Datasheet LT1513, LT1513-2 (Analog Devices) - 8

LT1513/LT1513-2
U U W U APPLICATIONS INFORMATION
shown in the maximum charging current graph. Higher in Figure 1. These are AVX type TPS or Sprague type 593D inductance values give slightly higher maximum charging surface mount solid tantalum units intended for switching current, but are larger and more expensive. A low loss toroid applications. Do not substitute other types without ensuring core such as Kool Mµ®, Molypermalloy or Metglas® is that they have adequate ripple current ratings. See Input recommended. Series resistance should be less than 0.04Ω Capacitor section for details of surge limitation on solid for each winding. “Open core” inductors, such as rods or tantalum capacitors if the battery may be “hot switched” to barrels are not recommended because they generate large the output of the charger. magnetic fields which may interfere with other electronics close to the charger.
Coupling Capacitor
C2 in Figure 1 is the coupling capacitor that allows a SEPIC
Input Capacitor
converter topology to work with input voltages either higher The SEPIC topology has relatively low input ripple current or lower than the battery voltage. DC bias on the capacitor is compared to other topologies and higher harmonics are equal to input voltage. RMS ripple current in the coupling especially low. RMS ripple current in the input capacitor is capacitor has a maximum value of about 1A at full charging less than 0.25A with L = 10µH and less than 0.5A with current. A conservative formula to calculate this is: L = 5µH. A low ESR 22µF, 25V solid tantalum capacitor (AVX type TPS or Sprague type 593D) is adequate for most I V ( V )( . ) I CHRG IN BAT = + 1 1 applications with the following caveat. Solid tantalum COUP RMS ( ) 2 V ( IN) capacitors can be destroyed with a very high turn-on surge current such as would be generated if a low impedance input (1.1 is a fudge factor to account for inductor ripple current source were “hot switched” to the charger input. If this and other losses) condition can occur, the input capacitor should have the With ICHRG = 1.2A, VIN = 15V and VBAT = 8.2V, ICOUP = 1.02A. highest possible voltage rating, at least twice the surge input voltage if possible. Consult with the capacitor manufacturer The recommended capacitor is a 4.7µF ceramic type from before a final choice is made. A 4.7µF ceramic capacitor such Marcon or Tokin. These capacitors have extremely low ESR as the one used for the coupling capacitor can also be used. and high ripple current ratings in a small package. Solid These capacitors do not have a turn-on surge limitation. The tantalum units can be substituted if their ripple current rating input capacitor must be connected directly to the V is adequate, but typical values will increase to 22µF or more IN pin and the ground plane close to the LT1513. to meet the ripple current requirements.
Output Capacitor Diode Selection
It is assumed as a worst case that all the switching output The switching diode should be a Schottky type to minimize ripple current from the battery charger could flow in the both forward and reverse recovery losses. Average diode output capacitor. This is a desirable situation if it is neces- current is the same as output charging current, so this will be sary to have very low switching ripple current in the battery under 2A. A 3A diode is recommended for most applications, itself. Ferrite beads or line chokes are often inserted in series although smaller devices could be used at reduced charging with the battery leads to eliminate high frequency currents current. Maximum diode reverse voltage will be equal to that could create EMI problems. This forces all the ripple input voltage plus battery voltage. current into the output capacitor. Total RMS current into the Diode reverse leakage current will be of some concern capacitor has a maximum value of about 1A, and this is during charger shutdown. This leakage current is a direct handled with the two paralleled 22µF, 25V capacitors shown drain on the battery when the charger is not powered. High Kool Mµ is a registered trademark of Magnetics, Inc. Metglas is a registered trademark of AlliedSignal Inc. sn1513 1513fas 8