LTC3550 Linear Technology, LTC3550 Datasheet - Page 19

• Manufacturer Part Number: LTC3550
• Description: Dual Input USB/AC Adapter Li-Ion Battery Charger
• Manufacturer: Linear Technology

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For simplicity, assume the buck regulator is disabled and
dissipates no power (P
DHC16 package, the thermal resistance (θ
Thus, the ambient temperature at which the LTC3550
charger will begin to reduce the charge current is:
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:
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
Because the regulator typically dissipates signifi cantly 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 modifi ed to take into account
the temperature rise due to the buck regulator:
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 2500mm
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
APPLICATIO S I FOR ATIO
T
T
T
I
I
I
I
BAT
BAT
BA
BAT
A
A
A
= 105°C – (1.3W • 40°C/W)
= 105°C – 52°C
= 53°C
T T
=
=
= 563
=
(
(
105
5
V
IN
V
105
105
mA
–
°
–
C T
3
(
V
V
°
–
V
°
BAT
C
IN
C T
) •
U
–
A
–
–
2
D(BUCK)
40
) • θ
60
−
double sided 1oz copper board, the
V
A
BAT
(
°
°
P
C W
U
C
JA
D BUCK
/
(
) •
= 0). For a properly soldered
θ
=
JA
80
)
45
•
W
°
θ
C A
°
JA
C
/
)
JA
) is 40°C/W.
U
(7)
(8)
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
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
equal to (ΔI
resistance of C
charge C
regulator loop then acts to return V
value. During this recovery time V
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
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
require a 250µs rise time, limiting the charging current
to about 130mA.
LOAD
). Thus, a 10µF capacitor charging to 3.3V would
OUT
OUT
LOAD
, which generates a feedback error signal. The
, causing a rapid drop in V
OUT
• ESR), where ESR is the effective series
. ΔI
OUT
CHG
LOAD
immediately shifts by an amount
= 50mA); however, additional
also begins to charge or dis-
OUT
OUT
can be monitored
to its steady state
LTC3550
OUT
. No regulator
19
3550fa