1 keeps VC low and charger off. 1µF if longer input start-up times are needed. In any switching regulator, conventional time-based soft
Charge Current Programming
starting can be defeated if the input voltage rises much The basic formula for charge current is (see Block slower than the time-out period. This happens because the Diagram): switching regulators in the battery charger and the com- 2. V 465 puter power supply are typically supplying a fixed amount I = I( )( ) BAT PROG 2000 = 2000 of power to the load. If the input voltage comes up slowly RPROG ( ) compared to the soft-start time, the regulators will try to where R deliver full power to the load when the input voltage is still PROG is the total resistance from PROG pin to ground. well below its final value. If the adapter is current limited, it cannot deliver full power at reduced output voltages and For example, 1A charge current is needed. the possibility exists for a quasi “latch” state where the (2. V 465 )( ) 2000 adapter output stays in a current limited state at reduced R = 4.9 k PROG = 3 output voltage. For instance, if maximum charger plus A 1 computer load power is 20W, a 24V adapter might be Charge current can also be programmed by pulse width current limited at 1A. If adapter voltage is less than (20W/1A modulating IPROG with a switch Q1 to RPROG at a frequency = 20V) when full power is drawn, the adapter voltage will be higher than a few kHz (Figure 3). Charge current will be pulled down by the constant 20W load until it reaches a lower proportional to the duty cycle of Q1 with full current at stable state where the switching regulators can no longer 100% duty cycle. supply full load. This situation can be prevented by utilizing When a microprocessor DAC output is used to control undervoltage lockout, set higher than the minimum adapter charge current, it must be capable of sinking current voltage where full power can be achieved. at a compliance up to 2.5V if connected directly to the A fixed undervoltage lockout of 7V is built into the LT1571. PROG pin. A higher lockout voltage can be implemented with a Zener diode D2 (see Figure 2). LT1571 PROG D3 300Ω VIN D2 R C D1 V PROG PROG V CC Z 4.64k 1µF 1N4148 VC LT1571 5V Q1 VN2222 0V 2k GND PWM 1571 F02 IBAT = (DC)(1A) 1571 F03
Figure 2. Undervoltage Lockout Figure 3. PWM Current Programming
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U U W U APPLICATIO S I FOR ATIO
is achieved with VC at 1.1V. With a 0.33µF capacitor, the The lockout voltage will be VIN = VZ + 1V. time to reach full charge current is about 9ms and it is For example, for a 24V adapter to start charging at 22V assumed that input voltage to the charger will reach full IN, choose V value in less than 3ms. Capacitance can be increased up to Z = 21V. When VIN is less than 22V, D1 keeps VC low and charger off. 1µF if longer input start-up times are needed. In any switching regulator, conventional time-based soft
Charge Current Programming
starting can be defeated if the input voltage rises much The basic formula for charge current is (see Block slower than the time-out period. This happens because the Diagram): switching regulators in the battery charger and the com- 2. V 465 puter power supply are typically supplying a fixed amount I = I( )( ) BAT PROG 2000 = 2000 of power to the load. If the input voltage comes up slowly RPROG ( ) compared to the soft-start time, the regulators will try to where R deliver full power to the load when the input voltage is still PROG is the total resistance from PROG pin to ground. well below its final value. If the adapter is current limited, it cannot deliver full power at reduced output voltages and For example, 1A charge current is needed. the possibility exists for a quasi “latch” state where the (2. V 465 )( ) 2000 adapter output stays in a current limited state at reduced R = 4.9 k PROG = 3 output voltage. For instance, if maximum charger plus A 1 computer load power is 20W, a 24V adapter might be Charge current can also be programmed by pulse width current limited at 1A. If adapter voltage is less than (20W/1A modulating IPROG with a switch Q1 to RPROG at a frequency = 20V) when full power is drawn, the adapter voltage will be higher than a few kHz (Figure 3). Charge current will be pulled down by the constant 20W load until it reaches a lower proportional to the duty cycle of Q1 with full current at stable state where the switching regulators can no longer 100% duty cycle. supply full load. This situation can be prevented by utilizing When a microprocessor DAC output is used to control undervoltage lockout, set higher than the minimum adapter charge current, it must be capable of sinking current voltage where full power can be achieved. at a compliance up to 2.5V if connected directly to the A fixed undervoltage lockout of 7V is built into the LT1571. PROG pin. A higher lockout voltage can be implemented with a Zener diode D2 (see Figure 2). LT1571 PROG D3 300Ω VIN D2 R C D1 V PROG PROG V CC Z 4.64k 1µF 1N4148 VC LT1571 5V Q1 VN2222 0V 2k GND PWM 1571 F02 IBAT = (DC)(1A) 1571 F03
Figure 2. Undervoltage Lockout Figure 3. PWM Current Programming
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