I am using the LM3478 in a conventional boost regulator with Vin nominally between 8V and 15VDC, Vout = 125V, 100mA load. F = 100KHz and nominal duty ratio is 33% but grows to 60% with Vin = 9V. It is operated from 12V lead-acid battery. It works to my satisfaction when input voltage is within specs (9V < Vin < 15V) but the circuit has the annoying habit of smoking front end components and switch transistor when battery runs low. This should rarely happen but in long run is unavoidable. If Vbatt runs down to about 7V then DR pin of LM3478 freezes HIGH holding the Xsistor ON and sucking the remaining battery charge down through a SB fuse, protection diode, inductor and switch. This can amount to several amps for several seconds. The fuse will eventually blow but not before being protected by the failing transistor, et al first. Typical input current is 1.3ADC.
I understand that the chips changes bias mode at about 7.2V but it is supposed to keep operating down to 3.3V or thereabouts. It doesn't. On testing, the DR pin WILL go low at Vin=3.3V but not before it has drained all of the charge to be had from the input first.
Three specimens behave this way.
Why doesn't the chip keep operating below 7V?
Why does the Isen pin not shut the DR signal down at that point?
Could ton+toff be in danger of crashing 100% with low Vin? Even if that did happen why would Isen feedback not kill DR?
Do I need to create low Vin detector and kill Vin at 7V approach?
Photo shows current through Q at vert = 2.5A/div and Voltage at drain of Q with Vin at vert = 50V/div, probably about 8V.
Any help would be appreciated.
Schematic follows: