LTM8062EVPBF datasheet(12/20 Pages) LINER

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LTM8062

8062f

APPLICATIONS INFORMATION

For example, given a common 36-cell solar panel that has

the following specified characteristics:

Open Circuit Voltage (VOC) = 21.7V

Maximum Power Voltage (VMP) = 17.6V

Open-Circuit Voltage Temperature Coefficient (VOC)

= –78mV/°C

As the temperature coefficient for VMP is similar to that

of VOC, the specified temperature coefficient for VOC

(TC) of –78mV/°C and the specified peak power voltage

(VMP(25°C)) of 17.6V can be inserted into the equations

to calculate the appropriate resistor values for the tem-

perature compensation network in Figure 4. With RSET

equal to 1k, then:

RSET = 1k

RIN1 =−1k •(−0.078 • 4405)= 344k

RIN2 =

344k

17.6 + 344k •(0.0674 / 1k)

2.7

− 1

= 24.4k

Battery Voltage Temperature Compensation

Some battery chemistries have charge voltage require-

ments that vary with temperature. Lead-acid batteries in

particular experience a significant change in charge volt-

age requirements as temperature changes. For example,

manufacturers of large lead-acid batteries recommend a

float charge of 2.25V/cell at 25°C. This battery float voltage,

however, has a temperature coefficient which is typically

specified at –3.3mV/°C per cell.

Figure 5. Lead-Acid 6-Cell Float Charge Voltage vs Temperature with a –19.8mV/°C

Temperature Coefficient Using LM234 with the Feedback Network

8062 F05a

LTM8062

LINEAR

TECHNOLOGY

LM234

RFB1

210k

RFB2

43k

V+

V–

RSET

2.4k

BAT

ADJ

RFB3

215k

6-CELL

LEAD-ACID

BATTERY

TEMPERATURE (°C)

–10

020

8062 F05b

12.6

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.3

–19.8mV/°C

In a manner similar to the MPPT temperature correction

outlined previously, implementation of linear battery charge

voltage temperature compensation can be accomplished by

incorporating a Linear Technology LM234 into the output

feedback network. For example, a 6-cell lead acid battery

has a float charge voltage that is commonly specified at

2.25V/cell at 25°C, or 13.5V, and a –3.3mV/°C per cell

temperature coefficient, or –19.8mV/°C. Using the feedback

network shown in Figure 5, with the desired temperature

coefficient (TC) and 25°C float voltage (VFLOAT (25°C))

specified, and using a convenient value of 2.4k for RSET,

necessary resistor values follow the relations:

RFB1 = –RSET • (TC • 4405)

= –2.4k • (–0.0198 • 4405) = 210k

RFB2 =

RFB1

VFLOAT(25°C)+RFB1 •(0.0674 / RSET)

VFB

− 1

210k

13.5 + 210k •(0.0674 / 2.4k)

3.3

− 1

= 43k

RFB3 = 250k – RFB1||RFB2

= 250k – 210k||43k = 215k

(see the Battery Float Voltage Programming section)

While the circuit in Figure 5 creates a linear tempera-

ture characteristic that follows a typical –3.3mV/°C per

cell lead-acid specification, the theoretical float charge

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