Datasheet
15
LTC4001
4001f
Battery charging current continues to drop as the V
BAT
rises, dropping to zero at V
FLOAT.
Because the voltage drop
in the LTC4001 is very low when charge current is highest,
power dissipation is also very low.
Thermal Calculations (PWM and Trickle Charging)
The LTC4001 operates as a linear charger when condition-
ing (trickle) charging a battery and operates as a high rate
buck battery charger at all other times. Power dissipation
should be determined for both operating modes.
For linear charger mode:
P
D
= (V
IN
– V
BAT
) • I
TRIKL
+ V
IN
• I
IN
where I
IN
is V
IN
current consumed by the IC.
Worst-case dissipation occurs for V
BAT
= 0, maximum V
IN
,
and maximum quiescent and trickle charge current. For
example with 5.5V maximum input voltage and 65mA
worst case trickle charge current, and 2mA worst-case
chip quiescent current:
P
D
= (5.5 – 0) • 65mA + 5.5 • 2mA = 368.5mW
LTC4001 power dissipation is very low if a current limited
wall adapter is used and allowed to enter current limit.
When the wall adapter is in current limit, the voltage drop
across the LTC4001 charger is:
V
DROP
= I
LIMIT
• R
PFET
where I
LIMIT
is the wall adapter current limit and R
PFET
is
the on resistance of the topside PMOS switch.
The total LTC4001 power dissipation during current lim-
ited charging is:
P
D
= (V
BAT
+ V
DROP
) • (I
IN
+ I
P
) + V
DROP
• I
LIMIT
where I
IN
is the chip quiescent current and I
P
is total
current flowing through the IDET and PROG programming
pins. Maximum dissipation in this mode occurs with the
highest V
BAT
that keeps the wall adapter in current limit
(which is very close to V
FLOAT
), highest quiescent current
I
IN
, highest PMOS on resistance R
PFET
, highest I
LIMIT
and
highest programming current I
P
.
Assume the LTC4001 is programmed for 2A charging and
200mA IDET and that a 1.5A wall adapter is being used:
I
LIMIT
= 1500mA, R
PFET
= 127mΩ, I
IN
= 2mA, I
P
= 4mA and
V
BAT
≈ V
FLOAT
= 4.242V
then:
V
DROP
= 1500mA • 127mΩ = 190.5mV
and:
P
D
= (4.242V + 0.1905V) • (2mA + 4mA) + 0.1905V
• 1500mA = 312mW
Power dissipation in buck battery charger mode may be
estimated from the dissipation curves given in the Typical
Performance Characteristics section of the data sheet.
This will slightly overestimate chip power dissipation
because it assumes all loss, including loss from external
components, occurs within the chip.
APPLICATIO S I FOR ATIO
WUUU
Figure 5. Charging Characteristic
LINEAR CHARGING
V
ADAPTER
V
IN
V
TRIKL
V
FLOAT
4001 F05
V
BAT
I
BAT
I
TRICKLE
WALL ADAPTER IN CURRENT LIMIT
V
BAT
+ V
DROP
I
LIMIT
PWM
CHARGING