9

LT1512

charging current.

Maximum diode reverse voltage will be

equal to input voltage plus battery voltage.

Diode reverse leakage current will be of some concern

during charger shutdown. This leakage current is a direct

drain on the battery when the charger is not powered. High

current Schottky diodes have relatively high leakage cur-

rents (2

µA to 200µA) even at room temperature. The latest

very-low-forward devices have especially high leakage cur-

rents. It has been noted that surface mount versions of some

Schottky diodes have as much as ten times the leakage of

their through-hole counterparts. This may be because a low

forward voltage process is used to reduce power dissipation

in the surface mount package. In any case, check leakage

specifications carefully before making a final choice for the

switching diode. Be aware that diode manufacturers want to

specify a maximum leakage current that is ten times higher

than the typical leakage. It is very difficult to get them to

specify a low leakage current in high volume production.

This is an on going problem for all battery charger circuits

and most customers have to settle for a diode whose typical

leakage is adequate, but theoretically has a worst-case

condition of higher than desired battery drain.

Thermal Considerations

Care should be taken to ensure that worst-case conditions

do not cause excessive die temperatures. Typical thermal

resistance is 130

°C/W for the S8 package but this number

will vary depending on the mounting technique (copper

area, air flow, etc).

Average supply current (including driver current) is:

ImA

VI

V

IN

BAT

CHRG

IN

=+

4

0 024

()(

)( .

)

Switch power dissipation is given by:

P

IR

V

V

V

V

SW

CHRG

SW

BAT

IN

BAT

IN

=

+

() (

)(

)(

)

()

2

2

Total power dissipation of the die is equal to supply current

times supply voltage, plus switch power:

The S8 package has a thermal resistance of 130

°C/W.

(Contact factory concerning 16-lead fused-lead package

with footprint approximately same as S8 package and with

lower thermal resistance.) Die temperature rise will be

(0.38W)(130

°C/W) = 49°C. A maximum ambient tempera-

ture of 60

°C will give a die temperature of 60°C + 49°C =

109

°C. This is only slightly less than the maximum junction

temperature of 125

°C, illustrating the importance of doing

these calculations!

Programmed Charging Current

LT1512 charging current can be programmed with a PWM

signal from a processor as shown in Figure 5. C6 and D2

form a peak detector that converts a positive logic signal to a

negative signal. The average negative signal at the input to

inverse PWM ratio. This assumes that the PWM signal is a

CMOS output that swings rail-to-rail with a source resis-

tance less than a few hundred ohms. The negative voltage is

converted to a current by R5 and R6 and filtered by C7. This

current multiplied by R4 generates a voltage that subtracts

from the 100mV sense voltage of the LT1512. This is not a

the diode voltage, but it can typically be used to adjust

charging current over a 20% to 100% range with good

repeatability (full charging current accuracy is not affected).

To reduce the load on the logic signal, R4 has been increased

C6

1

µF

C7

10

µF

C4

0.22

µF

R3

1512 F05

L1B

LT1512

R5

4.02k

PWM

INPUT

≥1kHz

D2

R6

4.02k

R4

200

Ω

+

Figure 5. Programming Charge Current