Datasheet

LTC3677-3
40
36773f
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 LTC3677-3 package be sol-
dered to a ground plane on the board. Correctly soldered
to 2500mm
2
ground plane on a double-sided 1oz copper
board the LTC3677-3 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 LTC3677-3 to reduce charge
current due to the thermal protection feedback can be ap-
proximated by considering the power dissipated in the part.
For high charge currents with a wall adapter applied to V
OUT
,
the LTC3677-3 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, and LDO regulators.
The power dissipated by a step-down switching regulator
can be estimated as follows:
P OUTx I
Eff
D SWx OUTx( )
• •
–
=
( )
100
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.
Thus the power dissipated by all regulators is:
P
DREGS
= P
DSW1
+ P
DSW2
+ P
DSW3
+ P
DLDO1
+ P
DLDO2
It is not necessary to perform any worst-case power dissi-
pation scenarios because the LTC3677-3 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 LTC3677-3 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 LTC3677-3 be-
gins to reduce the 1A charge current, is approximately
T
A
= 110°C – 2W • 45°C/W = 20°C
The LTC3677-3 can be used above 20°C, but the charge
current will be reduced below 1A. The charge current at a
given ambient temperature can be approximated by:
P
C T
V BAT I P
D
A
JA
OUT BAT D REGS
=
°
=
( )
+
110 –
– •
( )
θ
Thus:
I
C T
P
V BAT
BAT
A
JA D REGS
OUT
=
°
( )
110 –
–
–
( )
θ