LTC3550_15 Datasheet, PDF (19/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
For simplicity, assume the buck regulator is disabled and
dissipates no power (P_D(BUCK) = 0). For a properly soldered
DHC16 package, the thermal resistance (θ_JA) is 40°C/W.
Thus, the ambient temperature at which the LTC3550
charger will begin to reduce the charge current is:
T_A = 105°C – (1.3W • 40°C/W)
T_A = 105°C – 52°C
T_A = 53°C
The LTC3550 can be used above 53°C ambient, but the
charge current will be reduced from 650mA. Assum-
ing no power dissipation from the buck converter, the
approximate current at a given ambient temperature can
be approximated by:
I_BAT
=
105°C – T_A
(V_IN – V_BAT ) • θ_JA
(7)
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I_BAT
=
105°C – 60°C
(5V – 3V) • 40°C/W
=
45°C
80°C/A
I_BAT = 563mA
Because the regulator typically dissipates significantly less
power than the charger (even in worst-case situations),
the calculations here should work well as an approxima-
tion. However, the user may wish to repeat the previous
analysis to take the buck regulator’s power dissipation into
account. Equation (7) can be modified to take into account
the temperature rise due to the buck regulator:
I_BAT
=
105°C – T_A − (P_D(BUCK) •
(V_IN – V_BAT ) • θ_JA
θ_JA )
(8)
For optimum performance, it is critical that the exposed
metal pad on the backside of the LTC3550 package is
properly soldered to the PC board ground. When correctly
soldered to a 2500mm² double sided 1oz copper board, the
LTC3550 has a thermal resistance of approximately 40°C/W.
Failure to make thermal contact between the exposed pad
on the backside of the package and the copper board will
result in thermal resistances far greater than 40°C/W. As
an example, a correctly soldered LTC3550 can deliver over
800mA to a battery from a 5V supply at room temperature.
Without a good backside thermal connection, this number
would drop to much less than 500mA.
Battery Charger Stability Considerations
The constant-voltage mode feedback loop is stable without
any compensation provided a battery is connected to the
charger output. When the charger is in constant-current
mode, the charge current program pin (IDC or IUSB) is in
the feedback loop, not the battery. The constant-current
mode stability is affected by the impedance at the charge
current program pin. With no additional capacitance on
this pin, the charger is stable with program resistor val-
ues as high as 20k (I_CHG = 50mA); however, additional
capacitance on these nodes reduces the maximum allowed
program resistor value.
Checking Regulator Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V_OUT immediately shifts by an amount
equal to (ΔI_LOAD • ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT, which generates a feedback error signal. The
regulator loop then acts to return V_OUT to its steady state
value. During this recovery time V_OUT can be monitored
for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in paral-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
can deliver enough current to prevent this problem if the
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive
so that the load rise time is limited to approximately (25
• C_LOAD). Thus, a 10µF capacitor charging to 3.3V would
require a 250µs rise time, limiting the charging current
to about 130mA.
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