Help understanding switching regulator IC current

Thread Starter

Gorden

Joined Oct 30, 2019
25
Hi,

I am trying to understand more about switching regulator IC's and available current.
I was looking at a device MIC2605, which raised some questions. The datasheet says that the chip has a 40V/0.5A switch, and that the typical switch current limit is 0.5A / max 0.8A.
The datasheet talks about how to calculate the output voltage, but does not seem to describe how to determine the possible output current when the input / output voltage has been set.
Figure 2 of the datasheet shows a typical circuit, the input is 12V and the output is shown as 32V 30mA - but no reference to how that 32mA was derived.

The datasheet has a graph, switch current versus input voltage, but i do not understand the input voltages shown - 46 81 01 21 41 61 82 0.

Can anyone help me to understand how to calaculate the maximum output current of a switching regulator IC?

Thanks
 

ronsimpson

Joined Oct 7, 2019
2,083
Don't look at "max" current for this topic.
Let's talk boost converter.
5V in and 10V out. If the parts are 100% efficient, then 5V @ 200mA = 10V at 100mA.
The switch is on 50% of the time and the diode is on 50% of the time.

To get 100mA 100% of the time through the diode, you need to average 200mA for the 50% that the diode is on. But it is even worst. The current is in a ramp form where (depending on what mode the PWM is in) most likely the current ramps from 400mA down to 0mA over 50% of the time. Then the current is 0mA for the other 50% of the time. We just shown that the diode current has a peak of 400mA.

The current in the diode was stored on the inductor. To put the current into the inductor it is likely that the "switch" or MOSFET had to ramp the current from 0mA to 400mA in 50% of the time.

Here is an example where we used the switch at 400mA to get 100mA out of the supply.

Now I see I should have done the 12V to 32V example in Fig2. LOL
The power supply is 70 to 80% efficient so more input power than in the example.
hope this helps ronsimpson
 

Papabravo

Joined Feb 24, 2006
18,432
Hi,

I am trying to understand more about switching regulator IC's and available current.
I was looking at a device MIC2605, which raised some questions. The datasheet says that the chip has a 40V/0.5A switch, and that the typical switch current limit is 0.5A / max 0.8A.
The datasheet talks about how to calculate the output voltage, but does not seem to describe how to determine the possible output current when the input / output voltage has been set.
Figure 2 of the datasheet shows a typical circuit, the input is 12V and the output is shown as 32V 30mA - but no reference to how that 32mA was derived.

The datasheet has a graph, switch current versus input voltage, but i do not understand the input voltages shown - 46 81 01 21 41 61 82 0.

Can anyone help me to understand how to calaculate the maximum output current of a switching regulator IC?

Thanks
The simple concept is this:

Power out will always be less than power in. In some cases, it will be much less.

The ratio of power out to power in is referred to as the efficiency. It can approach 100%, but It can NEVER be greater than or equal to 100%. I use 80% for initial back of the envelope calculations. This approximation has never failed me.
 
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