Datasheet
LTC4425
13
4425fa
For more information www.linear.com/LTC4425
Board Layout Considerations
To be able to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC4425’s two packages have a good
thermal contact to the PC board ground. Correctly soldered
to a 2500mm
2
double-sided 1 oz. copper board, the DFN
package has a thermal resistance of approximately 43°C/W.
Failure to make thermal contact between the exposed pad
on the backside of the package and the copper board will
result in a thermal resistance far greater than 43°C/W.
Charge Current Reduction by the Thermal Regulator
To protect the part against excessive heat generated by
internal power dissipation, the LTC4425 is equipped with a
thermal regulator which automatically reduces the charge
current to maintain a maximum die temperature of 105°C.
Ignoring the quiescent current, the power dissipation can
be approximated by the following equation:
P
D
= (V
IN
– V
OUT
) • I
CHRG
If θ
JA
is the thermal resistance and T
A
is the ambient tem-
perature, then the die temperature can be calculated as:
T
DIE
= T
A
+ P
D
• θ
JA
When the part is in thermal regulation, the die temperature
is 105°C and for a given V
IN
and V
OUT
, the charge current
can be determined by the following equation:
I
CHRG
=
105– T
A
V
IN
– V
OUT
( )
•θ
JA
For example, if the LTC4425 in the DFN package is used
in LDO mode to charge a completely discharged supercap
stack (V
OUT
= 0V) at a room temperature of 25°C from a
applicaTions inForMaTion
5V source, the charge current, at first, will be limited to
approximately:
I
CHRG
=
105°C– 25°C
5– 0
( )
V • 43°C / W
=
80°C
215°C / A
= 372mA
As the output voltage rises, the charge current will gradually
rise to the full charge current programmed by the PROG pin
resistor as long as the constant-current loop is in control.
If the LTC4425 is programmed for a charge current of 2A,
the output voltage at which the part will deliver full charge
current can be determined by the following equation:
V
OUT
= V
IN
–
105– T
A
I
CHRG
•θ
JA
Using the previous example, for full charge current, the
output voltage has to rise to at least:
V
OUT
= 5V –
105– 25
( )
°C
2A • 43°C / W
= 5V –
80°C
86°C / V
= 4.07V
Figure 4 shows the graph of charge current vs output
voltage when the charge current profile is turned off by
shorting the FB pin to GND and the charge current is
limited by thermal regulation.
OUTPUT VOLTAGE (V)
0
0
CHARGE CURRENT(A)
2.0
1.5
1.0
0.5
2.5
1 5 62 3 4
4425 F04
V
IN
= 5V
R
PROG
= 500Ω
T
A
= 25°C
FB PIN GROUNDED
THERMAL REGULATION
4.07V
≈ 105°C
372mA
Figure 4. Charge Current vs Output Voltage under Thermal
Regulation (LDO Mode)
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