si8235 Silicon Laboratories, si8235 Datasheet - Page 14

• Manufacturer Part Number: si8235
• Description: 0.5 And 4.0 A Isodrivers
• Manufacturer: Silicon Laboratories

Si823x
5.3. 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 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.
5.4. 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 switching
losses, and power delivered to the load. Equation 1 shows total Si823x power dissipation. In a non-overlapping
system, such as a high-side/low-side driver, n = 1. For a dual low-side driver with each driver having an
independent load, n can have a maximum value of 2, corresponding to a 100% overlap between the two outputs.
The maximum power dissipation allowable for the Si823x is a function of the package thermal resistance, ambient
temperature, and maximum allowable junction temperature, as shown in Equation 2:
Substituting values for PDMAX TjMAX, TA, and ja into Equation 2 results in a maximum allowable total power
dissipation of 1.1 W. Maximum allowable load is found by substituting this limit and the appropriate datasheet
values from Table 4 on page 8 into Equation 1 and simplifying. The result is Equation 3 (0.5 A driver) and
Equation 4 (4.0 A driver), both of which assume VDDI = 5 V and VDDA = VDDB = 18 V.
14
P
where:
P
I
I
C
V
V
F is the switching frequency (Hz)
n is the overlap constant (max value = 2)
DDI
QOUT
D
D
DDI
DDO
int
is the total Si823x device power dissipation (W)
=
is the internal parasitic capacitance (75 pF for the 0.5 A driver and 370 pF for the 4.0 A driver)
is the input-side maximum bias current (3 mA)
is the input-side VDD supply voltage (4.5 to 5.5 V)
V
is the driver-side supply voltage (10 to 24 V)
P
where:
P
T
T
θja = Si823x junction-to-air thermal resistance (105 °C/W)
F = Si823x switching frequency (Hz)
is the driver die maximum bias current (5 mA)
DDI
Dmax
jmax
A
Dmax
= Ambient temperature (°C)
I
DDI
= Si823x maximum junction temperature (145 °C)
≤
= Maximum Si823x power dissipation (W)
T
---------------------------
+
jmax
2 V
(
θja
C
C
L(MAX)
L(MAX)
DDO
–
T
A
Preliminary Rev. 0.11
I
QOUT
=
=
Equation 1.
Equation 2.
Equation 3.
Equation 4.
1.4 10
------------------------- - 7.5
1.4 10
------------------------- - 3.7
+
C
×
×
F
F
int
V
–
–
3
3
DDO
–
–
2
F
×
×
)
+
10
10
2n C
–
–
11
10
(
L
V
DDO
2
F
)