LTC3577-4_15 Datasheet, PDF (46/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
LAYOUT AND THERMAL CONSIDERATIONS
Printed Circuit Board Power Dissipation
In order to be able to deliver maximum charge current
under all conditions, it is critical that the exposed ground
pad on the backside of the LTC3577-3/LTC3577-4 pack-
age be soldered to a ground plane on the board. Correctly
soldered to 2500mm² ground plane on a double-sided 1oz
copper board the LTC3577-3/LTC3577-4 have a thermal
resistance (θ_JA) of approximately 45°C/W. Failure to make
good thermal contact between the Exposed Pad on the
backside of the package and a adequately sized ground
plane will result in thermal resistances far greater than
45°C/W.
The conditions that cause the LTC3577-3/LTC3577-4
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 with a wall
adapter applied to V_OUT, the LTC3577-3/LTC3577-4 power
dissipation is approximately:
P_D = (V_OUT – BAT) • I_BAT + P_DREGS
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_DREGS is the sum of power dissipated
on-chip by the step-down switching, LDO and LED boost
regulators.
The power dissipated by a step-down switching regulator
can be estimated as follows:
( ) ^P_D(SWx) ⁼
OUTx • I_OUTx
• ^{100 – Eff}
100
where OUTx is the programmed output voltage, I_OUTx
is the load current and Eff is the % efficiency which can
be measured or looked up on an efficiency table for the
programmed output voltage.
The power dissipated on chip by a LDO regulator can be
estimated as follows:
P_DLDOx = (V_INLDOx – LDOx) • I_LDOx
where LDOx is the programmed output voltage, VI_NLDOx
is the LDO supply voltage and I_LDOx is the LDO output
load current. Note that if the LDO supply is connected to
one of the buck output, then its supply current must be
added to the buck regulator load current for calculating
the buck power loss.
The power dissipated by the LED boost regulator can be
estimated as follows:
P_DLED
=
I_LED
•
0.3V
+
R_NSWON
•
⎛
⎝⎜ ^ILED
•
BOOST ^⎞
V_OUT – 1_⎠⎟
2
where BOOST is the output voltage driving the top of
the LED string, R_NSWON is the on-resistance of the SW
N-FET (typically 330mΩ), I_LED is the LED programmed
current sink.
Thus the power dissipated by all regulators is:
P_DREGS = P_DSW1 + P_DSW2 + P_DSW3 + P_DLDO1 + P_DLDO2 + P_DLED
It is not necessary to perform any worst-case power dis-
sipation scenarios because the LTC3577-3/LTC3577-4 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 LTC3577-3/LTC3577-4 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_DREGS
= 0.3W, so the total power dissipation is:
P_D = (5V – 3.3V) • 1A + 0.3W = 2W
The ambient temperature above which the LTC3577-3/
LTC3577-4 will begin to reduce the 1A charge current, is
approximately
T_A = 110°C – 2W • 45°C/W = 20°C
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