SM3320-BATT-EV-NOPB Datasheet, PDF (7/21 Pages) Texas Instruments – AN-2121 SolarMagic™ SM3320-BATT-EV Charge Controller Reference Design

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Start-Up Circuitry
8 Start-Up Circuitry
If the panel voltage is lower than the battery voltage, a start up circuit (Figure 9) is required to force the
duty cycle high enough to create a flow of current to the battery. Once current is established, the circuit
can be turned off to allow MPPT operation to perform.
Figure 9. Start-Up Boost Circuitry
As long as the start-up circuit is activated, the duty cycle will increase every 1ms up to its maximum value.
However, the duty cycle will still be limited by the SM72442’s internal output voltage limiter.
The circuit is turned on when the anode of D101 and the cathode of D100 are kept at 5V. It is disabled
when that node is set at 0V.
The circuit should be disabled 5ms after current begins to flow into the battery to allow proper MPPT
operation.
If the current drops to 0 for any reason (no light, reset, and so on) the start-up circuit can be re-engaged
according to the timing diagram in Figure 10.
This circuit operates by sensing the average value of the gate voltage on the main buck switch (Q1) and
main boost switch (Q4). This value is fed back to the input current sense of the SM72442. At the same
time, a constant 4.4V is set at the input voltage sense pin of the SM72442. This results in the SM72442
measuring a virtual power that increases each time the duty cycle is increased and decreases each time
the duty cycle is decreased. The SM72442 will track this virtual power and increase the duty cycle of the
converter continuously. When this circuit is de-activated, the real input voltage and current appear at the
sensing pins of the SM72442 chip which will then perform regular MPPT operation.
SNOSB76C – December 2010 – Revised May 2013
AN-2121 SolarMagic™ SM3320-BATT-EV Charge Controller Reference
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