Datasheet

IFX24401
Application Information
Data Sheet 15 Rev. 1.02, 2009-12-10
6 Application Information
Figure 4 Application Diagram
Input, Output
An input capacitor is necessary for damping line influences. A resistor of approx. 1 Ω in series with C
I
, can damp
the LC of the input inductivity and the input capacitor.
The IFX24401 requires a ceramic output capacitor of at least 470 nF. In order to damp influences resulting from
load current surges it is recommended to add an additional electrolytic capacitor of 4.7
µF to 47 µF at the output
as shown in
Figure 4.
Additionally a buffer capacitor C
B
of > 10µF should be used for the output to suppress influences from load surges
to the voltage levels. This one can either be an aluminum electrolytic capacitor or a tantalum capacitor following
the application requirements.
A general recommendation is to keep the drop over the equivalent serial resistor (ESR) together with the discharge
of the blocking capacitor below the allowed Headroom of the Application to be supplied (e.g. typ. dV
Q
= 350mV).
Since the regulator output current roughly rises linearly with time the discharge of the capacitor can be calculated
as follows:
dVC
B
= dI
Q
*dt/C
B
The drop across the ESR calculates as:
dV
ESR
= dI*ESR
To prevent a reset the following relationship must be fullfilled:
dV
C
+ dV
ESR
< V
RH
= 350mV
Example: Assuming a load current change of dI
Q
= 100mA, a blocking capacitor of C
B
= 22µF and a typical
regulator reaction time under normal operating conditions of dt ~ 25µs and for special dynamic load conditions,
such as load step from very low base load, a reaction time of dt ~ 75µs.
dV
C
= dI
Q
*dt/C
B
= 100mA * 25µs/22µF = 113mV
So for the ESR we can allow
dV
ESR
= V
RH2
- dV
C
= 350mV - 113mV = 236mV
The permissible ESR becomes:
ESR = dV
ESR
/ dI
Q
= 236mV/100mA = 2.36Ohm
IFX24401
3, Tab
GND
Overtemperature
Shutdown
1
5Q
470
nF
V
CC
+ 4.7
µF
EN2
1
I
100
nF
V
Bat
e. g.
Ignition
Bandgap
Reference
Charge
Pump
Enable