Not knowing the particulars of your circuit, some general
things that you may want to consider doing are:
1. Using a pass transistor with a lower on-resistance (Rds).
2. Using multiple pass transistors in parallel so that the current
is distributed between the transistors along with lowering the total
on-resistance (due to paralleling of the pass transistors).
3. Using multiple sense resistors in parallel to achieve the same
net resistance as your one sensing resistor, thus lowering the
power dissipation through each resistor.
I also recommend Application Hint # 17 entitled "P.C. Board Heat
Sinking" located on the Micrel web site.
Alteon Networks, Inc.
Tim Parks wrote:
> Dear all
> We are designing a battery operated unit whose supply uses a Maxim 712
> switching regulator. Our pass transistor and sensing resistor is getting
> extremely hot (75 degrees C) along with some other key components of the
> analog power supply during battery charging. They are all working within
> specification but this causes the whole PCB board temperature to rise
> At present we have a single ground plane for analog and digital systems on
> the board. I'm considering a separate ground plane beneath the switching
> section in an attempt to isolate the high temperature from the digital
> section. I'd intend routing this Analog ground plane in isolation back to
> the DC input ground return.
> Any advice on use of separate ground planes and heat dissipation in surface
> mount devices would be greatly appreciated.