Datasheet LT1512 (Analog Devices) - 7

LT1512
operaTion
The LT1512 is a current mode switcher. This means that A unique error amplifier design has two inverting inputs switch duty cycle is directly controlled by switch current which allow for sensing both output voltage and current. rather than by output voltage or current. Referring to the A 1.245V bandgap reference biases the noninverting input. Block Diagram, the switch is turned “on” at the start of The first inverting input of the error amplifier is brought out each oscillator cycle. It is turned “off” when switch current for positive output voltage sensing. The second inverting reaches a predetermined level. Control of output voltage and input is driven by a “current” amplifier which is sensing current is obtained by using the output of a dual feedback output current via an external current sense resistor. The voltage sensing error amplifier to set switch current trip current amplifier is set to a fixed gain of –12.5 which level. This technique has the advantage of simplified loop provides a –100mV current limit sense voltage. frequency compensation. A low dropout internal regula- The error signal developed at the amplifier output is brought tor provides a 2.3V supply for all internal circuitry on the out externally and is used for frequency compensation. LT1512. This low dropout design allows input voltage to During normal regulator operation this pin sits at a voltage vary from 2.7V to 25V. A 500kHz oscillator is the basic between 1V (low output current) and 1.9V (high output clock for all internal timing. It turns “on” the output switch current). Switch duty cycle goes to zero if the V via the logic and driver circuitry. Special adaptive antisat C pin is pulled below the V circuitry detects onset of saturation in the power switch C pin threshold, placing the LT1512 in an idle mode. and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch.
applicaTions inForMaTion
The LT1512 is an IC battery charger chip specifically op- R2(V – 1 245 . ) timized to use the SEPIC converter topology. The SEPIC R BAT 1= topology has unique advantages for battery charging. It 1 245 . +R2 0 ( . µ 3 A) will operate with input voltages above, equal to or below V the battery voltage, has no path for battery discharge when BAT = battery float voltage turned off and eliminates the snubber losses of flyback 0.3µA = typical FB pin bias current designs. It also has a current sense point that is ground A value of 12.4k for R2 sets divider current at 100µA. referred and need not be connected directly to the battery. This is a constant drain on the battery when power to the The two inductors shown are actually just two identical charger is off. If this drain is too high, R2 can be increased windings on one inductor core, although two separate to 41.2k, reducing divider current to 30µA. This introduces inductors can be used. an additional uncorrectable error to the constant voltage A current sense voltage is generated with respect to ground float mode of about ±0.5% as calculated by: across R3 in Figure 1. The average current through R3 is 0.15 ± A( µ R1)(R2) always identical to the current delivered to the battery. The V Error= BAT LT1512 current limit loop will servo the voltage across R3 1.245(R1+R2) to –100mV when the battery voltage is below the voltage ±0.15µA = expected variation in FB bias current around limit set by the output divider R1/R2. Constant current the nominal 0.3µA typical value. charging is therefore set at 100mV/R3. R4 and C4 filter the current signal to deliver a smooth feedback voltage to with R2 = 41.2k and R1 = 228k, (VBAT = 8.2V), the error the I due to variations in bias current would be ±0.42%. FB pin. R1 and R2 form a divider for battery voltage sensing and set the battery float voltage. The suggested A second option is to disconnect the voltage divider with value for R2 is 12.4k. R1 is calculated from: a small NMOS transistor as shown in Figure 3. To ensure 1512fc For more information www.linear.com/LT1512 7 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts