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LTC4425

4425p

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:

PD = (VIN – VOUT) • ICHRG

θJA is the thermal resistance and TA is the ambient tem-

perature, then the die temperature can be calculated as:

TDIE = TA + PD • θJA

When the part is in thermal regulation, the die temperature

is 105°C and for a given VIN and VOUT, the charge current

can be determined by the following equation:

CHRG

OUT

= ()•

105 –

–

For example, if the LTC4425 is used in LDO mode to charge

a completely discharged supercap stack (VOUT = 0V) at a

room temperature of 25°C from a 5V source, the charge

current, at ﬁrst, will be limited to approximately:

VC W

CHRG =

°°

() •°

105

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:

OUT

CHRG

•

–

105

Using the previous example, for full charge current, the

output voltage has to rise to at least:

AC W

OUT =

()°

•°

105 25

243

407

–

APPLICATIONS INFORMATION

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)

2.0

1.5

1.0

0.5

2.5

234

4425 F04

VIN = 5V

RPROG = 500Ω

TA = 25°C

JA = 43° C/W

THERMAL

REGULATION = 105°C

FB PIN GROUNDED

THERMAL REGULATION

4.07V

372mA

Figure 4. Charge Current vs Output Voltage under Thermal

Regulation (LDO Mode)

CSUP

VOUT

C1 = C2 ≥ 100μF

4425 F05

LTC4425

VOUT

VMID

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

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