L5988D 데이터 시트보기 (PDF) - STMicroelectronics
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L5988D Datasheet PDF : 52 Pages
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L5988D
Functional description
Beating frequency noise is an issue when more than one voltage rail is on the same board.
A simple way to avoid this problem is to operate all the regulators at the same switching
frequency. The synchronization feature of a set of L5988D is simply get connecting together
their SYNCH pin. The device with highest switching frequency will be the MASTER and
provides the synchronization signal to the others. Therefore the SYNCH is a I/O pin to
deliver or recognize a frequency signal.
In order to minimize the RMS current flowing through the input filter, the L5988D provides a
phase shift of 180° between the master and the SLAVES. In cases where more than two
devices are synchronized, all slaves will have a common 180° phase shift with respect to
the master.
In case the synchronized set shares a switching frequency different to the nominal 400 kHz,
it is suggested to provide the proper FSW resistor to each device. In this way all the devices
will have a common peak amplitude of the internal sawtooth signal so the same oscillator
gain in the open loop gain transfer function. In this way the same compensation network is
valid for all the devices.
Taking in account the case of two synchronized L5988D regulating the same output voltage,
the RMS current in the input filter will be optimized and will observe the following formula:
Equation 3
IRMS
=
I--O---2-U----T-
-I-O---2-U----T-
2D 1 – 2D
2D – 1 2 – 2D
if D < 0.5
if D > 0.5
Multiple regulators can be also synchronized to an external frequency signal fed to the
SYNCH pin. In this case the set is phased to the reference and all the devices will work with
0° phase shift.
The graphical representation of the input RMS current of the input filter in the case of two
devices with 0° phase shift (synchronized to an external signal) or 180° phase shift
(synchronized connecting their SYNCH pins) regulating the same output voltage is provided
in Figure 8. To dimension the proper input capacitor please refer to Section 6.1 on page 25.
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