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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

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.