LTC3557_15 Datasheet, PDF (25/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
APPLICATIONS INFORMATION
no additional capacitance on the PROG pin, the battery
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 has a parasitic capacitance,
C_PROG, the following equation should be used to calculate
the maximum resistance value for R_PROG:
R_PROG
≤
2π
1
• 100kHz
• C_PROG
Printed Circuit Board Power Dissipation
Considerations
In order to be able to deliver maximum charge current
under all conditions, it is critical that the Exposed Pad on
the backside of the LTC3557/LTC3557-1 package is soldered
to a ground plane on the board. Correctly soldered to a
2500mm² ground plane on a double-sided 1oz copper
board, the LTC3557/LTC3557-1 has a thermal resistance
(θ_JA) of approximately 37°C/W. Failure to make good
thermal contact between the Exposed Pad on the backside
of the package and an adequately sized ground plane will
result in thermal resistances far greater than 37°C/W.
The conditions that cause the LTC3557/LTC3557-1 to
reduce charge current due to the thermal protection
feedback can be approximated by considering the power
dissipated in the part. For high charge currents and a wall
adapter applied to V_OUT, the LTC3557/LTC3557-1 power
dissipation is approximately:
P_D = (V_OUT – BAT) • I_BAT + P_D(SW1) + P_D(SW2) + P_D(SW3)
where, P_D is the total power dissipated, V_OUT is the supply
voltage, BAT is the battery voltage and I_BAT is the battery
charge current. P_D(SWx) is the power loss by the step-down
switching regulators. The power loss for a step-down
switching regulator can be calculated as follows:
P_D(SWx) = (OUTx • I_OUT) • (100 – Eff)/100
where OUTx is the programmed output voltage, I_OUT is
the load current and Eff is the % efficiency which can be
measured or looked up on an efficiency graph for the
programmed output voltage.
It is not necessary to perform any worst-case power
dissipation scenarios because the LTC3557/LTC3557-1
will automatically reduce the charge current to maintain
the die temperature at approximately 110°C. However, the
approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
T_A = 110°C – P_D • θ_JA
Example: Consider the LTC3557/LTC3557-1 operating
from a wall adapter with 5V (V_OUT) providing 1A (I_BAT)
to charge a Li-Ion battery at 3.3V (BAT). Also assume
P_D(SW1) = P_D(SW2) = P_D(SW3) = 0.05W, so the total power
dissipation is:
P_D = (5V – 3.3V) • 1A + 0.15W = 1.85W
The ambient temperature above which the LTC3557/
LTC3557-1 will begin to reduce the 1A charge current, is
approximately:
T_A = 110°C – 1.85W • 37°C/W = 42°C
The LTC3557/LTC3557-1 can be used above 42°C, but the
charge current will be reduced below 1A. The charge current
at a given ambient temperature can be approximated by:
P_D
=
110°C –
θ_JA
T_A
( ) ^{= V}_OUT ^{– BAT • I}_BAT ^{+ P}_D(SW1) ^{+ P}_D(SW2) ^{+ P}_D(SW3)
thus:
I_BAT
=
110°C – T_A
θ_JA
− P_D(SW1) – P_D(SW2) – P_D(SW3)
V_OUT – BAT
Consider the above example with an ambient temperature of
55°C. The charge current will be reduced to approximately:
110°C – 55°C _{− 0.15W}
I_BAT =
37°C/W
5V – 3.3V
= ^{1.49W – 0.15W} = 786mA
1.7V
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