Datasheet

VCC
MPPSET
bq24650
C21
2.2 Fm
Solar
Panel
R
3
R
4
R20
2Ω
R
SET
LM234
I
SET
I
1
I
2
V
IN
V
REG
C21
0.47 Fm
R21: 20Ω
SET
SET
227 μV/ K
I = Temp
R
°
´
SET
SET
0.0677V
I (25 C) =
R
°
REG IN REG
2 1 SET SET
4 3
V V - V
I = = I + I = + I
R R
)
IN REG SET
2
3
d(V - V dI
dI
1
= × + = 0
dT dT R dT
IN
3 SET
SET
-dV /dT
2mV × number of solar cells in series
R = = R ×
dI /dT 227μV
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( )
REG 3 MPPSET 3
4
IN 3 SET REG
MP 3 MPPSET
SET
V × R V × R
R = =
V + R × I - V
0.0677V
V (25°C) + R × - V
R
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bq24650
SLUSA75 –JULY 2010 www.ti.com
Figure 23. Feedback Network
In the circuit shown in Figure 23, for the LM234 temperature sensor,
(23)
Thus,
(24)
The current node equation is,
(25)
To have a zero temperature coefficient on V
REG
,
(26)
(27)
(28)
For example, given a common 18-cell solar panel that has the following specified characteristics:
Open circuit voltage (V
OC
) = 10.3V
Maximum power voltage (V
MP
) = 9V
Open-circuit voltage temperature coefficient (V
OC
) = –38mV/°C
Appling the following parameters into the equations of R
3
and R
4
:
1. Temperature coefficient for V
MP
(same as that of V
OC
) of –38mV/°C
2. Peak power voltage of 9V
3. MPPSET regulation voltage of 1.2V
And choosing R
SET
= 1000Ω.
The resistor values are R
SET
= 1kΩ, R
3
= 167.4kΩ, and R
4
=10.6kΩ. Selecting standard 1% accuracy resistors
and R
SET
= 1kΩ, R
3
= 169kΩ, and R
4
=10.7kΩ.
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