OM9371 International Rectifier Corp., OM9371 Datasheet - Page 7

• Manufacturer Part Number: OM9371
• Description: Full-featured Power Module For Direct Drive Of 3-phase Brushless Dc Motors
• Manufacturer: International Rectifier Corp.

OM9371
TACHOMETER OUTPUT (Pin 15) -- This output
provides a fixed width 5V pulse when any Hall-effect
Input (1, 2 or 3) changes state. The pulse width of the
Tachometer Output is set internally in the OM9371 to
113µs (nominal). The average value of the output
voltage on pin 15 is directly proportional to the motor's
speed, so this output may be used (with an external
averaging filter) as a true tachometer output, and fed
back to the Speed Input (pin 23) to sense the actual
motor speed.
Note: Whenever pin 15 is high, the internal Hall-effect
position latches are inhibited (i.e. "latched"), to reject
noise during the chopping portion of the commutation
cycle, and this makes additional commutations
impossible. This means that in order to prevent false
commutation at a speed less than the desired
maximum speed, the highest speed as observed at
the Tachometer Output should be set above the
expected maximum value.
BRAKE / TACH TIMING INPUT (Pin 16) -- The
Brake/Tach Timing Input is a dual-purpose input.
Internal to the OM9371 are timing components tied
from pin 16 to Ground (a 51kΩ resistor and a 3300pF
capacitor). These components set the minimum pulse
width of the Tachometer Output to 113µs, and this
time may be adjusted using external components,
according to the equation:
T(tach) = 0.67 x (C t + 3300pf) x
The recommended range of external resistance (to
Ground) is 15kΩ to ∞, and the range of external
capacitance (to Ground) is 0pF to 0.01uF. With each
Tachometer Output pulse, the capacitor tied to pin 16
is
approximately 1.67V by an internal timing resistor.
The Brake / Tach Timing Input has another function. If
this pin is pulled below the brake threshold voltage,
the OM9371 will enter the brake mode. The brake
mode is defined as the disabling of all three high-side
(pull-up) drivers associated with the Phase Outputs,
and the enabling of all three low-side (pull-down)
drivers.
OVERVOLTAGE / COAST INPUT (Pin 17) -- This
input may be used as a shutdown or an
enable/disable input to the OM9371. Also, since the
switching inhibit threshold is so tightly defined, this
input can be directly interfaced with a resistive divider
which senses the voltage of the motor supply, Vm, for
overvoltage conditions. A high level (greater than the
inhibit threshold) on pin 17 causes the coast condition
to occur, whereby all Phase Outputs revert to a Hi-Z
state and any motor current which flowed prior to the
Overvoltage / Coast command is commutated via the
power "catch" rectifiers associated with each Phase
Output.
discharged
from
approximately
(
Rt x 51kΩ
Rt + 51kΩ
3.33V
)
(µs)
to
2.1 - 7
SOFT-START INPUT (Pin 18) -- The Soft-Start input is
internally connected to a 10µA (nominal) current source,
the collector of an NPN clamp/discharge transistor, and
a voltage comparator whose soft-start/restart threshold
is 0.2Vdc (nominal). An external capacitor is connected
from this pin to Ground (pin 19). Whenever the Vcc
supply input drops below the turn-on threshold,
approximately 9Vdc, or the sensed current exceeds the
over-current threshold, approximately 0.3V at the current
sense amplifier, the soft-start latch is set. This drives the
NPN clamp transistor which discharges the external soft-
start capacitor. When the capacitor voltage drops below
the soft-start/restart threshold and a fault condition does
not exist, the soft-start latch is cleared; the soft-start
capacitor charges via the internal current source.
In addition to discharging the soft-start capacitor, the
clamp transistor also clamps the output of the error
amplifier internal to the controller IC, not allowing the
voltage at the output of the error amplifier to exceed the
voltage at pin 18, regardless of the inputs to the amplifier.
This action provides for an orderly motor start-up either
at start-up or when recovering from a fault condition.
GROUND (Pin 19) -- The voltages that control the
OM9371 are referenced with respect to this pin. All
bypass capacitors, timing resistors and capacitors, loop
compensation components, and the Hall-effect filter
capacitors must be referenced as close as possible to
pin 19 for proper circuit operation. Additionally, pin 19
must be connected as close as physically possible to the
Motor Return, pins 27 and 28.
HALL-EFFECT INPUTS (H1, Pin 22; H2, Pin 21; H3,
Pin 20) -- Each input has an internal pull-up resistor to
the +5V Reference. Each input also has an internal
180pF noise filter capacitor to Ground. In order to
minimize the noise which may be coupled from the motor
commutation action to these inputs, it is strongly
recommended that additional external filter capacitors,
whose value is in the range of 2200pF, be connected
from each Hall-Effect Input pin to Ground. Whatever
capacitor value is used, the rise/fall times of each input
must be guaranteed to be less than 20us for proper
tachometer action to occur.
position sensing may be used if one or two of the Hall-
effect sensor signals is inverted prior to connection to the
Hall-Effect Inputs.
Motors with 60 degree