LTC4425 Linear Technology, LTC4425 Datasheet - Page 13

• Manufacturer Part Number: LTC4425
• Description: Linear SuperCap Charger
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
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:
If θ
perature, then the die temperature can be calculated as:
When the part is in thermal regulation, the die temperature
is 105°C and for a given V
can be determined by the following equation:
For example, if the LTC4425 is used in LDO mode to charge
a completely discharged supercap stack (V
room temperature of 25°C from a 5V source, the charge
current, at fi rst, will be limited to approximately:
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:
Using the previous example, for full charge current, the
output voltage has to rise to at least:
P
T
I
I
V
V
JA
CHRG
CHRG
DIE
D
OUT
OUT
is the thermal resistance and T
= (V
= T
=
=
=
=
IN
A
5
V
(
(
IN
5 0
+ P
V
– V
V
IN
–
105
–
–
D
OUT
2
105 –
I
–
(
)
CHRG
• θ
105 25
A
°
V
105
V
) • I
C
•
OUT
•
JA
–
43
43
–
–
25
•
CHRG
T
)
°
A
°
T
θ
•
C W
C W
IN
A
°
JA
θ
)
C
/
/
°
and V
JA
C
=
=
5
OUT
215
V
A
80
–
, the charge current
is the ambient tem-
°
86
°
C A
C
80
/
°
C V
OUT
°
C
=
/
372
= 0V) at a
=
4 07
mA
.
V
Figure 4 shows the graph of charge current vs output
voltage when the charge current profi le is turned off by
shorting the FB pin to GND and the charge current is
limited by thermal regulation.
Charging a Single Supercapacitor
The LTC4425 can also be used to charge a single super-
capacitor by connecting two series-connected matched
ceramic capacitors with a minimum capacitance of 100μF
in parallel with the supercapacitor as shown in Figure 5.
Refer to Table 1 for supercapacitor manufacturers.
Table 1. Supercapacitor Manufacturers
CAP-XX
NESS CAP
Maxwell
Bussmann
AVX
Illinois Capacitor
Tecate Group
Figure 4. Charge Current vs Output Voltage under Thermal
Regulation (LDO Mode)
Figure 5. Charging a Single Supercapacitor
2.5
2.0
1.5
1.0
0.5
0
0
372mA
V
R
T
THERMAL
REGULATION = 105°C
FB PIN GROUNDED
C1 = C2 ≥ 100μF
A
IN
JA
PROG
LTC4425
THERMAL REGULATION
= 25°C
= 5V
= 43° C/W
V
V
1
GND
= 500Ω
OUT
MID
OUTPUT VOLTAGE (V)
2
3
www.cap-xx.com
www.nesscap.com
www.maxwell.com
www.cooperbussmann.com
www.avx.com
www.illcap.com
www.tecategroup.com
C1
C2
www.DataSheet4U.com
4
4.07V
4425 F05
C
SUP
5
V
OUT
LTC4425
4425 F04
6
13
4425p