BQ24650 Datasheet, PDF (24/33 Pages) Texas Instruments – Synchronous Switch-Mode Battery Charge Controller for Solar Power With Maximum Power Point Tracking

bq24650
SLUSA75 – JULY 2010
Solar ^R20
Panel
2Ω
C21
2.2mF
R21: 20Ω
V_IN
R₃
I₁
LM234
C21
0.47mF
VCC
R_SET
I_SET
V_REG
MPPSET
R₄
I₂
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Figure 23. Feedback Network
In the circuit shown in Figure 23, for the LM234 temperature sensor,
I_SET =
227 μV/°K
R_SET
´ Temp
(23)
Thus,
I_SET (25°C) =
0.0677V
R_SET
(24)
The current node equation is,
I₂ =
V_REG
R₄
= I₁ + I_SET =
V_IN - V_REG
R₃
+ I_SET
(25)
To have a zero temperature coefficient on V_REG,
dI_{2 =} d(V_IN - V_REG ) _× 1 ₊ dI_{SET = 0}
dT
dT
R₃
dT
(26)
R₃
=
æ
ç
-dV_IN
/dT
ö
÷
è ^dISET ^/dT ø
= R_SET
×
2mV × number of solar cells in series
227μV
(27)
( ) ^{R4 =}
V_REG × R₃
V_IN + R₃ × I_SET - V_REG
=
V_MPPSET × R₃
æ
ç
è
V_MP
(25°C)
+
R₃
×
0.0677V
R_SET
ö
÷
ø
- V_MPPSET
(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 R₃ and R₄:
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, R₃ = 167.4kΩ, and R₄=10.6kΩ. Selecting standard 1% accuracy resistors
and R_SET = 1kΩ, R₃ = 169kΩ, and R₄=10.7kΩ.
24
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