BQ24650 Datasheet, PDF (23/33 Pages) Texas Instruments – Synchronous Switch-Mode Battery Charge Controller for Solar Power With Maximum Power Point Tracking

bq24650
www.ti.com
SLUSA75 – JULY 2010
There are several methods to damping or limiting the over-voltage spike during adapter hot plug-in. An
electrolytic capacitor with high ESR as an input capacitor can damp the over-voltage spike well below the IC
maximum pin voltage rating. A high current capability TVS Zener diode can also limit the over-voltage level to an
IC safe level. However, these two solutions may not be lowest cost or smallest size.
A cost effective and small size solution is shown in Figure 21. R1 and C1 are composed of a damping RC
network to damp the hot plug-in oscillation. As a result, the over-voltage spike is limited to a safe level. D1 is
used for reverse voltage protection for the VCC pin. C2 is the VCC pin decoupling capacitor and it should be
placed as close as possible to the VCC pin. R2 and C2 form a damping RC network to further protect the IC from
high dv/dt and high voltage spike. The C2 value should be less than the C1 value so R1 can dominant the
equivalent ESR value to get enough damping effect for hot plug-in. R1 and R2 must be sized enough to handle
in-rush current power loss according to the resistor manufacturer’s datasheet. The filter component values
always need to be verified with a real application.
D1
Adapter
Connector
R1(2010)
2W
C1
2.2 mF
R2(1206)
4.7 - 30 W
VCC pin
C2
0.1 - 1 mF
Figure 21. Input Filter
MPPT TEMPERATURE COMPENSATION
A typical solar panel comprises of alot of cells in a series connection, and each cell is a forward-biased p-n
junction. So, the open-circuit voltage (V_OC) of a solar cell has a temperature coefficient that is similar to a
common p-n diode, or about –2mV/°C. A crystalline solar panel specification always provides both open-circuit
voltage V_OC and peak power point voltage V_MP. The difference between V_OC and V_MP can be approximated as
fixed and temperature-independent, so the temperature coefficient for the peak power point is similar to that of
V_OC. Normally, panel manufacturers specify the 25°C values for V_OC and V_MP, and the temperature coefficient for
V_OC, as shown in the following figure.
V_OC
V_MP
5
15
25
35
45
55
T_A - Free-Air Temperature - °C
Figure 22. Solar Panel Output Voltage Temperature Characteristics
The bq24650 employs a feedback network to the MPPSET pin to program the input regulation voltage. Because
the temperature characteristic for a typical solar panel V_MP voltage is almost linear, a simple solution for tracking
this characteristic can be implemented by using an LM234 3-terminal current source, which can create an easily
programmable, linear temperature dependent current to compensate the negative temperature coefficient of the
solar panel output voltage.
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