LM2676 External ON/OFF Control Implementation

Periyasamy Balasubramanian

Other Parts Discussed in Thread: LM2676

Hi,

I'm using LM2676S-3.3 in the design. I wanted to ON/OFF the regulator from external 12V (VREG_CTRL) signal.

Since the maximum allowed voltage at ON/OFF# pin is 6V, I'm using resistor divider network.

ON/OFF pin is connected to VREG_CTRL(12V) through 100K & ON/OFF pulled down to GND with 33.2k, so that when VREG_CTRL is >/=8V, voltage at ON/OFF pin would be 2.0V (2.0V is worst case threshold to switch ON the Regulator). For Typical threshold (1.4V), Regulator will ON when VREG_CTRL is >/= 6V.

The Requirement is if no volatge at VREG_CTRL(floating) then regulator should NOT ON.

My question,

since ON/OFF have weak current source 20uA hence Voltage at ON/OFF would be (20u*33.2k ) 664mV when VREG_CTRL is floating.

Question1: whether my above approach is correct

Question2: Is there possibility that regulator will get ON when VREG_CTRL is floating over entire operating temperature (till 85 dec C)?

Thanks for your help.

Bala.

over 9 years ago

0 Akshay Mehta over 9 years ago

TI__Genius 15810 points

Hello Bala,

The ION/OFF current listed in the LM2676 datasheet has a worst case current of 45uA. Based on your lower ON/OFF resistor, your worst case voltage generated could be 1.5V. This could turn the part on. There can be two options:

1) You set your lower ON/OFF resistor to set a much lower voltage than the minimum threshold on ON/OFF pin. Going with the worst case current of 45uA and a very conservative voltage of 0.1V to not turn on the LM2676, your resistor value could be 2k. You could set your upper resistor to give you 2V at 12V of VREG_CTRL. That results in a value of 10k ohms. The downside of this is that at 12V on VREG_CTRL, you could be burning about 1mA in the resistors. If efficiency of the system is not a big concern you could do this.

2) Alternatively you could use a JFET to control the EN pin. Please refer to the image I have sent. With this arrangement, when you have a voltage on VREG_CTRL, the JFET would be turned off and the ON/OFF pin would float, turning on the part. When there is no voltage at VREG_CTRL (i.e. it is floating), then the lower ON/OFF resistor pulls the gate of the JFET down and turns on the FET. This pulls the ON/OFF pin to ground and effectively turns off the part. The same happens if for some reason VREG_CTRL is shorted to ground. You should choose a FET that has the IDSS (saturation current) larger than the 45uA (worst case ION/OFF).