AN497 Datasheet, PDF (1/10 Pages) Silicon Laboratories – ADDING OVERCURRENT PROTECTION TO ISODRIVERS

AN497
ADDING OVERCURRENT PROTECTION TO ISODRIVERS
1. Introduction
The high peak currents in large switch mode power systems require fast response current protection (OCP) circuits
to ensure reliable operation. OCP circuits tend to be complex and often use power-inefficient resistive current
sensing. This application note discusses OCP circuits based on the Si850x/1x ac current sensor, including high-
side and high-side/low-side OCP techniques using latch-off protection and cycle-by-cycle current limiting.
2. Si850x/1x AC Current Sensor Overview
The Si850x/1x family of isolated ac current sensors operate over a 50 kHz to 1 MHz frequency range and are
available in full-scale measurement options of 5, 10, and 20 A. These devices offer smaller size (4 mm x 4 mm x
1 mm), lower loss (1.3 m primary series resistance), higher measurement accuracy (±5% of measurement), and
lower external BOM compared to current sense transformers.
AC Current
Gate Control
Timing
RESET
LOGIC
INTEGRATOR
Pick-Up
Coil
Si850x/1x
CHIP (DIE)
SIGNAL
CONDITIONING
Output
TEMP
SENSOR
ADC
AUTO
CALIBRATION
LOGIC
Figure 1. Si850x/1x Block Diagram
Referring to Figure 1, ac current flowing through the sensor primary (metal slug) induces voltage Lmdi/dt into the
on-die pick-up coil. Integrated signal processing circuitry “reconstructs” the current waveform by performing a finite
integral over the switching period (i.e. integration of the first derivative). The resulting current waveform is corrected
for temperature and offset error, then gained-up and buffered to produce a 2 Vpp full-scale, low-noise current
signal. Local system gate control signals are used to reset the on-chip integrator before each measurement cycle,
eliminating the need for external components commonly required for current transformer core reset. For more
information, see the Si85xx AC Current Sensor data sheet.
Rev. 0.1 5/10
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AN497