LT1512
APPLICATIONS INFORMATION
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碌A(chǔ) to 200碌A(chǔ)) 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擄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:
For V
IN
= 10V, V
BAT
= 8.2V, I
CHRG
= 0.5A, R
SW
= 0.65鈩?/div>
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擄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擄C/W) = 49擄C. A maximum ambient tempera-
ture of 60擄C will give a die temperature of 60擄C + 49擄C =
109擄C. This is only slightly less than the maximum junction
temperature of 125擄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
R5 is equal to the processor V
CC
level multiplied by the
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
high precision technique because of the errors in V
CC
and
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
I
IN
=
4mA
+
(V
BAT
)(I
CHRG
)(0.024)
V
IN
Switch power dissipation is given by:
(I
CHRG
)
2
(R
SW
)(V
BAT
+
V
IN
)(V
BAT
)
P
SW
=
(V
IN
)
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
+
PWM
INPUT
鈮?kHz
U
W
U
U
LT1512
I
FB
C6
1碌F
R5
4.02k
D2
R6
4.02k
C7
10碌F
R4
200鈩?/div>
C4
0.22碌F
L1B
+
R3
1512 F05
Figure 5. Programming Charge Current
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