SI8233 Datasheet, PDF (23/57 Pages) Silicon Laboratories – 0.5 AND 4.0 AMP ISODRIVERS

Si823x
3.4. Power Supply Connections
Isolation requirements mandate individual supplies for VDDI, VDDA, and VDDB. The decoupling caps for these
supplies must be placed as close to the VDD and GND pins of the Si823x as possible. The optimum values for
these capacitors depend on load current and the distance between the chip and the regulator that powers it. Low
effective series resistance (ESR) capacitors, such as Tantalum, are recommended.
3.5. Power Dissipation Considerations
Proper system design must assure that the Si823x operates within safe thermal limits across the entire load
range.The Si823x total power dissipation is the sum of the power dissipated by bias supply current, internal
parasitic switching losses, and power dissipated by the series gate resistor and load. Equation 1 shows total
Si823x power dissipation.
P_D = V_DDII_DDI + 2I_DD2V_DD2 + fQ_TLV_DD2
-------^R-----^p--------
R_p + R_g
+ fQ_TLV_DD2
-------^R-----ⁿ--------
R_n + R_g
+
2
fC
i
n
tV_D
D
2
2
where:
P_D is the total Si823x device power dissipation (W)
I_DDI is the input-side maximum bias current (3 mA)
I_DD2 is the driver die maximum bias current (2.5 mA)
C_int is the internal parasitic capacitance (75 pF for the 0.5 A driver and 370 pF for the 4.0 A driver)
V_DDI is the input-side VDD supply voltage (2.7 to 5.5 V)
V_DD2 is the driver-side supply voltage (10 to 24 V)
f is the switching frequency (Hz)
Q_TLis the total highside bootstrap charge (see Section 2.2 of AN486)
R_G is the external gate resistor
R_P is the R_{DSON}of the driver pull-up switch: (Rp=15 for the 0.5A driver; Rp=2.7 for the 4.0A driver)
R_n is the R_{DSON}of the driver pull-down switch: (Rn=5 for the 0.5A driver and 1 for the 4.0A driver)
Equation 1.
Power dissipation example for 0.5 A driver using Equation 1 with the following givens:
V_DDI = 5.0 V
V_DD2 = 12 V
f = 350 kHz
R_G = 22 
Q_G = 25 nC
Pd = 0.015 + 0.060 + 350  10³ï€©ï€¨25  10^–912
-------¹---⁵---------
15 + 22
+ 350  10³ï€©ï€¨25  10^–912
-------⁵--------
5 + 22
+ 2350  10³ï€©ï€¨75  10^–12144= 145 mW
From which the driver junction temperature is calculated using Equation 2, where:
Pd is the total Si823x device power dissipation (W)
_ja is the thermal resistance from junction to air (105 °C/W in this example)
T_A is the ambient temperature
Rev. 1.7
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