LTC3559-1_15 Datasheet, PDF (17/24 Pages) Linear Technology – Linear USB Battery Charger with Dual Buck Regulators

LTC3559/LTC3559-1
APPLICATIONS INFORMATION
Power Dissipation
The conditions that cause the LTC3559/LTC3559-1 to
reduce charge current through thermal feedback can be
approximated by considering the power dissipated in the
IC. For high charge currents, the LTC3559/LTC3559-1
power dissipation is approximately:
( ) ^P_D ^{= V}_CC ^{– V}_BAT ^•I_BAT
where P_D is the power dissipated, V_CC is the input supply
voltage, V_BAT is the battery voltage, and I_BAT is the charge
current. It is not necessary to perform any worst-case power
dissipation scenarios because the LTC3559/LTC3559-1
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:
T_A = 105°C – P_Dθ_JA
( ) ^T_A ^{= 105°C – V}_CC ^{– V}_BAT ^•I_BAT ^{• θ}_JA
Example: Consider an LTC3559/LTC3559-1 operating from
a USB port providing 500mA to a 3.5V Li-Ion battery.
The ambient temperature above which the LTC3559/
LTC3559-1 will begin to reduce the 500mA charge cur-
rent is approximately:
T_A = 105°C – (5V – 3.5V) •(500mA) • 68°C / W
T_A = 105°C – 0.75W • 68°C / W = 105°C – 51°
T_A = 54°C
The LTC3559/LTC3559-1 can be used above 70°C, but
the charge current will be reduced from 500mA. The
approximate current at a given ambient temperature can
be calculated:
( ) ^{IBAT =}
105°C – T_A
V_CC – V_BAT • θ_JA
Using the previous example with an ambient tem-
perature of 88°C, the charge current will be reduced to
approximately:
I_BAT
=
(5V
105°C – 88°C
– 3.5V) • 68°C
/
W
=
17°C
102°C /
A
I_BAT = 167mA
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/LTC3559-1
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/LTC3559-1 battery charger contains two
control loops: the constant-voltage and constant-cur-
rent loops. The constant-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 bat-
tery 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
resistor. The pole frequency at the PROG pin should be
kept above 100kHz. Therefore, if the PROG pin is loaded
with a capacitance, C_PROG, the following equation should
be used to calculate the maximum resistance value for
R_PROG:
R_PROG
≤
2π
•
1
10⁵ •
C_PROG
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