Datasheet

LTC4268-1
40
42681fc
The output capacitor should have an RMS current rating
greater than:
I
RMS(SEC)
=I
OUT
DC
MAX
1−DC
MAX
Continuing the example:
I
RMS(SEC)
= 5.3A
49.4%
1− 49.4%
= 5.24A
This is calculated for each output in a multiple winding
application.
ESR and ESL along with bulk capacitance directly affect the
output voltage ripple. The waveforms for a typical flyback
converter are illustrated in Figure 19.
The maximum acceptable ripple voltage (expressed as a
percentage of the output voltage) is used to establish a
starting point for the capacitor values. For the purpose
of simplicity we will choose 2% for the maximum output
ripple, divided equally between the ESR step and the
charging/discharging DV. This percentage ripple changes,
depending on the requirements of the application. You
can modify the equations below. For a 1% contribution
applicaTions inForMaTion
to the total ripple voltage, the ESR of the output capacitor
is determined by:
ESR
COUT
≤1% •
V
OUT
• 1−DC
MAX
( )
I
OUT
The other 1% is due to the bulk C component, so use:
C
OUT
≥
I
OUT
1% • V
OUT
• f
OSC
In many applications the output capacitor is created from
multiple capacitors to achieve desired voltage ripple,
reliability and cost goals. For example, a low ESR ceramic
capacitor can minimize the ESR step, while an electrolytic
capacitor satisfies the required bulk C.
Continuing our example, the output capacitor needs:
ESR
COUT
≤1% •
5V • 1− 49.4%
( )
5.3A
= 4m
W
C
OUT
≥
5.3A
1% • 5 • 200kHz
= 600µF
These electrical characteristics require paralleling several
low ESR capacitors possibly of mixed type.
OUTPUT VOLTAGE
RIPPLE WAVEFORM
SECONDARY
CURRENT
PRIMARY
CURRENT
I
PRI
∆V
COUT
42681 F19
RINGING
DUE TO ESL
I
PRI
N
∆V
ESR
Figure 19. Typical Flyback Converter Waveforms