SI8285 Datasheet, PDF (10/32 Pages) Silicon Laboratories – System Safety Features

Si8285/86 Data Sheet
Applications Information
3.1.3 Reset, RDY, and Fault
The Si828x has an active high ready (RDY) push pull output, an open drain fault (FLTb) output, and an active low reset input (RSTb)
that require pull-up resistors. Fast common-mode transients in high-power circuits can inject noise and glitches into these pins due to
parasitic coupling. Depending on the IGBT power circuit layout, additional capacitance (100 pF to 470 pF) can be included on these
pins to prevent faulty RDY and FLTb indications as well as unintended reset to the device.
The FLTb outputs from multiple Si828x devices can be connected in an OR wiring configuration to provide a single FLTb signal to the
MCU.
The Si828x gate driver will shut down when a fault is detected. It then provides FLTb indication to the MCU and remains in the shut-
down state until the MCU applies a reset signal.
3.1.4 Desaturation
The desaturation sensing circuit consists of the blanking capacitor (100 pF for Si8286 and 390 pF for Si8285), 100 Ω current limiting
resistor, and DSAT diode. These components provide current and voltage protection for the Si828x desaturation DSAT pin, and it is
critical to place these components as close to the IGBT as possible. Also, on the layout, make sure that the loop area forming between
these components and the IGBT is minimized for optimum desaturation detection.
3.1.5 Driver Outputs
The Si8285 has VH and VL gate drive outputs (see Figure 3.1 Recommended Si8285 Application Circuit on page 7). They work with
external RH and RL resistors to limit output gate current. The value of these resistors can be adjusted to independently control IGBT
collector voltage rise and fall time. The Si8286 only has one VO gate drive output with an external gate resistor to control IGBT collector
voltage rise and fall time (see Figure 3.2 Recommended Si8286 Application Circuit on page 7). To achieve independent rise and fall
time control, it is suggested to add a pair of fast diodes to the Si8286 VO circuit (see Figure 3.3 Recommended Si8286 Application
Circuit with RH and RL on page 8).
The CLMP output should be connected to the gate of the IGBT directly to provide clamping action between the gate and VSSB pin. This
clamping action dissipates IGBT Miller current from the collector to the gate to secure the IGBT in the off-state. Negative VSSB pro-
vides further help to ensure the gate voltage stays below the IGBT’s Vth during the off state.
3.2 Layout Considerations
It is most important to minimize ringing in the drive path and noise on the supply lines. Care must be taken to minimize parasitic induc-
tance in these paths by locating the Si828x as close as possible to the device it is driving. In addition, the supply and ground trace
paths must be kept short. For this reason, the use of power and ground planes is highly recommended. A split ground plane system
having separate ground and power planes for power devices and small signal components provides the best overall noise performance.
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