AS2941
APPLICATION HINTS
External Capacitors
A minimum capacitance of 22µF and conditions on ESR (Equivalent
Series Resistance) must be met. The minimum value for the
capacitance is 22µF and can be increased without limit. However the
ESR may cause loop instability if it is too high or too low. The
following graph shows the acceptable range for the ESR.
Output Capacitor ESR
100
COUT = 22
VO = 5V
µF
10
1
STABLE
REGION
0.1
0.01
0
200
400
600
800
1000
If the capacitor doOeUsTPnUoTtCUmReReENt Tt(hmeAs)e requirements oscillation can
result.
ESR is specified only at room temperature. Therefore the designer
must ensure the proper behavior of the ESR over the temperature
range. ESR, for electrolytic capacitor, will increase by about 30X as
the temperature is reduced from 25°C to -40°C. Aluminum
electrolytic capacitors are not well suited for low temperature
operation.
Solid tantalum capacitors’ ESR are more stable over temperature, but
expensive. A cost-effective approach is then to put in parallel solid
tantalum and aluminum electrolytic capacitors in the ratio25/75%.
Thermal Consideration
Although the AS2941 offers some limiting circuitry for overload
conditions, it is necessary not to exceed the maximum junction
temperature, and therefore to be careful about thermal resistance.
The heat flow will follow the lowest resistance path, which is the
Junction-to-case thermal resistance. In order to insure the best
thermal flow of the component, a proper mounting is required. Note
that the case of the device is electrically connected to the output. In
case the case has to be electrically isolated, a thermally conductive
spacer can be used. However do not forget to consider its
contribution to thermal resistance.
Rev. 9/29/00
SIPEX [ SIPEX CORPORATION ]
