Hello,
I use the MP2459 to step down a variable input voltage from 7V to 32V to a voltage of 3.3V.
The datasheet says that I should use a 100k pull-up resistor. If I use this resistor it works as described but I get about 9V at the enable pin.
Under the maximum ratings it is described that this may only have 6V. Is it still possible to use this?
A voltage divider does not work with this large range.
Another idea is to increase the pull up resistor and lower the voltage. Here I am not sure if it has unwanted effects.
We have a Pspice model for this part and an online DCDC designer tool that you can utilize to quickly simulate. The links are available on the parts page under Design Resources.
Using the values you provided, I ran couple configurations at various input voltages. The pull up resistor must be kept at 100k value, but R7 {shown below} should be ~32kohms.
Varying the divider resistors to a slightly higher value in order to lower the enable node voltage below the max rating of 6V should not result to unexpected issues.
Although using psice is great… simple maths.
considering the 100k / 32k voltage divider
32V => Ven = 7.76V (242uA)
7V => Ven = 1.7V (53uA)
Minimum turn on Voltage 5.57V
So you are still over voltage slightly.
if you change to 100k / 25k you are slightly closer, but still over at the top. (turn on 6.75V, Ven(max) 6.4V)
So why not simply change the circuit to 100k and a 4.7V Zener? You loose the turn off voltage, but that can be restored by leaving the divider and clamping the en pin at 4.7V. The clamp simply provides overvoltage protection in this case.
