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LT3652IDD-PBF

Power Tracking 2A Battery Charger for Solar Power

Linear Technology 

LT3652

APPLICATIONS INFORMATION

simple solution for tracking that characteristic can be

implemented using an LM234 3-terminal temperature

sensor. This creates an easily programmable, linear

temperature dependent characteristic.

In the circuit shown in figure 8,

VIN

RIN1

RIN2

LM234

V+

R

V–

RSET

VIN

VIN_REG

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Figure 8. MPPT Temperature Compensation Network

RIN1 = –RSET • (TC • 4405), and

RIN2 = RIN1/({[VMP(25°C) + RIN1 • (0.0674/RSET)]/VIN_REG} – 1)

Where: TC = temperature coefficient (in V/°C), and

VMP(25°C) = maximum power voltage at 25°C

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 tempera-

ture compensation network in Figure 8. With RSET equal

to 1000Ω, 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.

In a manner similar to the MPPT temperature correction

outlined previously, implementation of linear battery

charge voltage temperature compensation can be ac-

complished by incorporating an 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,

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