SI8285 Datasheet, PDF (12/32 Pages) Silicon Laboratories – System Safety Features

Si8285/86 Data Sheet
Applications Information
C_L
=
0.42
− (VDDA + VDDB)5 ×
0.111 × VDDB²( f )
10^−3
−
2.703 × 10^−8
VDDB
Equation 3
Power dissipation example for Si828x driver using Equation 1 with the following givens:
V_DDA = 5.0 V
V_DDB = 18 V
f = 30 kHz
R_H = 10 Ω
R_L = 15 Ohms
Q_G = 85 nC
( )( )( ) ^PD
=
(5)(0.005)
+
(15)(0.005)
+
(2
×
10⁴)(3
×
10^−9)(18)
+
1
2
2 × 10⁴
25 × 10^−9
18
2.6
2.6 + 10
+
0.8
0.8 + 15
= 100mW
From which the driver junction temperature is calculated using Equation 2, where:
Pd is the total Si828x device power dissipation (W)
θja is the thermal resistance from junction to air (60 °C/W in this example)
TA is the maximum ambient temperature (125 °C)
Tj = Pd × θja + TA
Tj = (0.1) × (90) + 125 = 134°C
Calculate maximum loading capacitance from equation 3:
1. VDDA = 5 V and VDDB = 12.5 V.
C_L
=
1.92
× 10^−2
f
− 2.16 × 10^−9
2. VDDA = 5 V and VDDB = 18 V.
C_L
=
8.48
×
f
10^−3
−
1.5
×
10^−9
3. VDDA = 5 V and VDDB = 30 V.
C_L
=
2.45
×
f
10^−3
−
9.01
×
10^−10
Graphs are shown in the following figure. All points along the load lines in these graphs represent the package dissipation-limited value
of C_L for the corresponding switching frequency.
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