This type of reg does not work all that well. There are a number of issues:
1. The reg cannot regulate very effectively. The inductor current may be set at 1 amp, but the average current is still proportional to duty cycle and duty cycle depends on input voltage. So if the DC input voltage isn't regulated the LED intensity will change with input voltage.
2. The LEDs depend on a cap to keep from turning off while the inductor is charging. The isn't an ideal situation because an LED has a sharp IV curve, the cap need only discharge slightly before the LED current decreases substantially. And it's going to be discharging at 1 amp. So there are 3 requirements for effective filtering- high capacitance (and/or high freq), very low ESR, and a high ripple rating (generally only big quality caps are rated for 1 amp). I thought another inductor and schottky would do a better job but I haven't seen this design suggested.
3. There's a flaw if the power supply is having trouble producing the specified current. The latch will stay on so the MOSFET just becomes a dead short, as does the inductor.
It would be much, much easier if you could do 4 strings of 5. You really, really sure you can't? Just some sharing ballast resistors.
The HV9930 is better designed for boost conversion from what I can tell, but I don't know much about it. I think Zetex may have made a good one too. Ideally I'd look for one whose average output current doesn't depend on duty cycle. My statements about the stability of larger inductors may or may not stay true with a different circuit.
I'm afraid you've lost me here ... I am following Figure 3 in the hv9910 datasheet, I have no resistor between the gate pin and the mosfet gate. Is the resistor internal to the chip?
No. Generallly a gate resistor isn't necessary (it'll slow the transitions and increase the heat) and it wasn't what I was describing. I'm taking about adding an R and C on the feedback voltage from the shunt.