Datasheet

LTC1730-4/LTC1730-4.2
10
sn1730 1730fs
Surface mount packages rely primarily on the copper leads
(pins) to conduct the heat from the package to the sur-
rounding PC board copper which is acting as a heat sink.
The ground pin is especially important for conduct ing heat
as well as providing an electrical connection. Use gener-
ous amounts of copper around the ground pin and also
consider feedthrough vias (plated through holes) to back-
side or inner copper layers to maximize power dissipation.
Table 1 lists the thermal resistance for the SO-8 package.
Measured values of thermal resistance for several differ-
ent board sizes and copper areas are listed. All measure-
ments were taken in still air on 0.062" FR-4 board with one
ounce copper.
Calculating Junction Temperature
Example: Find the maximum junction temperature for a
battery voltage of 4V (V
CC
will collapse to approximately
V
BAT
in the fast charge mode), charge current of 1A and a
maximum ambient temperature of 75°C.
The power dissipated by the IC is:
P=(I
BAT
• I
BAT
• R
DS(ON)
) + (V
CC
• I
CC
)
= (1A • 1A • 0.35Ω) + (4V • 1mA)
= 0.35 + 0.004
= 0.354W
The SO-8 package for the LTC1730 features a special lead
frame with a lower thermal resistance and higher allow-
able power dissipation. The junction-to-ambient thermal
resistance of this package when soldered to a PC board is
approximately 80°C/W depending on the copper area. So
the junction temperature rise above ambient will be
approximately:
(0.354W)(80°C/W) = 28.3°C
The maximum junction temperature is equal to the maxi-
mum junction temperature rise above ambient plus the
maximum ambient temperature or:
T
JMAX
= 75°C + 28.3°C = 103.3°C
Table 1. SO-8 Package Thermal Resistance
COPPER AREA TOTAL THERMAL RESISTANCE
TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
80°C/W
1000mm
2
2500mm
2
2500mm
2
80°C/W
250mm
2
2500mm
2
2500mm
2
85°C/W
*Device is mounted on topside.
Output Voltage Selection (LTC1730EGN-4)
The float voltage at the BAT pin can be selected by the SEL
pin. Shorting the SEL pin to ground will set the float
voltage to 4.1V, while connecting it to V
CC
sets it to 4.2V.
This feature allows the charger to be used with different
types of Li-Ion cells.
ACPR Output Pin (LTC1730EGN-4)
When the input voltage is 40mV higher than the voltage at
the BAT pin, the ACPR pin is pulled low to ground to
indicate that the input supply (wall adapter) is applied.
After the input supply is removed, this pin becomes high
impedance. An internal 80ms delay prevents the LTC1730
from turning off if the voltage at the V
CC
pin rings and gets
too close to V
BAT
because of the parasitic inductance.
FAULT Output Pin (LTC1730EGN-4)
The FAULT pin is pulled to ground when either one of the
following conditions is detected:
1. The voltage at the NTC/SHDN pin is out of the operation
range. With a 10k negative temperature coefficient
thermistor placed near the battery pack, this pin indi-
cates that the temperature of the pack is either too cold
or too hot and the charger is suspended. Once the
temperature is back to the operational range, the FAULT
pin becomes high impedance and the charging re-
sumes.
2. The voltage at the BAT pin stays below 2.45V for 1/4 of
the programmed charge time. If V
BAT
remains low even
with 35mA of trickle charge current for 1/4 of the total
APPLICATIO S I FOR ATIO
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