LT1512
10
1512fc
For more information www.linear.com/LT1512
applicaTions inForMaTion
With I
CHRG
= 0.5A, V
IN
= 15V and V
BAT
= 8.2V, I
COUP
= 0.43A
The recommended capacitor is a 2.2µF ceramic type from
Marcon or Tokin. These capacitors have extremely low ESR
and high ripple current ratings in a small package. Solid
tantalum units can be substituted if their ripple current
rating is adequate, but typical values will increase to 22µF
or more to meet the ripple current requirements.
Diode Selection
The switching diode should be a Schottky type to minimize
both forward and reverse recovery losses. Average diode
current is the same as output charging current , so this
will be under 1A. A 1A diode is recommended for most
applications, although smaller devices could be used at
reduced 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 currents
(2µA to 200µA) even at room temperature. The latest very-
low-forward devices have especially high leakage currents.
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 typi
-
cal 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
IRVVV
V
SW
CHRG SW BAT IN BAT
=
+()()
()
2
2
R
SW
= output switch ON resistance
Total power dissipation of the die is equal to supply current
times supply voltage, plus switch power:
P
D(TOTAL)
= (I
IN
)(V
IN
) + P
SW
For V
IN
= 10V, V
BAT
= 8.2V, I
CHRG
= 0.5A, R
SW
= 0.65
I
IN
= 4mA + 10mA = 14mA
P
SW
= 0.24W
P
D
= (0.014)(10) + 0.24 = 0.38W
The S8 package has a thermal resistance of 130°C/W.
(Contact factory concerning 16-lead fused-lead pack
-
age 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
temperature 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
+
C6
1µF
C7
10µF
C4
0.22µF
R3
1512 F05
L1B
I
FB
LT1512
R5
4.02k
PWM
INPUT
≥1kHz
D2
R6
4.02k
R4
200Ω
+
Figure 5. Programming Charge Current
