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Thermal Management

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Thermal Management Thermal Management New JRC’s stepper motor ICs are power Ics encapsulated in Dual in Line (DIP), EMP and PLCC (Plastic Leaded Chip Carrier) packages. The silicon die is directly bonded to a heat-spreading lead-frame for efficient heat-transfer to an external heat sink, or to a copper ground plane on the printed circuit board. Theory of power dissipation To explain thermal management and power dissipation ohm’s law could be used, see figure 1. The power dissipation PD could be compared to the current in ohm’s law. The power dissipation is generating heat, which is flowing from the chip out of the package. In its way out it’s “held back” by the thermal resistance. The thermal resistance is depending on what kind of “connection” there is from the chip to the air surrounding the circuit. There are two “connections” out from the chip to the surrounding air, one metallic and one plastic. The metallic “connection” is the lead frame in the package; the pins on the circuit, the soldering to the copper area on the PCB and the copper area it self to the air see figure 2. There is also a connection from the chip through the plastic package out in the air. Considering ohm’s law in figure 1 with a constant voltage at V2 it could easily be seen that if the current, I, is increased with a constant resistance, R, the voltage V1 will be increased compared to the constant voltage V2. This could now be compared to the thermal management. If there is a constant ambient temperature, TA, and the power dissipation is increased with a constant thermal resistance, RthJ-A, the temperature on the chip, TJ will be increased. If there are defined maximum ambient temperature, maximum chip temperature and a known maximum power loss the needed thermal resistance could be calculated. Determine the amount of PCB area. In figure 3 the thermal resistance could be found for PLCC, DIP and EMP packages depending on how big the copper area is connected to the batwing pins. The graph shows the thermal resistance when the PCB cooling is made on the same side as the circuit. If there is no space on the circuit side vias could be used for PLCC and EMP’s to lead the heat through the card to copper areas on the other side of the PCB. Determining the needed amount of PCB copper area for heat sinking is a simple procedure, following a few basic guidelines. T J TA PD TA RthJ-A TJ RthJ-A Equivalent TJ PD TA “Circuit” RthC-A T-T=RthJ A J-A PD TA RthJ-C RthJ-pin RthB-A Rthpin-B R TJ V1 I V2 Ohm’s Law V12 -V =R I Figure 1. Comparing thermal Figure 2. Complete transport ways resistance with ohm’s law. for heat out of the circuit. 1. Establish a value of the circuit’s power dissipation, PD. Graphs showing typical power dissipation vs. output current are found in our data sheets. 2. Specify maximum operating ambient temperature, TA. 3. Specify maximum junction temperature, TJ, the temperature of the chip at maximum operating current. No strict rules exist— typically one should design for a maximum continuous junction temperature of 100 °C to 130 °C. Maximum rating is 150 °C. The maximum value of thermal resistance junction-to-ambient, RthJ-A, can now be calculated as follows: RThJ-A = (TJ - TA) / PD The graph in figure 3 shows thermal resistance junction-to-ambient vs. the area of a copper ground plane of a PC board. Use this graph to determine the area required to achieve the value of the thermal resistance as calculated. Figure 4 summarizes the output current capability of New JRC’s stepper motor drivers for various levels of heat sinking. RthJ-A Thermal resistance [o C/W] Example of PCB layout. For example on other packages see 90 the data sheet for the specific circuit. 80 70 60 24 pin EMP 50 40 30 5 10 15 20 25 30 35 2 PCB copper foil area [cm ] 24 pin EMP 28 pin PLCC 16 pin DIP 22 pin DIP 22 pin DIP Figure 3. Typical thermal resistance vs. PC Board copper area and suggested layout. Dual Drivers (both channels on) NJM3771 NJM3772 NJM3773/3775 NJM3774 NJM3777 I Single Drivers 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 (A) NJM3717 NJM3770A With PCB-heatsinking. Rth = 40°C/W, P = 2 W, T =70°C, T = 150°C. J-A D A J ° ° ° With extensive heatsinking. Rth J-A =27 C/W, PD = 3 W, TA =70 C, TJ = 150 C. Maximum output current. Figure 4. Output current capability for New JRC’s stepper motor driver ICs. Verifying In order to verify the thermal design, it is useful to make an estimate of the real chip temperature. Attaching a thermocouple or some other miniature temperature sensor right onto the bat-wing ground pins of the IC under test, and measuring the temperature of the batwing pin (TBW) does this. The chip (junction) temperature can now be calculated: TJ = TBW + PD x RThJ-BW where RthJ-BW is the thermal resistance from junction to the batwing pin. Package RThJ-BW PLCC 9°C/W DIP 11°C/W EMP 13°C/W Extra heatsink Using extra heatsink for the DIP packages we recommend heatsinks from Colombia-Staver UK, se figure 5. For the 16 pin DIP package V7-3S or V8-3S and for the 22 pin DIP package V7-7, V8-7 or V7-7ST, V8-7ST. For further information please contact: V7 V8 38,5mm 11,6 mm 11,6 Figure 5. Heatsinks, type V7 and V8 by Columbia-Staver UK..
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