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LM3478: Designing with LM3478MA/NOPB

Shibin Varghese
Intellectual 835 points
Part Number: LM3478
Other Parts Discussed in Thread: LM5155

Hi Team,
We are using LM3478MA/NOPB as a DC-DC boost converter in one of our projects.
The input is from a Li-ion battery of nominal voltage 3.7V (2.8V to 4.2V).
The required output current is about 260mA.
Some 8 shape memory valves are connected in series at the output of the DC-DC converter.
The Shape memory valves require the consent current driver.
So we made a constant current sink like in the application note SLAA868.
The resistance of the shape memory valve is 2.2 - 3 ohms.
So the voltage drop across any valve is 3*0.260=0.78V.
A maximum of only 4 valves will be operational at any time. The remaining valves will be bypassed using FETs like in the image below.

So the maximum voltage needed at the output of the blower is the sum of the voltage across 4 operational valves and the voltage drop across FETs = (4*.78) + (4*[0.55*0.25]) =3.67V (adding voltage drop across the sense resistor and the current limiting FET, for 10 Ohms sense resistor, the voltage drop is 2.5V).
When only one valve is operational the voltage needed at the output of the DC-DC converter is  (1*.78) + (7*[0.55*0.25]) = 1.74V (adding voltage drop across the sense resistor and the current limiting FET, for 10 Ohms sense resistor, the voltage drop is 2.5V).
1). Is my above calculation correct?. Please correct me if I am wrong.
2). Since the DC-DC converter is giving a constant voltage, the excess voltage is dropped across the current limiting FET, which is not good for a battery-operated device. So can I adjust the voltage at the output of the DC-DC converter using the DAC of my microcontroller connected to the FB pin along with the resistor divider?.
3). In both the above calculations, sometimes the voltage needed at the output is less than the voltage at the input. Are there any issues with this?.
4). Is the IC LM3478MA/NOPB able to deliver 250mA of current?. How is the switching current related to the output current?.
5). 

  • 0
    TI__Mastermind 31315 points

    Hi Shibin,

    Sorry for the delay in response. It slipped through my notifications. 

    Please see my response below:

    1) Yes, your calculation of the load is correct. Voltage across the load is equivalent to n*V_valve + (8-n)*(Iout*Rds), where n is the number of valves used, and V-Valve is the voltage drop across the valve. 

    2) You should be able to adjust the voltage at the output of a DC-DC converter. I think a better application for you would be to use a scheme as shown in section 5.1: https://www.ti.com/lit/an/slvaej4/slvaej4.pdf?ts=1646064769149&ref_url=https%253A%252F%252Fwww.google.com%252F

    3) The boost topology intrinsically supports this case, which is called bypass operation, where if the input is higher than the output voltage, then the output is just the input minus a diode drop (in the case of the LM3478).

    4) The LM3478 is able to deliver this load. The LM3478 is a controller and can supply any load required, since it does not have any of the power-stage current flowing through the controller. In this case, I would recommend using a more updated controller, such as the LM5155, which is the next-gen variation of the LM3478. The average current flowing through the low-side MOSFET is equivalent to the input current. 

    Regards,

    Richard