Datasheet

EDGE
FINDER
SHIFT
REG
S Q
QR
S Q
QR
PUA
PDA
PULL
DOWN
PULL UP
FROM
DECODER
PWM
CLK
UC1625
UC2625
UC3625
www.ti.com SLUS353C –NOVEMBER 2003–REVISED JUNE 2013
APPLICATION INFORMATION
Cross Conduction Prevention
The UC3625 inserts delays to prevent cross conduction due to overlapping drive signals. However, some thought
must always be given to cross conduction in output stage design because no amount of dead time can prevent
fast slewing signals from coupling drive to a power device through a parasitic capacitance.
The UC3625 contains input latches that serve as noise blanking filters. These latches remain transparent through
any phase of a motor rotation and latch immediately after an input transition is detected. They remain latched for
two cycles of the PWM oscillator. At a PWM oscillator speed of 20 kHz, this corresponds to 50 μs to 100 μs of
blank time which limits maximum rotational speed to 100 kRPM for a motor with six transitions per rotation or 50
kRPM for a motor with 12 transitions per rotation.
This prevents noise generated in the first 50 μs of a transition from propagating to the output transistors and
causing cross-conduction or chatter.
The UC3625 also contains six flip flops corresponding to the six output drive signals. One of these flip flops is set
every time that an output drive signal is turned on, and cleared two PWM oscillator cycles after that drive signal
is turned off. The output of each flip flop is used to inhibit drive to the opposing output (Figure 8). In this way, it is
impossible to turn on driver PUA and PDA at the same time. It is also impossible for one of these drivers to turn
on without the other driver having been off for at least two PWM oscillator clocks.
Figure 8. Cross Conduction Prevention
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Product Folder Links: UC1625 UC2625 UC3625