I want to calculate variation of power dissipation for two situations. But when
I use equations in datasheet of the product, I noticed two problems.
In the datasheet, in page 29, it is mentioned that ton and toff are 10ns for a
24V input. In page 3, in Table 1,it is mentioned that on time is minimum 50ns
and off time is minimum 165ns. According to the notes 3 and 4, I think given
ton is for 12V Vin, 1,2V Vout and 1A load only. And given toff is for 15V Vin,
12Vout and 1A load only. Where can I find ton and toff values for different
input voltages,output voltages and loads?
According to the expression in page 29, it seems ton and toff are only depends
on Vin. But according to Table 1(Notes 3 and 4) it seems it depends on Vin,
Vout, and Iload. Which one is true?
Another subject is total power dissipation calculation. When I looked equations
27, 28, 29 and 30; it seems power dissipation is independent from Vin-Vout. I
couldn' t see anything about Vout in equations. Am I missing something or is
the power dissipation independent from Vout?
D = Vout / Vin
D= ton / Fsw
So ton = Fsw * Vout / Vin
This is assuming no losses, in reality ton will be slightly higher (maybe 1-2%)
to compensate for the losses.
The datasheet specifies also the capability of the controller, meaning it can
only provide a range of D between about 8% and 93%, this is affected by the min
on and off time + some other timing limitations inside the IC.
The datasheet equations are not very accurate to calculate the power
dissipation. We have a tool for this device which calculates very accurate
losses including additional parameters that may not be specified in the
datasheet such as Driver strength, MOSFET capacitance etc.
Please use our tool to calculate the operating conditions losses.
It can be found here:
- ADP244x Buck Regulator Design Tool (Version: 1.45 Size: 5.33 MB)
- ADP244x Inverting Buck Boost Designer (Version: 0.37 Size: 5.80 MB)
- I am not sure which topology you use but we have the tool for regular buck
(most likely) or inverting buck-boost to generate negative voltages.
- the tool calculates the power losses across 0 to full load and at 2 Vin
operating points (Vinmin and Vinmax).