Datasheet

TMC5160 DATASHEET (Rev. 1.01 / 2017-NOV-29) 19
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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 MOSFET or high-side MOSFET. Diode conduction occurs
during break-before make time when the load current is inverse to the prior bridge polarity, i.e.
following a fast decay cycle. 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. It must be ensured, that the driver
IC does not see spikes on its BM pins to GND going below -5V. 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.
To improve behavior
- Tune MOSFET switching slopes (measure without inductive load) to be slower than the MOSFET
bulk diode reverse recovery time. This will reduce cross conduction.
- Add optional resistors and capacitors to ensure clean switching by minimizing ringing and
reliable operation. Figure 3.5 shows different options.
- Some MOSFETs eliminate this problem by integrating a Schottky diode from source to drain.
Figure 3.7 shows performance of the basic circuit after adapting switching slope and adding 1nF
bridge output capacitors.
LS
GNDD
GNDA
DIE PAD
LS
BMA2
SRAH
Capacitor reduces
ringing on sense resistor.
R
S
SRAL
LA1
LA2
HA1
HA2
BMA1
HS
HS
CA1
C
B
CA2
C
B
+V
M
R
G
‘: optional position for high side slope control resistor.
In case, severe undershooting < -5V of BM occurs at BM
terminal, R
G
’ will protect the driver.
1µF
R
G
R
G
R
G
R
G
Coil
out
47R
47R
R
G
‘
R
G
‘
Decide use and value of the additional components based on measurements of the actual circuit using the final layout!
100n
1n,
100V
1n,
100V
470pF to a few nF output
capacitors close to bridge reduce
ringing and improve EMC
2n2
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 R
G
’ eliminate undershooting of
BM and allow for lower values of R
G
‘ (e.g., 2R2 to 4R7).
Figure 3.5 Bridge protection options for power routing inductivity