CYStech Electronics Corp.
Spec. No. : C504N3
Issued Date : 2003.03.21
Revised Date : 2005.10.07
Page No. : 7/9
OUT
Iz
10£gF
+
R
Iz
15k
PL432
LM432
232
R1
LM432
PL432
8.25k
R2
CL
GND
Applications Information - Stability
Selection of load capacitance when using LM432 as a shunt regulator
When the LM432 is used as a shunt regulator, two options for selection of C
L
(see diagram on page 6)
are recommended for optimal stability:
1) No load capacitance across the device, decouple at the load.
2) Large capacitance across the device, optimal decoupling at the load.
The reason for this is that LM432 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) capaci-
tance, 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 LM432 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.
LM432XA3/N3
CYStek Product Specification
CYSTEKEC [ CYSTECH ELECTONICS CORP. ]
