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Hmc988 Clodck divider and Hmc835 PLL running hot

Question asked by matangk on Aug 11, 2017


I have an integrated board, that contains both parts. The 3.3V rail of the PLL is generated off-chip, as recommended using Hmc1060 LDO regulator. Both parts are laid out above complete ground plane with thermal vias.

The divider works at 1/8 and drives the PLL with CML biased outputs. The PLL works in differential mode with only LO1 enabled, its input clock is 120MHz, divided by 3 and multiplied by 32 to output of 1.28GHz. The parts program fine and functional with my MCU.


the combined current consumption of them both is ~310mA, which fits, according to datasheets to

PLL: 6 (CP)+ 105(5V)+ 48(3.3V)=160mA

Divider: 122mA (min) - 173mA (max)


Both parts get a bit too hot in my opinion (cannot touch them for more than a few seconds). they both are assembled above a large copper plate (no heat sinks on the package though). I estimate they are both in the 70-80 degC range. More accurate tests are under-way. The temp. rating for both part are:

PLL: Thermal resistance: 9 C/W, recommended temp (backplate)=85degC

Divider: Thermal resistance: 25 C/W, max junction temp = 125degC.


The divider certainly feels too hot.

The PLL feels hot as well.


Does anyone have any input/experience regarding those heating issues?

If needed I'll use a heat sink, but I don't remember the PLL eval-board getting that hot, and it doesn't have a heat sink on.


Thanks ahead