LTC3559 Linear Technology, LTC3559 Datasheet - Page 17

• Manufacturer Part Number: LTC3559
• Description: Linear USB Battery Charger
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Power Dissipation
The conditions that cause the LTC3559 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. For high
charge currents, the LTC3559 power dissipation is
approximately:
where P
voltage, V
current. It is not necessary to perform any worst-case
power dissipation scenarios because the LTC3559 will
automatically reduce the charge current to maintain the
die temperature at approximately 105°C. However, the
approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
Example: Consider an LTC3559 operating from a USB port
providing 500mA to a 3.5V Li-Ion battery. The ambient
temperature above which the LTC3559 will begin to reduce
the 500mA charge current is approximately:
The LTC3559 can be used above 70°C, but the charge cur-
rent will be reduced from 500mA. The approximate current
at a given ambient temperature can be calculated:
Using the previous example with an ambient tem-
perature of 88°C, the charge current will be reduced to
approximately:
P
I
I
I
T
T
T
T
T
BAT
BAT
B B AT
D
A
A
A
A
A
=
=
=
=
=
=
=
=
= 167
(
D
105
105
105
105
54
V
(
(
is the power dissipated, V
BAT
CC
5
°
V
C
°
°
°
°
CC
V
–
C P
C
C
C
105
mA
is the battery voltage, and I
– .
105
V
–
–
–
– . .
–
3 5
BAT
(
(
0
V
°
5
D JA
V
BAT
°
C T
75
θ
CC
V
C
V
)
–
)
– .
–
•
W
)
–
•
I
3 5
88
BAT
• θ
A
68
V
•
BAT
68
°
JA
°
V
C
C W
) (
°
)
/
•
C W
•
/
I
500
BAT
=
=
mA
90
CC
•
105
17
θ
°
is the input supply
JA
)
C A
°
•
BAT
C
°
/
68
C
–
is the charge
°
C W
45
/
°
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
It is important to remember that LTC3559 applications do
not need to be designed for worst-case thermal conditions
since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately
105°C.
Battery Charger Stability Considerations
The LTC3559 battery charger contains two control loops:
the constant voltage and constant current loops. The con-
stant voltage loop is stable without any compensation when
a battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1.5μF from BAT
to GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
High value capacitors with very low ESR (especially
ceramic) reduce the constant voltage loop phase margin,
possibly resulting in instability. Ceramic capacitors up to
22μF may be used in parallel with a battery, but larger
ceramics should be decoupled with 0.2Ω to 1Ω of series
resistance.
In constant current mode, the PROG pin is in the feedback
loop, not the battery. Because of the additional pole created
by the PROG pin capacitance, capacitance on this pin must
be kept to a minimum. With no additional capacitance on
the PROG pin, the charger is stable with program resistor
values as high as 25K. However, additional capacitance on
this node reduces the maximum allowed program resis-
tor. The pole frequency at the PROG pin should be kept
above 100kHz. Therefore, if the PROG pin is loaded with a
capacitance, C
to calculate the maximum resistance value for R
R
PROG
≤
2
π •
PROG
10
, the following equation should be used
5
1
•
C
PROG
LTC3559
PROG
17
3559f
: