TMC5160 DATASHEET (Rev. 1.08 / 2018-NOV-19)
19
3.4 Tuning the MOSFET Bridge
A clean switching event is favorable to ensure low power dissipation and good EMC behavior.
Unsuitable layout or components endanger stable operation of the circuit. Therefore, it is important to
understand the effect of parasitic trace inductivity and MOSFET reverse recovery.
Stray inductance in power routing will cause ringing whenever the opposite MOSFET is in diode
conduction prior to switching on a low-side or high-side MOSFET. Diode conduction occurs during
break-before make time whenever the load current is inverse to the following bridge polarity. The
MOSFET bulk diode has a certain, type specific reverse recovery time and charge. This time typically is
in the range of a few 10ns. During reverse recovery time, the bulk diode will cause high current flow
across the bridge. This current is taken from the power supply filter capacitors (see thick lines Figure
3.5). Once the diode opens parasitic inductance tries to keep the current flowing. A high, fast slope
results and leads to ringing in all parasitic inductivities (see Figure 3.6). This may lead to bridge
voltage undershooting the GND level as well as fast pulses on VS and all MOSFET connections. It
must be ensured, that the driver IC does not see spikes on its BM pins to GND going below -5V.
Severe VS ripple might overload the charge-pump circuitry. Measure the voltage directly at the driver
pins to driver GND. The amount of undershooting depends on energy stored in parasitic inductivities
from low side drain to low side source and via the sense resistor RS to GND.
When using relatively small MOSFETs, a soft slope control requires a high gate series resistance. This
endangers safe MOSFET switch off. Add additional diodes to ensure safe MOSFET off conditions with
slow switch-on slopes (shown for right MOSFET pair in Figure 3.5).
Figure 3.7 shows performance of the basic circuit after adapting switching slope and adding 1nF
bridge output capacitors.
RG: Reduce slope and protect the driver against ringing in the
interconnections between MOSFET and driver
+VM
VS
RG ꢀꢁAdditional position for high side slope control resistor. In case,
1R
220nF
severe undershooting < -5V of BM occurs at BM terminal, RGꢂꢁwill
Optional RC filter
protect the driver.
against VS ringing
CB
LOW-
ESR
4.7µF
CA2
Filter capacitors placed near bridge
HA2
HS
CA1
Optional gate diodes in combination
with very high value of RG
CB
HA1
HS
RG
RG
RG
BMA1
Coil
out
BMA2
LA1
RG
470pF to a few nF output
capacitors close to bridge
and / or output reduce
LS
LS
1n,
100V
1n,
100V
RG
RG
LA2
ringing and improve EMC
SRAH
47R
2n2
Capacitor reduces
ringing on sense resistor.
RS
100n
SRAL
47R
RC-Filter protects SRAH /
SRAL and reduces spikes
seen by the chopper
Additional 1A type Schottky Diodes (selected for full VM range) in
combination with RGꢂꢁꢃꢄꢅꢆꢁto 4.7 Ohm) eliminate undershooting of BM.
Decide use and value of the additional components based on measurements of the actual circuit using the final layout!
Figure 3.5 Bridge protection options for power routing inductivity
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