TMC6200 DATASHEET (Rev. 1.01 / 2018-NOV-15)
12
3.3 MOSFETs and Slope Control
The selection of power MOSFETs depends on a number of factors, like package size, on-resistance,
voltage rating and supplier. It is not true, that larger, lower RDSon MOSFETs will always be better, as
a larger device also has higher capacitances and may add more ringing in trace inductance and power
dissipation in the gate drive circuitry. Adapt the MOSFETs to the required motor voltage (adding 5-10V
of reserve to the peak supply voltage) and to the desired maximum current, in a way that resistive
power dissipation still is low for the chosen MOSFET package. The TMC6200 drives the MOSFET gates
with roughly 10V, so normal, 10V specified types are sufficient. Logic level FETs (4.5V specified RDSon)
will also work but may be more critical with regard to bridge cross-conduction due to lower VGS(th)
.
The gate drive current and MOSFET gate resistors RG (optional) should basically be adapted to the
MOSFET gate-drain charge (Miller charge) in order to yield reasonable slope times. Figure 3.3 shows
the influence of the Miller charge on the switching event. Figure 3.4 additionally shows the switching
events in different load situations (load pulling the output up or down), and the required bridge
brake-before-make time.
The following table shall serve as a thumb rule for programming the MOSFET driver current
(DRVSTRENGTH setting) and the selection of gate resistors:
MOSFET MILLER CHARGE VS. DRVSTRENGTH AND RG
Miller Charge
[nC] (typ.)
<10
DRVSTRENGTH
setting
0 or 1
Value of RG [Ω]
≤ 10 (recommended)
≤ 5 (optional)
10…20
0 to 2
20…80
>80
1 to 3
3
≤ 2.5 (optional)
≤ 1 (optional)
The TMC6200 provides increased gate-off drive current to avoid bridge cross-conduction induced by
high dV/dt. This protection will be less efficient with gate resistors exceeding the values given in the
table. For larger values of RG, a parallel diode may be required to ensure keeping the MOSFET safely
off during switching events of the opposite MOSFET.
MOSFET gate charge vs. switching event
10
8
25
VM
20
15
10
5
6
4
2
0
0
25
0
5
10
QMILLER
15
20
QG – Total gate charge (nC)
Figure 3.3 Miller charge determines switching slope
Hints
-
-
Choose modern MOSFETs with fast and soft recovery bulk diode and low reverse recovery charge.
A small, SMD MOSFET package allows compacter routing and reduces parasitic inductance effects.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
