LB11921T Datasheet, PDF (16/18 Pages) Sanyo Semicon Device – Monolithic Digital IC Three-Phase Brushless Motor Driver

LB11921T
12. Low-Voltage Protection Circuit
The LB11921T includes a low-voltage protection circuit to protect against incorrect operation when VCC power is
applied or if the power supply voltage falls below its operating level. The (external) all output transistors are turned
off if VCC falls under about 3.74 volts, and the protection function is cleared at about 3.93 volts or higher.
13. Power Supply Stabilization
Since this IC is used in applications that draw large output currents, the power-supply line is subject to fluctuations.
Therefore, capacitors with capacitances adequate to stabilize the power-supply voltage must be connected between
the VCC pin and ground. If diodes are inserted in the power-supply line to prevent IC destruction due to reverse
power supply connection, since this makes the power-supply voltage even more subject to fluctuations, even larger
capacitors will be required.
14. Ground Lines
The signal system ground and the output system ground must be separated and a single ground point must be taken at
the connector. Since the output system ground carries large currents, this ground line must be made as short as
possible.
Output system ground --- Ground for Rf, output diodes, and 24V line capacitors
Signal system ground --- Ground for the IC, external components, and 5V line capacitors
15. FG Amplifier
The FG amplifier is normally implemented as a filter amplifier such as that shown in the application circuits to reject
noise. Since a clamp circuit has been added at the FG amplifier output, the output amplitude is clamped at about
3Vp-p, even if the gain is increased.
Since a Schmitt comparator is inserted after the FG amplifier, applications must set the gain so that the amplifier
output amplitude is at least 250mVp-p. (It is desirable that the gain be set so that the amplitude is over 0.5Vp-p at
the lowest controlled speed to be used.)
The capacitor inserted between the FGIN+ pin (pin 20) and ground is required for bias voltage stabilization. To make
the connected capacitor as immune from noise as possible, connect this capacitor to the GND pin (pin 23) with a line
that is as short as possible.
16. Integrating Amplifier
The integrating amplifier integrates the speed error pulses and phase error pulses and converts them to the speed
controlling voltage. At the same time it also sets the control loop gain and frequency characteristics.
External components necessary for the integrating amplifier must be placed as close to the IC as possible to reduce
influence of noise.
17. FIL Pin External Components
The capacitor inserted between the FIL pin and ground is used to suppress ripple on the FIL pin voltage. Therefore,
application designers must select a capacitance value that provides fully adequate smoothing of the FIL pin voltage
even at the lowest external clock input frequency used. Also, the FIL pin voltage convergence time (the time until the
reference signal stabilizes) when the input clock frequency is switched is shortened by connecting a resistor and a
capacitor in series between the FIL pin and ground. Therefore, designers must select values for the resistor and
capacitor that give the required convergence time.
No.A0604-16/18