Datasheet
TMC2590 DATASHEET (V1.0 / 2019-FEB-22) 44
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MOSFETs and adjusting the output slope of the controlled gate charge and discharge. A slower slope
reduces electromagnetic emissions, but it increases power dissipation in the MOSFETs. The actual
choice should be tried out in the application when using the desired MOSFETs.
The duration of the complete switching event depends on the total gate charge of the MOSFETs. In
Figure 11.1, the voltage transition of the gate-charge output (dotted line) takes place during the so-
called Miller plateau. The Miller plateau results from the gate-to-drain capacitance of the MOSFET
charging or discharging during switching. The datasheet for the MOSFETs typically will show a Miller
plateau that only covers a part (for example, one quarter) of the complete charging/discharging event.
The gate voltage level at which the Miller plateau starts depends on the threshold voltage of the
MOSFET and on the actual load current.
MOSFET gate charge vs. switching event
Q
G
– Total gate charge (nC)
V
GS
– Gate to source voltage (V)
5
4
3
2
1
0
0 2 4 6 8 10
V
DS
– Drain to source voltage (V)
25
20
15
10
5
0
V
S
Q
MILLER
Figure 11.1 MOSFET gate charge vs. V
DS
for a typical MOSFET during a switching event
The slope time t
SLOPE
can be calculated as:
Where:
Q
MILLER
is the charge the MOSFET needs for the switching event.
I
GATE
is the driver current setting.
The chopper frequency is typically slightly above the audible range, around 18 kHz to 40 kHz. The
lower limit for the slope is dictated by the reverse recovery time of the MOSFET internal diodes,
unless additional Schottky diodes are used in parallel to the MOSFETs source-drain diode. For most
applications a switching time between 20ns and 250ns is sufficient
Example:
A circuit using the transistor in Figure 11.1 is operated with a gate current setting of 15mA.
The Miller charge of the transistor is about 2.5nC.
Hint
Test the best driver strength setting in the application. A good value is found, if a further increment
of driver strength does not lead to lower supply current (and thus reduced power dissipation).