Texas Instruments | Low Voltage Motor Drive Operation With Smart Gate Drive | Application notes | Texas Instruments Low Voltage Motor Drive Operation With Smart Gate Drive Application notes

Texas Instruments Low Voltage Motor Drive Operation With Smart Gate Drive Application notes
Low-Voltage Motor Drive Operation with a Smart Gate
Driver
Introduction
Low voltage operation is a crucial consideration for
motor drive system design. Motor drivers operate in
harsh applications, many of which have inconsistent
loads and operating conditions. For example, battery
powered hand tools have frequent changes in load
which create changes in power delivery from the
battery to the motor. In these situations, the motor may
experience transient periods of low voltage. The
battery supplied system experiences a high load and
the voltage from the battery will drop as the device
tries to draw more current. This voltage drop may
cause the device to trigger undervoltage lockout
(UVLO) and stop operating. Some gate drivers, such
as TI’s Smart Gate Drivers, can avoid problems of low
voltage dropouts with minor modification to maintain
consistent operation during transient periods of low
voltage.
VDS Monitors and VDRAIN
Undervoltage Lockout (UVLO)
Undervoltage lockout is one of many safety features
employed in TI motor driver solutions. Motor drivers
require a certain amount of input voltage to operate
properly. If a certain voltage is not met, the
functionality and performance of the device can be
unknown, making it difficult to predict the system’s
behavior. Some examples of the detrimental effects
include logic functions generating the wrong control
signals and incorrect gate switching, both which could
damage the device and the system. UVLO protection
is included to protect against the negative effects of
supply drop-out and disconnection. Whenever the onchip monitor detects the voltage level is below the
operable threshold, the device outputs are disabled
and the device performs a controlled shut down,
reporting a fault condition. Using the earlier situation
as an example, if a high load causes a voltage drop,
UVLO can trigger and protect the device from
erroneous operation.
Modified Circuit to Improve Low-Voltage Operation
By using a simple diode in series with the driver supply
pin, the device can be operational for transient
durations when a voltage drop occurs or if the supply
is momentarily disconnected. The diode will allow
energy to be stored in the VM capacitor which can
hold up the device supply in the case of battery droop
and avoid triggering UVLO. (see Figure 1). This
modification can easily be used in gate drivers that
implement a VDRAIN pin.
SLVAE64 – December 2018
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Figure 1. Modified Circuit with Diode
TI Smart Gate Drivers implement adjustable VDS
voltage monitors used to detect short-circuit and
overcurrent events on the external power MOSFETs.
These monitors will trigger a fault if the VDS trip point
is reached. The high-side MOSFET monitors measure
the voltage between the VDRAIN and SHx pins. A
VDRAIN pin ensures the sense line for the overcurrent
monitors and the power supply stay separate and
prevents the overcurrent monitor triggering
inaccurately due to measuring the incorrect voltage
difference across the switch. Some gate drivers do not
integrate a VDRAIN pin. These types of devices use
the gate driver power pin (VM for example) as the
reference for the VDS monitor. These devices will not
be operable using this modified circuit because it will
cause the overcurrent monitor to trip incorrectly.
Transient Operation Time
An estimate can be made of the transient time that the
device can stay on despite the battery voltage falling
below UVLO by using basic charge and current
relationships.
(1)
(2)
(3)
The transient time dt is the time that the capacitor will
discharge from nominal voltage to the UVLO threshold
voltage. The difference between these two voltages is
denoted by VNOM – VUVLO. The total current, ITOT, is the
sum of the given internal device quiescent current plus
the total average current drawn from the FETs that are
Low-Voltage Motor Drive Operation with a Smart Gate Driver
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switching. Additionally, the total current may also
include the current from the internal clamping diodes
within the device which activate when the high side
VGS exceed a certain voltage level. The number of
FETs switching is denoted by n which varies by
system. In sinusoidal systems, there are typically six
FETs switching at once whereas for trapezoidal
systems, two are switching in brake mode and one in
coast.
Figure 4 shows a DRV832x device tested with a diode
and repeated voltage drops with an off time of about
0.9 ms. The driver continues to run and the motor
continuously spins. Without the diode, the driver does
not operate.
Complete Turn Off vs Momentary Turn Off
A DRV832x device with Smart Gate Driver technology
drives a motor and uses a diode at VM to implement
the low voltage operation method. Figure 2 displays
what occurs when power is suddenly removed from
the system (for example to 0 V). With this
configuration, the driver remains operational for a little
over 1.6 ms. The gate driver is clamping the gate drive
voltage on the high-side FETs to 16 V using internal
gate clamps. There is a VGS enhancement of about
10 V (on a VM of 0 V) right before the DRV832x
triggers UVLO.
• FETs: CSD18540Q5B QG = 41 nC
• PWM: 25 kHz
• VM = 12 V > 0 V
• Trap Control
Figure 2. Complete Turn Off With Diode
Figure 3 displays what occurs when the supply is
suddenly removed (for example to 0 V) and then
reconnected after 1 ms. The driver continues to run
but the motor stops in this test.
Figure 4. Recurring Voltage Drops with Diode
Conclusion
Reliability is a key factor in systems that involve
motors. With minimal components, the motor drive
system can be improved and optimized for many
applications. Having features that ensure the system
operates consistently and efficiently brings a lot of
value for the system designer. TI’s BLDC motor drivers
with Smart Gate Drive provide this reliability, as well as
improved efficiency, protection, and optimization
features.
Related Links
• BLDC Motor Driver Portfolio
• Understanding IDRIVE and TDRIVE in TI Smart
Gate Drivers
• Reduce Motor Drive BOM and PCB Area with TI
Smart Gate Drive
• Six weird ways to design with a brushless-DC
driver
Devices Featuring Smart Gate Drive Technology
DRV8343-Q1
DRV8353, DRV8353R, DRV8350, DRV8350R
DRV8323, DRV8323R, DRV8320, DRV8320R
DRV8306
DRV8304
DRV8305-Q1
DRV8305
Figure 3. Momentary Turn Off With Diode
2
Low-Voltage Motor Drive Operation with a Smart Gate Driver
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