TMC9660-STP-EVKIT: Y2 strong pull-down not working on high-side mosfet

Question

Hi, I have got an additional problem with the TMC9660. The Smart Gate-Drivers are working fine, except for the Gate-Driver of Y2 . It looks like it is not activating the STRONG PD (pull down of 2A) on the High-Side-Gate while the Low-Side-Gate is being pulled high. The rapid decrease of Phase-Voltage does turn on the High-Side-Mosfet. This can only be seen on Y2 and on none of the other 3 Phases. 
 I was able to reproduce the error on the custom PCB as well as on the evaluation kit TMC9660-STP-EVKIT using TMCL-IDE with the configuration wizard.

Is it a configuration that I am missing? In register mode I have configured the following: I have set for UVW and Y2: - ADAPTIVE_MODE: 1 - IGATE_SOURCE: 2 - IGATE_SINK: 2 - T_DRIVE_SOURCE: 0xFF - T_DRIVE_SINK: 0xFF - BBM_H: 0 - BBM_L: 0 Additionally: - MCC_PWM_CONFIG -> Y2_HS_SRC : 0 - MCC_GDRV_HW -> HS_AS_LS_Y2 : 0 Yellow: Phase Voltage Pink: Low-Side-Gate Voltage (with Miller-Plateau) Blue: High-Side-Gate Voltage to Gnd Violet: (Math): High-Side-Gate Voltage 
 Phase Y2:

Phase Y1:

Best Regards 
 Gabriel

BernhardDwersteg · Accepted Answer

Hi Gabriel, I have an update: My colleagues have been able to replicate the issue. Unfortunately it is a bug within the IC, skipping the strong discharge phase for bridge Y2_HS output. I sketched a solution in the attached whitepaper that should resolve the issue. A fixed TMC9660 version will become available in 2027coming together with a bunch of additional enhancements that are currently in work. 
 I'm very sorry for the issue. 
 TMC9660 Gate-Driver Output Bridge Y2 Issue and Recommended Fix 
 Purpose. This whitepaper summarizes a practical approach for identifying and fixing an issue observed on bridge Y2 of the TMC9660 gate-driver output stage. It is intended for the first silicon revision when driving a 2-phase motor, or when using bridge Y2 as an additional half-bridge. It does not apply for use-cases with individual control of one or two MOSFETs using bridge Y2 HS and LS outputs. 
 1. Background 
 The TMC9660 is a new, complex device mainly intended for the control of 3-Phase BLDC motors. It adds a fourth half-bridge control for 2-phase, four wire motors, as well as for different purpose that allows driving additional high-side and low-side MOSFETs. 
 2. Issue Description 
 The reported issue concerns bridge Y2 of the TMC9660 gate-driver output. The observed problem is that the strong pull-down function on the Y2 high-side MOSFET gate does not engage and keep the high-side gate sufficiently discharged when the low-side MOSFET becomes active. As a result, the high-side MOSFET gate can remain partially charged or become unintentionally biased while the low-side MOSFET is being switched on, creating a risk of unwanted high-side conduction, shoot-through current, or abnormal bridge behaviour. This can be observed by increased power draw when activating PWM on bridge Y2. The severity of the issue depends on supply voltage, and ratio of Gate-Drain capacitance of the external MOSFET to its Gate-Sourve capacitance. 
 The key distinction during debug is whether the high-side gate is being discharged quickly and robustly enough when the complementary low-side device turns on. For this issue, the proposed mitigation is not only a parameter change; it is a small external hardware addition that reinforces the high-side gate discharge path using the existing low-side gate-driver signal as the control reference. 
 3. Root Cause 
 
 Insufficient high-side gate discharge on Y2: The TMC9660 high-side MOSFET gate strong dischage is not becoming engaged while the low-side MOFET switches on. 
 
 4. Diagnostic Procedure 
 
 Reproduce the Y2 condition: Operate the TMC9660 application with no motor attached but MOSFET bridges switching with chopper. Monitor current draw at the power supply with bridge Y2 disabled vs. operation with bridge Y2 enabled. A substantial increase of power supply current shows that cross conduction occurs. Carefully test with reduced supply voltage before increasing voltage to the desired operation point. Do not further increase supply voltage if current draw increases more than a few 10mA. 
 Probe the Y2 high-side gate-to-source voltage: Measure directly between the high-side gate and source pins with a low-inductance probing method. 
 Compare low-side turn-on with high-side discharge: Verify whether the high-side gate remains above the safe off-level when the Y2 low-side MOSFET is switched on. 
 
 Additional steps when fix is applied 
 
 Validate the auxiliary MOSFET patch: Confirm that the added MOSFET turns on with the low-side gate signal and clamps the high-side gate below its gate-threshold voltage during low-side conduction. 
 Check driver loading and timing margin: Ensure that the auxiliary MOSFET gate does not overload the low-side gate driver or create timing overlap with normal high-side switching. 
 
 5. Recommended Fix 
 The recommended fix is to add an additional MOSFET close to the Y2 high-side MOSFET gate network. This auxiliary MOSFET is controlled by the TMC9660 low-side gate-driver output. When the low-side MOSFET is commanded on, the auxiliary MOSFET also turns on and provides an active discharge path from the high-side MOSFET gate to GND. In effect, the low-side gate signal enforces a hardware interlock: whenever the low-side device is being driven on, the high-side gate is actively clamped low. 
 The auxiliary MOSFET should be a small switching device capable of conducting a few Ampere and able to resist the full bridge voltage plus high-side gate driver voltage. Ensure adequate gate-threshold margin, identical to or slightly lower than that of the power MOSFET bridge devices, and sufficient pulsed current capability to discharge the high-side gate charge quickly. The same voltage rating as the power bridge MOSFETs plus roughly 10V will fit. A 100V device fits the full application range, in case 80V power MOSFETs are used. Its drain-source path should be connected so that it shorts or strongly pulls down the high-side MOSFET gate to GND. A series source resistor R may be added to limit peak current during the switching events to the MOSFETs rating. In most constellations this resistor should not be required. As the low-side MOSFET engages at the same time as the auiliary MOSFET, conduction occurs at the low-side MOSFETs Miller Plateau. In case the low-side MOSFET has a substantially higher VGTH than the auxiliary MOSFET, a resistor of 1Ohm or less could be applied. A 1A 30V Schottky diode over the high-side MOSFET source to gate prevents the gate from charging to a negative voltage. 
 This fix should be implemented as a local hardware patch or board revision close to the Y2 power MOSFETs. The routing from the auxiliary MOSFET to the high-side gate should be short and low inductance, because the purpose of the circuit is to remove gate charge during the fast transient interval when the low-side MOSFET turns on. 
 Figure 1. Schematic concept for the Y2 fix: a small auxiliary MOSFET is driven from the TMC9660 low-side gate-driver signal and actively clamps the Y2 high-side MOSFET gate to GND while the low-side MOSFET is switched on.

Component

Selection

Auxiliary MOSFET

Voltage rating: Minimum peak supply voltage plus 20V, or bridge MOSFET rating plus 10V 
 Current rating: ~1A continuous, >5A peak. 
 Gate treshold: Identical to, or slightly lower than bridge MOSFET. 
 E.g. BUK6D385-100E for a full 100V device

Schottky protection diode

1A, 30V type, e.g. MSS1P3

Auxiliary MOSFET source resistor R

0 Ohm or up to 1Ohm to limit peak current of auxiliary MOSFET during low-side switch on. Start with 0Ohm. Increase resistor only if high-side gate-to source voltage undershoots 0V and Schottky diode becomes active during low-side MOSFET going active switching event.

6. Validation Checklist

Check

Expected Result

Y2 high-side gate-to-source voltage during low-side turn-on

Gate remains clamped safely below the MOSFET threshold

Auxiliary MOSFET drive signal

Turns on whenever the Y2 low-side gate driver turns on

High-side gate voltage

Gate to source voltage does not increase beyond the gate threshold during low-side MOSFET turn on

Bridge switching under load

No unwanted high-side conduction or shoot-through indication

7. Conclusion 
 The Y2 issue is best understood as a high-side gate-discharge robustness problem. The proposed fix adds a small auxiliary MOSFET controlled by the TMC9660 low-side gate-driver output. During low-side turn-on, this auxiliary device actively discharges and clamps the Y2 high-side MOSFET gate to GND, preventing residual or Miller-induced gate charge from turning the high-side MOSFET on unintentionally. This hardware reinforcement directly addresses the missing strong-pull-down behaviour and should be validated with gate-to-source probing and load switching tests.)

BernhardDwersteg · Answer

Hi Gabriel, great! Thank you for your the feedback! The fixes will be announced with the new device and will be made in a way that allows full compatibility. I do not expect that we change anything that requires re-wiring.

Popular Forums

Product Forums

Application Forums

Design Center Forums

Highlighted Webinar

Places

Libraries

Upcoming Learning & Events

Challenge Yourself!

What's Brewing

Places

Resources

Visit ContentZone

The Latest Read

New Release

Recent Technical Insights

Forums

Learn

Community Hub

ContentZone

Products Mentioned

Thread Details

TMC9660-STP-EVKIT: Y2 strong pull-down not working on high-side mosfet

Thread Summary

Before You Switch