8 2008 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC431L
Applications Information - Stability
Selection of load capacitance when using SC431L as
a shunt regulator
When the SC431L is used as a shunt regulator, two
options for selection of C
L
(see diagram on page 7) are
recommended for optimal stability:
1) No load capacitance across the device, decouple at
the load.
2) Large capacitance across the device, optional
decoupling at the load.
The reason for this is that SC431L exhibits instability with
capacitances in the range of 10nF to 1µF (approx.) at
light cathode currents (up to 3mA typical). The device is
less stable the lower the cathode voltage has been set
for. Therefore while the device will be perfectly stable
operating at a cathode current of (say) 10mA with a 0.1µF
capacitor across it, it will oscillate transiently during start-
up as the cathode current passes through the instability
region. Selecting a very low (or preferably, no)
capacitance, or alternatively a high capacitance (such
as 10µF) will avoid this issue altogether. Since the user
will probably wish to have local decoupling at the load
anyway, the most cost effective method is to use no
capacitance at all directly across the device. PCB trace/
via resistance and inductance prevent the local load
decoupling from causing the oscillation during the
transient start-up phase. Note: if the SC431L is located
right at the load, so the load decoupling capacitor is
directly across it, then this capacitor will have to be ≤
1nF or ≥ 10µF.
