SM72445_15 Datasheet, PDF (8/20 Pages) Texas Instruments – SM72445 Programmable Maximum Power Point Tracking Controller With Adjustable PWM Frequency

SM72445
SNVS795 – MARCH 2012
www.ti.com
If the output current is greater than the current threshold set at A/D Channel 6 (A6) during soft-start, the chip will
then engage in MPPT mode and will not be subject to the start-up delay.
Figure 9. Startup Sequence
MAXIMUM OUTPUT VOLTAGE
The maximum output voltage on the SM72445 is set by the resistor divider ratio on pin A0. (Please refer to
Figure 2 Typical Application Circuit).
The value of the voltage on pin A0 is sampled and stored by the ADC of the SM72445 at start-up and after reset
events. While voltage on pin AVOUT is above the voltage set at pin A0, the duty cycle of the converter will be
reduced every MPPT cycle (1ms-2ms depending on the set switching frequency). This is true when the converter
is running in MPPT state or during Soft-Start. When the unit is in Panel Mode (PM) or in Startup Panel Mode
(PM_Startup) there is no control on the output voltage and the device will not react to the presence of a voltage
on AVOUT higher than the A0 setpoint. See Figure 8 for more details on the different states of operation.
This means that the voltage limit setting cannot be used to ensure overall maximum output voltage for the
system: there will be times during Panel Mode operation and Stand-by mode operation when the output will
increase above the programmed output voltage if the input (solar panel) gets over that voltage limit. Therefore,
the maximum output voltage threshold set by programming A0 is only valid if its value is higher than the
maximum input voltage (solar panel in open circuit at coldest operating point). If over-voltage protection needs to
be implemented, it must be done using external components. For exampe, a voltage comparator with its output
connected to the reset pin of the SM72445 is one possible implementation.
The maximum output voltage is always enforced during MPPT operation of the IC.
The following equation sets the maximum output voltage:
RB1
(RFB1 + RFB2)
V_{OUT_MAX} = 5 x _{RT1 + RB1} x
RFB2
Where RT1 and RB1 are the resistor divider on the ADC pin A0 and RFB1 and RFB2 are the output voltage
sense resistors. A typical value for RFB2 is about 2 kΩ
CURRENT LIMIT SETTING
Maximum output current can be set by changing the resistor divider on A4 (pin 18). Refer to Figure 2.
Overcurrent at the output is detected when the voltage on AIOUT (pin 21) equals the voltage on A4 (pin 18). The
voltage on A4 can be set by a resistor divider connected to 5V whereas the voltage on AIOUT can be set by a
current sense amplifier.
AVIN PIN
AVIN is an A/D input to sense the input voltage of the SM72445. A resistor divider can be used to scale max
voltage to about 4V, which is 80% of the full scale of the A/D input.
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