DRIVER & CONTROLLER FOR STEPPER MOTORS INTEGRATED CIRCUITS
N-MOS and P-MOS power stages
Valid for TMC262, TMC389, TMC239, TMC249
This application note compares the mixed N and P channel MOSFET power stages supported by the
TRINAMIC ICs to the more commonly used pure N channel power stages.
Table of Contents
1 Understanding the Difference .................................................................................................................................. 1
2 Effective Resistance of the N&P Channel Half-Bridge........................................................................................ 3
3 Revision History ............................................................................................................................................................ 5
1 Understanding the Difference
DISCRETE MOSFET POWER BRIDGES
Driving a two phase bipolar stepper motor requires four half bridges able to drive the motor current.
The motor current is regulated using chopper operation at a frequency of several ten kHz. In order to
drive a stepper motor at higher motor current, a driver chip using external MOSFETs is the most
efficient solution. This is, because discrete MOSFETs use a dedicated semiconductor technology
different from the technology in integrated circuits. Further, a discrete MOSFET is an extremely rugged
and robust device. It is safe against overvoltage spikes up to a certain energy (due to zener effect), it
is not easily destroyed by high temperatures (the devices will survive short term operation far in
excess of 200°C) and provides an integrated reverse diode.
P CHANNEL IS THE NATURAL CHOICE FOR HIGH SIDE
Basically, there are two types of power MOSFETs: N channel and P channel devices. While an N
channel device is the natural choice for a low side switch, a P channel device is the natural choice for
a high side switch. In both configurations the MOSFETs source terminal is connected to the supply
voltage terminals (GND or VS) and the gate drive voltage is identical to this supply voltage for an off-
state, or a gate control voltage in between of GND and VS for the on-state. This gate to source
voltage typically lies in between 4.5V and 20V (N), respectively -4.5V to -20V (P). The TMC262, TMC389
and TMC239/249 family of stepper driver ICs use N&P channel power MOSFETs.
Many competitors’ products use N-channel MOSFETs also as high-side driver. In this configuration,
Source and Drain are swapped when compared to the P channel MOSFET. The MOSFET then operates
in a voltage follower circuit. Driving this MOSFET is more complex, because its gate now is related to
the motor driver output, i.e. to a floating level rather than to a fixed level like a power supply rail.
Additionally, its gate must be driven to a voltage higher than the positive supply voltage in order to
switch on the MOSFET completely. Figure 1.1 shows both kinds of power stages for a half bridge.
DIFFERENCE IN THE DRIVER
A P channel MOSFET is the natural choice for a high-side driver. It is efficient, rugged and easy to
control. Operation is possible up to the maximum supply voltage of the driver chip. A high-side N-
MOS gate driver is more complex and subject to issues at high dU/dt, because the driver itself is
floating, i.e. its relative GND potential is identical to the driver output. Sometimes this is perceived
when an unforeseen event (e.g. ESD event on the outer side of the motor) leads to an accelerated
slope. As the slope is seen by the N channel high side driver control chip directly, it might exceed the
maximum allowable dU/dt. This can destroy the driver and lead to a sudden defect.
An N-channel high side driver also requires additional passive components to be supplied by the user:
One bootstrap capacitor per high-side MOSFET is necessary in order to provide the high-side control
TRINAMIC Motion Control GmbH & Co. KG
Hamburg, Germany
Application Note 006 (Rev. 0.10 / 2013-OCT-22) 20
voltage. A few devices use an additional common rail charge pump, which again requires two
capacitors.
+VM +VM
Floating gate
driver
High side
D N-MOS S
G
G
UG P-MOS (high-side)
S
CBOOT D
Out Out
D D
UG G
Low side N-MOS UG G
N-MOS (low-side)
S S
N channel power stage N & P channel power stage
Figure 1.1: Comparing N&N-MOS power stage to N&P-MOS power stage
Parameter N&P power bridge N&N power bridge
MOSFET Types Many manufacturers offer half-bridge A single device type can be used for
devices or single devices in the voltage all MOSFETs. There are a huge number
range 30V to 60V. Examples are shown of single devices and a number of half
in the TRINAMIC chip datasheets. bridge devices available.
Cooling via The MOSFETs for each power half- The low-side MOSFETs drain is tied to
MOSFET drain bridge have a common drain terminal. the half-bridge output, while the high-
terminal Each half bridge has a single electrical side MOSFET drain is at the positive
potential at the heat slug. An efficient motor supply voltage. There are five
single-heat slug package is possible for different electrical potentials for heat
a half bridge device. dissipation. When building a medium
current driver with dual devices, a
separated heat slug per MOSFET is still
required.
On Resistance P channel devices are less efficient than On resistance is determined by the
N channel devices. They have about choice of MOSFET. Typically all
150% to 200% of the on-resistance MOSFETs regardless of high-side or
RDSon at the same die size (i.e. same low-side use are the same type and
cost). This results from a P-channel provide the same on resistance.
device using minority carrier
conduction.
This fact has only minor influence on
the half bridge power dissipation, due
to much lower duty cycle on the high-
side MOSFET.
Gate Driver Same principle as low side driver, Floating driver is sensitive to damage
rugged device. by high dU/dt transients.
Additional parts No additional components required. Floating driver requires bootstrap
for high-side capacitors. This makes up for typically
control four additional passive components
per stepper motor.
Voltage Range The application benefits from the full The application voltage of the gate
voltage range covered by the gate driver is reduced by the bootstrap
driver technology. voltage, typically 5 to 12V.
Table 1.1 Comparison chart
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Application Note 006 (Rev. 0.10 / 2013-OCT-22) 30
AVAILIBILITY
P channel MOSFETs have been rare on the market several years ago, but extended use in monitor
backlighting inverters and switch regulators has led to an increasingly good availability within the
last years. Many devices are available from large discrete component suppliers like Alpha and Omega,
Fairchild, Infineon or Vishay. Increasingly a number of smaller and Asian suppliers offer a wide choice
of devices with extremely competitive data, e.g. Diodes, Central semiconductor, APEC or UBIQ.
2 Effective Resistance of the N&P Channel Half-Bridge
In order to understand the influence of the P channel MOSFET on the actual driver power dissipation,
it is important to check the amount of time it conducts the motor current. The actual duty cycle
depends on motor velocity, supply voltage and chopper settings. Typical duty cycles of a stepper
motor coil in chopper operation within a microstep application range from 20% to 50% on-state duty
cycle (for standby, low and medium velocity operation). The rest is slow decay duty cycle (i.e. 50% to
80%). The on-state duty cycle gives the conduction time for any high-side MOSFET. That means either
one or the other high side MOSFET conducts. For the low side MOSFETs, at least one MOSFET is on all
the time, i.e. 100% duty cycle for this MOSFET. The other MOSFET has a duty cycle of 100% (the high-
side duty cycle), i.e. 50% to 80%.
See Figure 2.1 for an example measured in an actual application (motor running) using a TMC262 and
spreadCycle operation. The chopper cycle spans for 24µs, made up from roughly 7µs of on-time, 7µs
slow decay time, 3µs negative on-state time (fast decay) and another 7µs slow decay time. The mean
on-state duty cycle of a high-side MOSFET thus is (7+3)/24/2 = 20.8%. The mean on-state duty cycle for
a low-side MOSFET is (24+14)/24/2 = 79.2%
This means:
A high-side MOSFET has a typical medium duty cycle of 10% to 25%.
A low-side MOSFET has a typical medium duty cycle of 75% to 90%.
Thus the effective bridge resistance for an N and P channel bridge is mainly determined by the low
side, N channel MOSFET and has a lower dependence on the P channel MOSFET.
Figure 2.1 On, slow decay and fast decay states as seen on the sense resistor
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Application Note 006 (Rev. 0.10 / 2013-OCT-22) 40
This is, because the chopped operation of a motor uses the coil inductivity to store the energy. The
voltage drop on the motor coil resistance therefore must be substantially less than the nominal
supply voltage. Typically only a few volts of voltage drop in comparison to several 10 volts of supply
voltage. This means that the medium duty cycle of the chopper (in operation at low velocities) is
determined by the quotient of coil voltage drop and supply voltage. A slightly higher duty cycle
results from mixed decay operation and from back-EMF at higher velocity motion. The TRINAMIC ICs
use the low side MOSFETs for re-circulation during slow decay phases. At high motor velocity, the
chopper switches on for higher duty cycles up to about 90%, because the motor current cannot follow
the desired target current anymore. On the other hand, the overall current drops in this mode and the
number of chopper cycles reduces, thus reducing both static and dynamic power dissipation.
EXAMPLE
A pure N channel MOSFET half-bridge based stepper driver is compared to an N&P channel half bridge
based stepper driver. Both designs shall show the same effective on-resistance:
Assumption:
Duty cycle high-side MOSFET = 20%
Duty cycle low-side MOSFET = 80%
The P-channel MOSFET has a 1.75 times higher on-resistance than the N-channel MOSFET
Parameter N&P power bridge N&N power bridge for comparison
Low-Side switch resistance Rn Rn1
High-Side switch Rp=1.75Rn Rn1
Resistance (assumption)
Low-Side switch resistive power I*Rn*80% I*R1*80%
dissipation
High-Side switch resistive power I*Rp*20% I*R1*20%
dissipation
Resistive power dissipation for a half I*Rn*(0.8+1.75*0.2) I*R1
bridge = I*Rn*1.15
Required MOSFET resistance for same Rn=87%*R1 R1
power dissipation
Result: In a mixed N&P bridge with the given assumptions, the actual on-resistance of the N-MOS
must be 87% of the comparative pure N-channel bridge MOSFET, the actual on-resistance of the P-MOS
can be 152% of the comparative pure N-channel bridge MOSFET in order to yield the same bridge
power dissipation.
2.1 Calculation of Actual Power Dissipation
Please refer our product specific spread sheet to calculate the actual power dissipation of a bridge
(e.g. TMC262_calculations.xls for the device TMC262). It also allows calculation of power dissipation
using any MOSFET type described by its electrical data.
Figure 2.2 Example excerpt showing N&N bridge vs. N&P bridge at 4A with identical NMOS
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Application Note 006 (Rev. 0.10 / 2013-OCT-22) 50
3 Revision History
Version Date Author Description
BD – Bernhard Dwersteg
0.10 2013-OCT-22 BD First preliminary version.
Table 3.1 Document revisions
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