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L6910G Step Down

Question asked by hamilton.john on Jul 23, 2012
I am trying to use a L6910G as a step down regulator for a 0-12V 10A power supply.  I have designed a board very similar to the demo board.  At first I could not get it to stop hic cupping until I tied the Vref to the SS pin.  Now it starts the PWM to the Mosfets for one or two pulses and then stops.  Then it will try again after about 250ms.  By placing a finger (expert engineering method of desperation)  across the cap that goes between the Boot and Phase pins I can increase the number of pulses.  However, It appears the oscillator is running at 1Mhz. - The OSC pin 2 is left open and I expect the oscillator to run at 200KHz.
 
I am feeding my board with 12V.  With no load.  The feedback is set with two resistors 4.7K to Vout behind the inductor and 1K to ground.    On the Boot pin I see a pulse of 24V (is this a voltage doubler circuit?) for the MOSFET.  Again this is only a single pulse at 1MHz and repeats in 250ms. 

The Over Current is set for 20A, and I have zero load on the system so far. 

With that one pulse I can get as much as 3 volts on the output with no load.  As soon as I add a load (should be about 2 A ) it goes away. 

Any one else using this chip?  Any suggestions?

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