Stability Boundary Conditions

A Note About Stability

When using the GM432 as a shunt regulator, you can

optimize stability by setting C either (1) no load ca-

L

pacitance across the GM432, decouple at the load; or

(2) large capacitance across the GM432, optional de-

coupling at the load. The GM432 can become unsta-

ble with capacitances of approximately 10nF to 1µF

when cathode currents are less than 3mA or so, with

instability increasing as cathode current is reduced.

So, while the GM432 is happily stable at, for example,

cathode current of 10mA with a 0.1µF capacitor

across it, it can oscillate transiently as the cathode cur-

rent rises through the region of instability. To avoid

this problem completely, simply eliminate the capaci-

tor or select a very low or very high (e.g. 10µF) capac-

itor C . Since you will probably want local decoupling

L

at the load, the best idea is to use no capacitance

across the device. Just the resistance and capaci-

tance of the PCB traces and vias will prevent local

load decoupling from causing transient oscillation dur-

ing start-up. If you place the GM432 right next to the

load, with the load decoupling capacitor directly

across it, you must use a capacitor of

nF or

10µF.

+

_

R

GM432

R1

R2

C - Load Capacitance - µF

L

Tantalum Capacitors

* Stability boundary condition test shows that tantalum capacitors

are recommended to minimize the conditions that may cause

the device to oscillate.

16

14

12

10

8

6

4

2

0

0.001

0.01

0.1

1

10

C

- Load Capacitance - µF

L

Tantalum Capacitors

T =25°C

A

V=V

KA

ref

Stable

V =2V

KA

V =3V

KA

1.0

0.8

0.6

0.4

0.2

0

0.001

0.01

0.1

1

10

For V

= 2 & 3V

KA

Device does not

Oscillate

T =25°C

A

Stable

V=V

KA

ref

8

A

A

MICROELECTRONICS

GM432

1.24V ADJUSTABLE PRECISION SHUNT REGULATORS

Power Management