Hi Neil, If you take, in this case, roughly 0.5 ohm total (high-side plus low-side) resistance, 5 A and 3 Kelvin per Watt junction to case thermal impedance, you get a temperature difference between junction and case of (5 A)^2 * (0.5 ohm) * (3 K/W) =3D 37.5 degrees C (or K, makes no difference for a delta-T). This means that you need a heatsink which can remove (5 A)^2*(0.5 ohm)=3D12.5 Watts of heat while keeping the IC case below (150-37.5)=3D112.= 5 deg C. That is going to be a challenge given the small area of contact between heatsink and the part (also, the thru-hole part would be better because it would allow direct mounting of the metal back of the part to the heatsink). In real life, you actually will want more margin than this for life, reliability, and also because there will be some additional sources of heat generation inside the part, AND because the Rds_on will get higher, as you point out, as the temperature rises. In addition, you will want to account for the max ambient temp being higher than 25 C. Where did you see the 3.5 Watt max power spec? I looked at the Infineon datasheet and didn't see it. In general, you MUST perform the full thermal budget calculation (like we are doing here) when selecting a part like this. You cannot just say "the part is rated 5 Amps continuous so I can use it at 5 Amps" because power semiconductor datasheets are notorious for giving VERY optimistic max continuous current ratings based on keeping the case of the part at very close to 25 C, which would require and amazing heatsink and often liquid cooling. In this case, it looks like they were not quite so overly optimistic and 5 A may be achievable but very challenging. You are absolutely correct that the Rds_on for this part is terrible. Is this a very old part? When you are driving a brushed DC motor, the motor looks kinda inductive, kinda not. This is because some of the inductive energy is dumped at every commutation step (governed by the mechanical brush commutator inside the motor). Still, I would think that the goal would be to use a PWM frequency which was several times higher than the commutation step rate, AND high enough that the PWM period is much shorter than the L/R time constant of the motor, so that the PWM does not cause a large current ripple but instead operates the motor like a buck SMPS. This would be more efficient than allowing the current to be chopped on and off completely during each PWM cycle. The above may not be possible given the frequency limitations of this device (if it is indeed 2kHz as you suggest). Sean On Mon, Jul 28, 2014 at 1:34 AM, Neil wrote: > This is the first time I'm playing with motor control... using a > TLE5205-2 motor controller/driver chip to run a large throttle body, and > it's getting very hot. I'm figuring there are a few reasons for this... > > (1) The throttle body is large -- drawing up to 4-5A continuous. > > (2) The PWM frequency is high (~4Khz currently, but that's lowest it > seems I can get right now w/o lowering the osc frequency). The > datasheet doesn't specify the max PWM frequency, but I came across an > app note (more of a sales sheet) that talks about the merits of their > different motor control devices, and there it says that these devices > can be controlled by PWM up to 2Khz. The datasheet specifies switching > times of up to 30us, but that's for resistive loads, and I'm driving a > large inductive load. > > (3) The "Low Rds on, typ 200mohm per switch" is actually quite high I > think. That quote is on the first page "Features" section, but the > electrical characteristics show that typical is 220 & 230 mohms per > switch, and max is 350 mohms each switch at 25 deg-C. Those only get > higher as temp rises. Ballpark bestcase math here =3D 5A x (220+230) =3D > 2.25W. FWIW. max power =3D 3.5W. That's significant for an SMD part. > > Is this correct? Or am I doing something wrong here. The only thing I > can really control at this point is the PWM frequency, but not sure if > that will bring it to "normal" operating temps, due to the power calcs. > Somehow though, they specify that this chip can do 5A continuous, so > what might I be missing here? > > Cheers, > -Neil. > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .