TMC246 / TMC246A DATA SHEET (V2.05 / 2009-Jul-30)
13
Protection Functions
Overcurrent protection and diagnosis
The TMC246 uses the current sense resistors on the low side to detect an overcurrent: Whenever a
voltage above 0.61V is detected, the PWM cycle is terminated at once and all transistors of the bridge
are switched off for the rest of the PWM cycle. The error counter is increased by one. If the error
counter reaches 3, the bridge remains switched off for 63 PWM cycles and the error flag is read as
“active”. The user can clear the error condition in advance by clearing the error flag. The error counter
is cleared, whenever there are more than 63 PWM cycles without overcurrent. There is one error
counter for each of the low side bridges, and one for the high side. The overcurrent detection is
inactive during the blank pulse time for each bridge, to suppress spikes which can occur during
switching.
The high side comparator detects a short to GND or an overcurrent, whenever the voltage between VS
and VT becomes higher than 0.15 V at any time, except for the blank time period which is logically
ORed for both bridges. Here all transistors become switched off for the rest of the PWM cycle,
because the bridge with the failure is unknown.
The overcurrent flags can be cleared by disabling and re-enabling the chip either via the ENN pin or by
sending a telegram with both current control words set to “0000”. In high side overcurrent conditions
the user can determine which bridge sees the overcurrent, by selectively switching on only one of the
bridges with each polarity (therefore the other bridge should remain programmed to “0000”).
Over temperature protection and diagnosis
The circuit switches off all output power transistors during an over temperature condition. The over
temperature flag should be monitored to detect this condition. The circuit resumes operation after cool
down below the temperature threshold. However, operation near the over temperature threshold
should be avoided, if a high lifetime is desired.
Overvoltage protection and ENN pin behavior
During disable conditions the circuit switches off all output power transistors and goes into a low
current shutdown mode. All register contents is cleared to “0”, and all status flags are cleared. The
circuit in this condition can also stand a higher voltage, because the voltage then is not limited by the
maximum power MOSFET voltage. The enable pin ENN provides a fixed threshold of ½ VCC to allow a
simple overvoltage protection up to 40V using an external voltage divider (see schematic).
+VM
for switch off at 26 - 29V:
at VCC=5V: R1=100K; R2=10K
at VCC=3.3V: R1=160K; R2=10K
ENN
µC-Port (opt.)
low=Enable,
high=Disable
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
