Datasheet LT1512 (Analog Devices) - 10

LT1512
applicaTions inForMaTion
with ICHRG = 0.5A, VIN = 15V and VBAT = 8.2V, ICOUP = 0.43A Average supply current (including driver current) is: The recommended capacitor is a 2.2µF ceramic type from (V ) I( ) 0 ( 024 . ) Marcon or Tokin. These capacitors have extremely low ESR I = m 4 A BAT CHRG IN + V and high ripple current ratings in a small package. Solid IN tantalum units can be substituted if their ripple current Switch power dissipation is given by: rating is adequate, but typical values will increase to 22µF or more to meet the ripple current requirements. (I )2 R ( )(V + V )(V ) P CHRG SW BAT IN BAT SW = 2
Diode Selection
(VIN) The switching diode should be a Schottky type to minimize RSw = output switch ON resistance both forward and reverse recovery losses. Average diode current is the same as output charging current , so this Total power dissipation of the die is equal to supply current will be under 1A. A 1A diode is recommended for most times supply voltage, plus switch power: applications, although smaller devices could be used at PD(TOTAL) = (IIN)(VIN) + PSw reduced charging current. Maximum diode reverse voltage For V will be equal to input voltage plus battery voltage. IN = 10V, VBAT = 8.2V, ICHRG = 0.5A, RSw = 0.65Ω I Diode reverse leakage current will be of some concern IN = 4mA + 10mA = 14mA during charger shutdown. This leakage current is a direct PSw = 0.24w drain on the battery when the charger is not powered. High P current Schottky diodes have relatively high leakage currents D = (0.014)(10) + 0.24 = 0.38w (2µA to 200µA) even at room temperature. The latest very- The S8 package has a thermal resistance of 130°C/w. low-forward devices have especially high leakage currents. (Contact factory concerning 16-lead fused-lead pack- It has been noted that surface mount versions of some age with footprint approximately same as S8 package Schottky diodes have as much as ten times the leakage of and with lower thermal resistance.) Die temperature rise their through-hole counterparts. This may be because a low will be (0.38w)(130°C/w) = 49°C. A maximum ambient forward voltage process is used to reduce power dissipation temperature of 60°C will give a die temperature of 60°C + in the surface mount package. In any case, check leakage 49°C = 109°C. This is only slightly less than the maximum specifications carefully before making a final choice for the junction temperature of 125°C, illustrating the importance switching diode. Be aware that diode manufacturers want to of doing these calculations! specify a maximum leakage current that is ten times higher than the typical leakage. It is very difficult to get them to
Programmed Charging Current
specify a low leakage current in high volume production. LT1512 charging current can be programmed with a PwM This is an on going problem for all battery charger circuits signal from a processor as shown in Figure 5. C6 and D2 and most customers have to settle for a diode whose typi- form a peak detector that converts a positive logic signal cal leakage is adequate, but theoretically has a worst-case to a negative signal. The average negative signal at the condition of higher than desired battery drain.
Thermal Considerations
LT1512 IFB Care should be taken to ensure that worst-case conditions C6 L1B R6 R4 PWM +1µF R5 4.02k 4.02k 200Ω do not cause excessive die temperatures. Typical thermal INPUT ≥1kHz resistance is 130°C/w for the S8 package but this number C7 C4 D2 10µF 0.22µF R3 + will vary depending on the mounting technique (copper 1512 F05 area, air flow, etc).
Figure 5. Programming Charge Current
1512fc 10 For more information www.linear.com/LT1512 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