TMC262 DATASHEET (Rev. 2.07 / 2013-FEB-14)
39
10.3 Slope Control
The TMC262 provides constant-current gate drivers for slope control. This allows adapting the driver
strength to the drive requirements of the power 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 duration of the complete switching event depends on the total gate charge of the MOSFETs. In
Figure 10.1, 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
5
4
3
2
1
0
25
VS
20
15
10
5
0
10
0
2
4
6
8
QMILLER
QG – Total gate charge (nC)
Figure 10.1 MOSFET gate charge vs. VDS for a typical MOSFET during a switching event
The slope time tSLOPE can be calculated as:
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ꢌ
ꢪꢼꢸꢵ
Where:
QMILLER is the charge the MOSFET needs for the switching event.
IGATE 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 100ns and 750ns is chosen.
Example:
A circuit using the transistor in Figure 10.1 is operated with a gate current setting of 15mA.
The Miller charge of the transistor is about 2.5nC.
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