Datasheet LT1512 (Analog Devices) - 9

LT1512
applicaTions inForMaTion
especially low. RMS ripple current in the input capacitor is forces all the ripple current into the output capacitor. Total less than 0.1A with L = 33µH and less than 0.2A with L = RMS current into the capacitor has a maximum value of 15µH. A low ESR 22µF, 25V solid tantalum capacitor (AVx about 0.5A, and this is handled with a 22µF, 25V capacitor type TPS or Sprague type 593D) is adequate for most appli- shown in Figure 1. This is an AVx type TPS or Sprague cations with the following caveat. Solid tantalum capacitors type 593D surface mount solid tantalum unit intended can be destroyed with a very high turn-on surge current for switching applications. Do not substitute other types such as would be generated if a low impedance input source without ensuring that they have adequate ripple current were “hot switched” to the charger input. If this condition ratings. See Input Capacitor section for details of surge can occur, the input capacitor should have the highest pos- limitation on solid tantalum capacitors if the battery may sible voltage rating, at least twice the surge input voltage if be “hot switched” to the output of the charger. possible. Consult with the capacitor manufacturer before a final choice is made. A 2.2µF ceramic capacitor such as
Coupling Capacitor
the one used for the coupling capacitor can also be used. C2 in Figure 1 is the coupling capacitor that allows a SEPIC These capacitors do not have a turn-on surge limitation. converter topology to work with input voltages either The input capacitor must be connected directly to the VIN higher or lower than the battery voltage. DC bias on the pin and the ground plane close to the LT1512. capacitor is equal to input voltage. RMS ripple current in the coupling capacitor has a maximum value of about
Output Capacitor
0.5A at full charging current. A conservative formula to It is assumed as a worst case that all the switching out- calculate this is: put ripple current from the battery charger could flow in the output capacitor. This is a desirable situation if it is I (V + V )( . 1 ) 1 I CHRG IN BAT = necessary to have very low switching ripple current in COUP RM ( S) ( 2 VIN) the battery itself. Ferrite beads or line chokes are often inserted in series with the battery leads to eliminate high (1.1 is a fudge factor to account for inductor ripple current frequency currents that could create EMI problems. This and other losses) GND VIN 1 4 +VIN R4 R1 4 3 L1B 2 WINDING L1A L1B INDUCTOR R3 C4 R2 2 3 D1 C2 1 L1A 2 D1 C3 C2B C2A VBATT S/S + C1 C3 + R5 C5 C1 VBATT U1 C5 R3 R4 GND GND V SW R1 R2 GND V IN R5 GND S 1512 F04a C C4 V FB I FB S/S S/S
a. Double-Sided (Vias Connect to the Backside of Ground Plane. Dash
1512 F04b
Lines Indicate Interconnects on Backside. Demo Board Uses This Layout, Except that R5 Has Been Added to Increase Phase Margin) b. Single-Sided Alternative Layout Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths
1512fc For more information www.linear.com/LT1512 9 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