
LB8649W
No.7894-6/9
(3) Startup correction function (ISH, OUT9, and OUT10)
Coil current
ISH discharge
When VCC is high (no ISH capacitor)
When VCC is low (no ISH capacitor)
SH close operation
Startup correction coil current
Startup correction coil current
Startup correction is applied to the coil waveform by setting the ISH pin input voltage to a time constant larger than that
of the coil with an external RC circuit. This makes it possible to provide stable shutter operation even in the presence of
power supply fluctuations.
Note : For the ISH startup correction, the capacitance is determined by, in the state where the ISH capacitor is not
present, verifying the coil current startup waveform when VCC is at a reduced level and choosing a capacitance
such that the time constant is lower than that of this waveform.
Note, however, that in cases where, for example, the supply voltage is stabilized and a startup correction
function is not needed, this startup correction capacitor is not needed.
(4) Phase correction capacitor (FC1, FC2)
Consider values in the range 0.0015 to 0.033μF for the FC1/2 capacitors, and select values such that oscillation in the
output is not a problem. If a coil with a particularly high impedance is used, an adequate margin must be provided in the
capacitor value. Note that since the constant current control block is connected to PGND internally to the IC, the
ground sides of the FC1/2 capacitors must be connected to PGND.
Notes to determine the value of FC1 and FC2 capacitors
FC1 is the connection for the phase compensation capacitor for the OUT9/10 output constant current control circuit.
Similarly, FC2 is the connection for the OUT11/12 phase compensation capacitor.
To determine the value of these capacitors, observe the output waveform and select a value such that the output does
not oscillate.
The FC pin is connected in the IC circuit to the constant current control amplifier output blocs and the output transistor
is driven by the rise in the FC potential. Therefore, since the FC pin initial state influences the output drive timing,
before applying power to the shutter, this IC discharges (with the rapid discharge circuit) the FC pin to a certain fixed
potential internally and then when starting to apply power to the shutter, the IC charges (with the rapid charge circuit)
the FC pin to a fixed potential internally so that the FC pin state is always fixed when driving the shutter.
This stabilizes the input to output delay time.
However, if the capacitor value is made too large, the time required for the above circuit to charge and discharge that
capacitor will become longer and the input to output delay time fluctuations will become larger due to variations in the
capacitor value (due both to sample-to-sample variations and to temperature characteristics).
Another disadvantage of making this capacitor larger is that the coil current rising slope will become less steep.
Although the rising slope of the coil current is essentially determined by the inductance component of the coil, if the
capacitor is made larger and its time constant increases, the slope of the rise of the coil current will become dependent
on the capacitor value.
For the above reasons, especially if high-speed shutter drive is required, the value of the capacitor connected to the FC
pin should as small as possible as long as the output does not oscillate (the range roughly from 0.0015 to 0.033 μF).