Multiphase Buck Converter Controller

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I'm thinking of using the National Semiconductor LM2639 Multiphase PWM
Controller to design a high voltage Buck Converter. The chip is
designed to provide outputs from 1.3 to 3.5V, depending on the
programming code. I have two questions:
1) In order to provide higher voltages Im thinking of providing a
separate 5V Vcc supply, and connecting the switching MOSFETs to a high
voltage unregulated supply. I would also insert a voltage divider in
the feedback path to "fool" the controller into providing a higher
output voltage. Has anyone tried this?
2) The LM2639 is listed by National as "Not recommended for new
design". Does anyone know why, and know of a suitable replacement.
Basically, I need a multi-phase PWM buck converter controller capable
of providing around 70VDC.

 

[legg]

I'm thinking of using the National Semiconductor LM2639 Multiphase PWM
Controller to design a high voltage Buck Converter. The chip is
designed to provide outputs from 1.3 to 3.5V, depending on the
programming code. I have two questions:
1) In order to provide higher voltages Im thinking of providing a
separate 5V Vcc supply, and connecting the switching MOSFETs to a high
voltage unregulated supply. I would also insert a voltage divider in
the feedback path to "fool" the controller into providing a higher
output voltage. Has anyone tried this?
2) The LM2639 is listed by National as "Not recommended for new
design". Does anyone know why, and know of a suitable replacement.
Basically, I need a multi-phase PWM buck converter controller capable
of providing around 70VDC.

So long as you provide housekeeping power and isolate the drive signal
from switches operating on the HV bus, you are free to do whatever you
want with LV controllers.

Synchronizing commodity controllers for interleaved operation is no
great expense.

I suggest, however that a 2MHz conversion frequency is poorly suited
to higher voltage applications, regardless of the control method used.

Originally aimed for local VRMM and point of load regulators for PII
processors in Y2K, using the VRM 8X standard. The new standard is
VRM10 for P4 or later.

RL