LT1172 Boost Converter 10 Volt only

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
LT1172.png

I follow exactly the above diagram and using 5 volt but the output is only around 8 to 10 volt instead of the 12 volt -- any advice what went wrong?
 

LesJones

Joined Jan 8, 2017
3,707
Have you used the exact inductor specified ? Does the 5 volt spply drop in voltage ? What output current are you drawing ?

Les.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
Have you used the exact inductor specified ? Does the 5 volt spply drop in voltage ? What output current are you drawing ?

Les.
When using inductor less than 100uH the circuit doesn't work at all and because of that sometimes i need to use like 200uH inductor and the current draw is very small like 80mA -- 8 LEDs
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
I have removed the resistor R1= 10.7K and R2 = 1.2K from the feedback and everything turns good...whats the point of the schematic from linear technology make it that way i don't understand
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
My first guess is that one of the resistors is the wrong value. My second guess would be that you have a subtle miswire.
I measured exactly the value of the resistor using a multimeter on the feedback 10.7K (+-) and also 1.24K (+-) and

would you mind to explain what is the point of the Vc resistor R3 and also the capacitor 1uF -- when i took it off or put it in it does virtually nothing.
 

Alec_t

Joined Sep 17, 2013
12,812
They are compensation components to ensure stability, to cater for varying loads. Instability may account for why you were only getting an apparent 8V output. I say 'apparent' because a digital multimeter on a DC Volts range requires a steady input to give an accurate reading.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
They are compensation components to ensure stability, to cater for varying loads. Instability may account for why you were only getting an apparent 8V output. I say 'apparent' because a digital multimeter on a DC Volts range requires a steady input to give an accurate reading.
Actually i have another step up circuit using LM2577 IC and it works exactly as expected...in comparison the output effect between LM2577 and LT1172 using 5 volt is way so much different. And LT1172 is very weak...i try find all sort of ways to change connection including diode, inductor, resistor, capacitor...seems nothing changed while the LM2577 is pretty easy and work very very well - not much all sort of tweaking...but i want to get the LT1172 to work so that i can compare their performance..
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
I tested again maybe i am closer to the answer...without load it generate 11.7 volt but when load 8 volt LED is being used the voltage drop down to 8 volt...can anyone advise where do you think i should tweak this time
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
229
I tested again maybe i am closer to the answer...without load it generate 11.7 volt but when load 8 volt LED is being used the voltage drop down to 8 volt...can anyone advise where do you think i should tweak this time
Now i can see clearly maybe the inductor value need higher test on 100uH gives 9.6 volt with load...increase to 240uH increase to 10.5 volt so maybe need higher like 300uH or 470uH
 
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