LTC4001-1_15 Datasheet, PDF (15/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

APPLICATIONS INFORMATION
LINEAR CHARGING
V_ADAPTER
V_IN
WALL ADAPTER IN CURRENT LIMIT
PWM
CHARGING
V_BAT + V_DROP
LTC4001-1
I_BAT
I_TRICKLE
I_LIMIT
V_TRIKL
V_BAT
40011 F05
V_FLOAT
Figure 5. Charging Characteristic
to V_ADAPTER. Battery charging current continues to drop
as the V_BAT rises, dropping to zero at V_FLOAT. Because the
voltage drop in the LTC4001-1 is very low when charge
current is highest, power dissipation is also very low.
Thermal Calculations (PWM and Trickle Charging)
The LTC4001-1 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
LTC4001-1 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 LTC4001-1 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 LTC4001-1 power dissipation during current
limited 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 cur-
rent 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 LTC4001-1 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 = 4.141V
then:
V_DROP = 1500mA • 127mΩ = 190.5mV
and:
P_D = (4.141V + 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.
40011fa
15