MC33035, NCV33035
http://onsemi.com
13
Figure 21. Pulse Width Modulator Timing Diagram
Current
Sense Input
Capacitor C
T
Error Amp
Out/PWM
Input
Latch “Set"
Inputs
Top Drive
Outputs
Bottom Drive
Outputs
Fault Output
Reference
The on−chip 6.25 V regulator (Pin 8) provides charging
current for the oscillator timing capacitor, a reference for the
error amplifier, and can supply 20 mA of current suitable for
directly powering sensors in low voltage applications. In
higher voltage applications, it may become necessary to
transfer the power dissipated by the regulator off the IC. This
is easily accomplished with the addition of an external pass
transistor as shown in Figure 22. A 6.25 V reference level
was chosen to allow implementation of the simpler NPN
circuit, where V
ref
− V
BE
exceeds the minimum voltage
required by Hall Effect sensors over temperature. With
proper transistor selection and adequate heatsinking, up to
one amp of load current can be obtained.
Figure 22. Reference Output Buffers
The NPN circuit is recommended for powering Hall or opto sensors, where
the output voltage temperature coefficient is not critical. The PNP circuit is
slightly more complex, but is also more accurate over temperature. Neither
circuit has current limiting.
To
Control
Circuitry
6.25 V
Sensor
Power
≈5.6 V
MPS
U51A
V
in
MPS
U01A
V
in
To Control Circuitry
and Sensor Power
6.25 V
UVLO
17
39
REF
8
0.1
REF
8
18
UVLO
17
18
Undervoltage Lockout
A triple Undervoltage Lockout has been incorporated to
prevent damage to the IC and the external power switch
transistors. Under low power supply conditions, it
guarantees that the IC and sensors are fully functional, and
that there is sufficient bottom drive output voltage. The
positive power supplies to the IC (V
CC
) and the bottom
drives (V
C
) are each monitored by separate comparators that
have their thresholds at 9.1 V. This level ensures sufficient
gate drive necessary to attain low R
DS(on)
when driving
standard power MOSFET devices. When directly powering
the Hall sensors from the reference, improper sensor
operation can result if the reference output voltage falls
below 4.5 V. A third comparator is used to detect this
condition. If one or more of the comparators detects an
undervoltage condition, the Fault
Output is activated, the top
drives are turned off and the bottom drive outputs are held
in a low state. Each of the comparators contain hysteresis to
prevent oscillations when crossing their respective
thresholds.
Fault Output
The open collector Fault Output (Pin 14) was designed to
provide diagnostic information in the event of a system
malfunction. It has a sink current capability of 16 mA and
can directly drive a light emitting diode for visual indication.
Additionally, it is easily interfaced with TTL/CMOS logic
for use in a microprocessor controlled system. The Fault
Output is active low when one or more of the following
conditions occur:
1) Invalid Sensor Input code
2) Output Enable at logic [0]
3) Current Sense Input greater than 100 mV
4) Undervoltage Lockout, activation of one or more of
the comparators
5) Thermal Shutdown, maximum junction temperature
being exceeded
This unique output can also be used to distinguish between
motor start−up or sustained operation in an overloaded
condition. With the addition of an RC network between the
Fault
Output and the enable input, it is possible to create a
time−delayed latched shutdown for overcurrent. The added
circuitry shown in Figure 23 makes easy starting of motor
systems which have high inertial loads by providing
additional starting torque, while still preserving overcurrent
protection. This task is accomplished by setting the current
limit to a higher than nominal value for a predetermined time.
During an excessively long overcurrent condition, capacitor
C
DLY
will charge, causing the enable input to cross its
threshold to a low state. A latch is then formed by the positive
feedback loop from the Fault
Output to the Output Enable.
Once set, by the Current Sense Input, it can only be reset by
shorting C
DLY
or cycling the power supplies.
