Datasheet LT1513, LT1513-2 (Analog Devices) - 9

LT1513/LT1513-2
U U W U APPLICATIONS INFORMATION
current Schottky diodes have relatively high leakage cur- vary depending on the mounting technique (copper area, rents (5µA to 500µA) even at room temperature. The latest airflow, etc.). very-low-forward devices have especially high leakage cur- Average supply current (including driver current) is: rents. It has been noted that surface mount versions of some Schottky diodes have as much as ten times the leakage of V ( ) I ( ) 0 ( 024 . ) I = m 4 A BAT CHRG IN + their through-hole counterparts. This may be because a low VIN forward voltage process is used to reduce power dissipation in the surface mount package. In any case, check leakage Switch power dissipation is given by: specifications carefully before making a final choice for the I ( )2 R ( ) V ( V ) V CHRG SW BAT IN ( BAT) switching diode. Be aware that diode manufacturers want PSW = + 2 to specify a maximum leakage current that is ten times V ( IN) higher than the typical leakage. It is very difficult to get them R to specify a low leakage current in high volume production. SW = Output switch ON resistance This is an on going problem for all battery charger circuits Total power dissipation of the die is equal to supply current and most customers have to settle for a diode whose typical times supply voltage, plus switch power: leakage is adequate, but theoretically has a worst-case PD(TOTAL) = (IIN)(VIN) + PSW condition of higher than desired battery drain. For VIN = 10V, VBAT = 8.2V, ICHRG = 1.2A, RSW = 0.3Ω,
Thermal Considerations
IIN = 4mA + 24mA = 28mA Care should be taken to ensure that worst-case conditions PSW = 0.64W do not cause excessive die temperatures. Typical thermal P resistance is 30°C/W for the R package but this number will D = (10)(0.028) + 0.64 = 0.92W GROUND PLANE VBAT + + C1 C1 D1 LT1513 TAB AND GROUND PIN SOLDERED TO GROUND PLANE C2 C5 2 WINDING L1A L1B INDUCTOR + R5 R3 C3 C1,C3,C5 AND R3 TIED DIRECTLY TO GROUND PLANE LT1513 • F04 VIN
Figure 4. LT1513 Suggested Partial Layout for Critical Thermal and Electrical Paths
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