LT1511 UUWUAPPLICATIONS INFORMATION capacitor. A resistor divider is used to set the desired VCC being charged without complex load management algo- lockout voltage as shown in Figure 2. A typical value for R6 rithms. Additionally, batteries will automatically be charged at is 5k and R5 is found from: the maximum possible rate of which the adapter is capable. R6(V – V ) This feature is created by sensing total adapter output R5 = IN UV current and adjusting charging current downward if a VUV preset adapter current limit is exceeded. True analog V control is used, with closed loop feedback ensuring that UV = Rising lockout threshold on the UV pin adapter load current remains within limits. Amplifier CL1 VIN = Charger input voltage that will sustain full load power in Figure 2 senses the voltage across RS4, connected Example: With R6 = 5k, V between the CLP and CLN pins. When this voltage exceeds UV = 6.7V and setting VIN at 12V; 100mV, the amplifier will override programmed charging R5 = 5k (12V – 6.7V)/6.7V = 4k current to limit adapter current to 100mV/RS4. A lowpass The resistor divider should be connected directly to the filter formed by 500Ω and 1µF is required to eliminate adapter output as shown, not to the VCC pin to prevent switching noise. If the current limit is not used, both CLP battery drain with no adapter voltage. If the UV pin is not and CLN pins should be connected to VCC. used, connect it to the adapter output (not VCC) and connect a resistor no greater than 5k to ground. Floating Charging Current Programming the pin will cause reverse battery current to increase from The basic formula for charging current is (see Block 3µA to 200µA. Diagram): If connecting the unused UV pin to the adapter output is not possible for some reason, it can be grounded. Al- RS2 2.465V R I S2 ( ) RS1 ( )( ) BAT = IPROG = though it would seem that grounding the pin creates a RPROG RS1 permanent lockout state, the UV circuitry is arranged for phase reversal with low voltages on the UV pin to allow the where RPROG is the total resistance from PROG pin to ground. 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 + CLP directly to the sense resistor (RS1) as shown in the Block + Diagram. CL1 1µF CLN 500Ω – For example, 3A charging current is needed. To have low AC ADAPTER OUTPUT power dissipation on RS1 and enough signal to drive the VCC RS4* VIN amplifier CA1, let RS1 = 100mV/3A = 0.033Ω. This limits + LT1511 R R5 S1 power to 0.3W. Let RPROG = 5k, then: UV (I R6 R BAT)(RPROG)(RS1) S2 = RS3 = *RS4 = 100mV ADAPTER CURRENT LIMIT 2.465V 1511 • F02 (3A)(5k)(0.033) = = 200Ω Figure 2. Adapter Current Limiting 2.465V Adapter Limiting Charging current can also be programmed by pulse width modulating IPROG with a switch Q1 to RPROG at a frequency An important feature of the LT1511 is the ability to higher than a few kHz (Figure 3). Charging current will be automatically adjust charging current to a level which proportional to the duty cycle of the switch with full current avoids overloading the wall adapter. This allows the at 100% duty cycle. product to operate at the same time that batteries are 10
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