Datasheet LT1769 (Analog Devices) - 10

LT1769
U U W U APPLICATIONS INFORMATION
VUV = Rising lockout threshold on the UV pin adapter load current remains below the limit. Amplifier V CL1 in Figure 2 senses the voltage across R IN = Charger input voltage that will sustain full load power S4, connected between the CLP and CLN pins. When this voltage exceeds Example: With R6 = 5k, VUV = 6.7V and setting VIN at 12V; 100mV, the amplifier will override the programmed charge R5 = 5k (12V – 6.7V)/6.7V = 4k current to limit adapter current to 100mV/RS4. A lowpass filter formed by 500Ω and 1µF is required to eliminate The resistor divider should be connected directly to the switching noise. If the input current limit is not used, both adapter output as shown, not to the VCC pin, to prevent CLP and CLN pins should be connected to V battery drain with no adapter voltage. If the UV pin is not CC. used, connect it to the adapter output (not VCC) and
Charge Current Programming
connect a resistor no greater than 5k to ground. Floating this pin will cause reverse battery current to increase from The basic formula for charge current is (see Block 3µA to 200µA. Diagram): If connecting the unused UV pin to the adapter output is not RS2 2.465V RS2 ( ) RS1 ( )( ) possible, it can be grounded. Although it would seem that IBAT = IPROG = R R grounding the pin creates a permanent lockout state, the PROG S1 UV circuitry is arranged for phase reversal with low volt- where RPROG is the total resistance from PROG pin to ground. ages on the UV pin to allow the grounding technique to work. For the sense amplifier CA1 biasing purpose, RS3 should have the same value as RS2 and SPIN should be connected 100mV directly to the sense resistor (R + CLP S1) as shown in the Block + Diagram. CL1 1µF CLN 500Ω For example, 2A charge current is needed. For low power – AC ADAPTER OUTPUT dissipation on RS1 and enough signal to drive the amplifier VCC RS4* VIN CA1, let RS1 = 100mV/2A = 0.05Ω. This limits RS1 power + to 0.2W. Let RPROG = 5k, then: LT1769 R5 UV (I R BAT)(RPROG)(RS1) R6 S2 = RS3 = *RS4 = 100mV 2.465V ADAPTER CURRENT LIMIT 1769 F02 (2A)(5k)(0.05) = = 200Ω 2.465V
Figure 2. Adapter Input Current Limiting
Charge current can also be programmed by pulse width
Adapter Current Limiting
modulating IPROG with a switch Q1 to RPROG at a frequency An important feature of the LT1769 is the ability to higher than a few kHz (Figure 3). Charge current will be automatically adjust charge current to a level which avoids proportional to the duty cycle of the switch with full current overloading the wall adapter. This allows the product to at 100% duty cycle. operate at the same time the batteries are being charged
Lithium-Ion Charging
without complex load management algorithms. Addition- ally, batteries will automatically be charged at the maximum The 2A Lithium-Ion Battery Charger (Figure 1) charges at possible rate of which the adapter is capable. a constant 2A until battery voltage reaches a limit set by R3 This is accomplished by sensing total adapter output and R4. The charger will then automatically go into a current and adjusting the charge current downward if a constant-voltage mode with current decreasing to near preset adapter current limit is exceeded. True analog zero over time as the battery reaches full charge. This is the control is used, with closed-loop feedback ensuring that normal regimen for lithium-ion charging, with the charger 1769fa 10