LTC3550 Datasheet, PDF (18/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate
is switched from high to low to high again, a packet of
charge, dQ, moves from V_CC to ground. The resulting
dQ/dt is the current out of V_CC that is typically larger
than the DC bias current. In continuous mode, I_GATECHG
= f(Q_T + Q_B) where Q_T and Q_B are the gate charges of
the internal top and bottom switches. Both the DC bias
and gate charge losses are proportional to V_CC and
thus their effects will be more pronounced at higher
supply voltages.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R_DS(ON) and the duty cycle
(DC) as follows:
R_SW = (R_DS(ON)TOP)(DC) + (R_DS(ON)BOT)(1 – DC)
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I²R losses, simply add R_SW to R_L
and multiply the result by the square of the average output
current. Other losses including C_IN and C_OUT ESR dissipa-
tive losses and inductor core losses generally account for
less than 2% total additional loss.
Thermal Considerations
The battery charger’s thermal regulation feature and the
buck regulator’s high efficiency make it unlikely that enough
power will be dissipated to exceed the LTC3550 maximum
junction temperature. Nevertheless, it is a good idea to
do some thermal analysis for worst-case conditions.
The junction temperature, T_J, is given by: T_J = T_A + T_RISE
where T_A is the ambient temperature. The temperature
rise is given by:
T_RISE = P_D • θ_JA
where P_D is the power dissipated and θ_JA is the thermal
resistance from the junction of the die to the ambient
temperature.
18
In most applications the buck regulator does not dissi-
pate much heat due to its high efficiency. The majority of
the LTC3550 power dissipation occurs when charging a
battery. Fortunately, the LTC3550 automatically reduces
the charge current during high power conditions using
a patented thermal regulation circuit. Thus, there is no
need to design for worst-case power dissipation scenarios
because the LTC3550 ensures that the battery charger
power dissipation never raises the junction temperature
above a preset value of 105°C. In the unlikely case that
the junction temperature is forced above 105°C (due to
abnormally high ambient temperatures or excessive buck
regulator power dissipation), the battery charge current will
be reduced to zero and thus dissipate no heat. As an added
measure of protection, even if the junction temperature
reaches approximately 150°C, the buck regulator’s power
switches will be turned off and the SW node will become
high impedance.
The conditions that cause the LTC3550 to reduce charge
current through thermal feedback can be approximated by
considering the power dissipated in the IC. The approxi-
mate ambient temperature at which the thermal feedback
begins to protect the IC is:
T_A = 105°C – T_RISE
T_A = 105°C – (P_D • θ_JA)
T_A = 105°C – (P_D(CHARGER) + P_D(BUCK)) • θ_JA
(5)
Most of the charger’s power dissipation is generated from
the internal charger MOSFET. Thus, the power dissipation
is calculated to be:
P_D(CHARGER) = (V_IN – V_BAT) • I_BAT
(6)
V_IN is the charger supply voltage (either DCIN or USBIN),
V_BAT is the battery voltage and I_BAT is the charge cur-
rent.
Example: An LTC3550 operating from a 5V wall adapter
(on the DCIN input) is programmed to supply 650mA
full-scale current to a discharged Li-Ion battery with a
voltage of 3V.
The charger power dissipation is calculated to be:
P_D(CHARGER) = (5V – 3V) • 650mA = 1.3W
3550fa