# Heatsinks for LM4780 in dual mono

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#### mattjuk81

Could anyone recommend 2x suitable heatsinks (size and/or thermal rating) to mount 2 lm4780 chips on their own heatsinks, running from two 300va 2x25v transformers please?

#### jackinnj

Thermal impedance information is in the datasheet (with a formular) -- C/W depends upon the rail voltage, target output impedance, etc., etc.

#### mattjuk81

Thermal impedance information is in the datasheet (with a formular) -- C/W depends upon the rail voltage, target output impedance, etc., etc.

The thing is I don't really understand how to use that excel file to work it out, would you mind helping me at all?

#### Digitalscrap

The datasheet for the LM4870 is quite nice...it really does spell things out pretty explicitly, what is giving you trouble?

The chip is designed to dissipate heat, and the amount of heat it must dissipate depends on how much power you are trying to push through the chip (which is a function of supply voltage, load, etc).

The junction temperature should not rise above 150 degC. So use equation 2 to calculate Pdmax. Then you can use equation 4 to calculate THETAsa. Buy a heatsink that has less thermal resistance than THETAsa, and you're all set. (small thermal resistance = heat flows easily = large thermal dissipation)

Many people just buy a big honking heatsink. An oversized heatsink won't hurt anything.

#### mattjuk81

The datasheet for the LM4870 is quite nice...it really does spell things out pretty explicitly, what is giving you trouble?

The chip is designed to dissipate heat, and the amount of heat it must dissipate depends on how much power you are trying to push through the chip (which is a function of supply voltage, load, etc).

The junction temperature should not rise above 150 degC. So use equation 2 to calculate Pdmax. Then you can use equation 4 to calculate THETAsa. Buy a heatsink that has less thermal resistance than THETAsa, and you're all set. (small thermal resistance = heat flows easily = large thermal dissipation)

Many people just buy a big honking heatsink. An oversized heatsink won't hurt anything.

Ok thanks i've given it a go but still find it confusing, when you say an oversized heatsink, what size would that be haha? i am using 35v rails and 8ohm speakers, i was going to buy some 300mm x 40mm x 80mm heatsinks

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