Datasheet

LTC4425
13
4425f
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
temperature, 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
T
VV
CHRG
A
IN OUT JA
=
()
•
105 –
– θ
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 5V source,
the charge current, at fi rst, will be limited to approximately:
I
CC
VCW
C
CA
mA
CHRG
=
°°
()
•°
=
°
°
=
105 25
50 43
80
215
372
–
–/
/
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:
VV
T
I
OUT IN
A
CHRG JA
=
•
–
–105
θ
Using the previous example, for full charge current, the
output voltage has to rise to at least:
VV
C
ACW
V
C
CV
V
OUT
=
()
°
•°
=
°
°
=5
105 25
243
5
80
86
407–
–
/
–
/
.
APPLICATIONS INFORMATION
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.
OUTPUT VOLTAGE (V)
0
0
CHARGE CURRENT(A)
2.0
1.5
1.0
0.5
2.5
156234
4425 F04
V
IN
= 5V
R
PROG
= 500
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)
C1
C
SUP
V
OUT
C1 = C2 ≥ 100µF
C2
4425 F05
LTC4425
V
OUT
V
MID
GND
Figure 5. Charging a Single Supercapacitor
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 www.cap-xx.com
NESS CAP www.nesscap.com
Maxwell www.maxwell.com
Bussmann www.cooperbussmann.com
AVX www.avx.com
Illinois Capacitor www.illcap.com
Tecate Group www.tecategroup.com