Datasheet

LTC3577/LTC3577-1
48
3577fa
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 package is soldered
to a ground plane on the board. Correctly soldered to
2500mm
2
ground plane on a double-sided 1oz. copper
board the LTC3577 has a thermal resistance (θ
JA
) of ap-
proximately 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 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 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 sys-
tem 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)
= BOUTx •I
OUT
()
•
100 – Eff
100
where BOUTx is the programmed output voltage, I
OUT
is the load current and Eff is the % effi ciency which can
be measured or looked up on an effi ciency table for the
programmed output voltage.
The power dissipated on chip by a LDO regulator can be
estimated as follows:
P
DLDOx
= (V
INLDOx
– LOUTx) • I
OUT
where LOUTx is the programmed output voltage, V
INLDOx
is the LDO supply voltage and I
OUT
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
•I
LED
•
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 will automatically
reduce the charge current to maintain the die temperature
at approximately 110°C. However, the approximate ambi-
ent temperature at which the thermal feedback begins to
protect the IC is:
T
A
= 110°C – P
D
• θ
JA
Example: Consider the LTC3577 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 will
begin to reduce the 1A charge current, is approximately
T
A
= 110°C – 2W • 45°C/W = 20°C