E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
The output capacitance typically increases with load transient requirements. During the time between the
load transient and turn-on of the internal P-channel MOS transistor, the output capacitor must supply all the
current required by the load. This current supplied by the output capacitor results in a voltage drop across
the ESR that is substracted from the output voltage.
For a load step from near zero output current (discontinuous conduction mode) to its maximum (continuous
conduction mode), the following equation can be used to calculate the required output capacitance.
2
COUT
≥
LCK
⋅
IOUTMAX
+
I RIPPLE
2
2 ⋅VOUTPUT ⋅VRIPPLE CVAL
⋅
VINPUT
VINPUT − VOUTPUT
[26]
If the worst case load step is not so extremely large and the E910.27 stays always in continuous conduction
mode, a much smaller capacitance value for COUT is possible.
COUT
≥
LCK
⋅
I
2
∆L
VOUTPUT ⋅VRIPPLE CVAL
⋅
VINPUT
VINPUT − VOUTPUT
[27]
Where I∆L is the change in choke current, therefore minimum IRIPPLE.
Obtain an output ripple voltage lower than the system hysteresis (VHYST) is not practical, since the E910.27
will switch at slower frequencies and increases choke ripple current.
The output capacitor COUT also needs therefore to be at least big enough to handle the worst case RMS
current through it.
ICOUTRMS = IOUTMAX ⋅
r
12
[28]
Estimate the input capacitor capacitance value for a given maximum voltage ripple in continuous mode as
follows:
CIN
≥
LCK
⋅
IOUTMAX
+
I RIPPLE
2
2 ⋅VINPUT ⋅VRIPPLE CIN
2
[29]
In a buck converter the input capacitor CIN must carry a high RMS current because of the currents pulsating
nature. The input filter capacitor reduces peak currents drawn from the power source and reduces noise and
voltage ripple at VINPUT. Use a low ESR capacitor type. For good EMI behaviour, the ESL of the input
capacitor should be also very low. Two or more smaller value low ESR capacitors connected in parallel work
often better than a single larger one. Place ceramic capacitors very close from VSA and VSP to GND. In
continuous conduction mode, the input capacitor must be able to handle the whole RMS current of the con-
verter with the worst case duty cycle of your application.
ICIN RMS = IOUTMAX ⋅
Duty
⋅
1
−
Duty
+
r2
12
[30]
ELMOS Semiconductor AG
Specification 29 / 50
03SP0357E.00 05.09.2006
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