LTC3557-1 Linear Technology, LTC3557-1 Datasheet - Page 25

• Manufacturer Part Number: LTC3557-1
• Description: USB Power Manager
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
capacitance on this pin must be kept to a minimum. With
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
the maximum resistance value for R
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 sol-
dered to a ground plane on the board. Correctly soldered
to a 2500mm
board, the LTC3557/LTC3557-1 has a thermal resistance
(θ
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
dissipation is approximately:
P
where, P
voltage, BAT is the battery voltage and I
charge current. P
switching regulators. The power loss for a step-down
switching regulator can be calculated as follows:
where OUTx is the programmed output voltage, I
the load current and Eff is the % effi ciency which can be
measured or looked up on an effi ciency graph for the
programmed output voltage.
PROG
D
JA
P
R
= (V
D(SWx)
) of approximately 37°C/W. Failure to make good
PROG
, the following equation should be used to calculate
OUT
D
≤
is the total power dissipated, V
= (OUTx • I
– BAT) • I
2
2
π •
ground plane on a double-sided 1oz copper
D(SWx)
100
OUT
kHz C
BAT
OUT
1
is the power loss by the step-down
, the LTC3557/LTC3557-1 power
+ P
•
) • (100 – Eff)/100
PROG
D(SW1)
+ P
PROG
D(SW2)
BAT
OUT
:
is the battery
is the supply
+ P
D(SW3)
OUT
is
It is not necessary to perform any worst-case power dis-
sipation 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:
Example: Consider the LTC3557/LTC3557-1 operating
from a wall adapter with 5V (V
to charge a Li-Ion battery at 3.3V (BAT). Also assume
P
dissipation is:
The ambient temperature above which the LTC3557/
LTC3557-1 will begin to reduce the 1A charge current, is
approximately:
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 approxi-
mated by:
thus:
Consider the above example with an ambient temperature
of 55°C. The charge current will be reduced to approxi-
mately:
P
I
BAT
D(SW1)
D
T
P
T
I
=
=
BAT
A
A
D
=
=
(
= 110°C – P
= 110°C – 1.85W • 37°C/W = 42°C
110
= (5V – 3.3V) • 1A + 0.15W = 1.85W
V
1 49
OUT
37
=
.
= P
110
θ
°
θ
°
C T
JA
5
C W
D(SW2)
–
JA
W
V
–
/
1 7
°
BAT I
.
C
–
– .
– .1 1 5
A
LTC3557/LTC3557-1
–
– .
V
P
3 3
0
D SW
D
)
0 15
55
= P
(
•
• θ
V
BAT
°
W
110
V
C
D(SW3)
JA
1
W
OU
)
=
–
+
T T
°
P
786
C T
P
–
D SW
D SW
–
(
= 0.05W, so the total power
BAT
(
mA
OUT
A
2
1
)
)
+
–
) providing 1A (I
P
P
D SW
D S
(
(
W W
3
2
)
)
+
P
D SW
25
(
BAT
35571f
3
)
)