ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 43

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

ripple, divided equally between the ESR step and the
charging/discharging ΔV. This percentage ripple changes,
depending on the requirements of the application. You
can modify the equations below. For a 1% contribution
to the total ripple voltage, the ESR of the output capacitor
is determined by:
The other 1% is due to the bulk C component, so use:
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 satisfi es the required bulk C.
Continuing our example, the output capacitor needs:
These electrical characteristics require paralleling several
low ESR capacitors possibly of mixed type.
Most capacitor ripple current ratings are based on 2000
hour life. This makes it advisable to derate the capacitor
or to choose a capacitor rated at a higher temperature
than required.
One way to reduce cost and improve output ripple is to
use a simple LC fi lter. Figure 19 shows an example of the
fi lter.
APPLICATIONS INFORMATION
ESR
ESR
C
C
OUT
OUT
COUT
COUT
≥
≥
1% •
1
% •
≤
≤
1
1
5 5 200
% •
V
% •
5 3
I
OUT
OUT
.
•
A
5
V
OUT
V
•
f
kHz
OSC
•
(
1 49 4
•
5 3
−
(
I
.
=
1
OUT
−
600
A
DC
. %
μ
MAX
F
)
=
)
4
m
Ω
The design of the fi lter is beyond the scope of this data
sheet. However, as a starting point, use these general
guidelines. Start with a C
solution. Make C1 1/4 of C
pole independent of C
with multiple ceramic capacitors. Make L1 smaller than
the output inductance of the transformer. In general, a
0.1μH fi lter inductor is suffi cient. Add a small ceramic
capacitor (C
those interested in more details refer to “Second-Stage
LC Filter Design,” Ridley, Switching Power Magazine, July
2000 p8-10.
Circuit simulation is a way to optimize output capacitance
and fi lters, just make sure to include the component
parasitic. LTC SwitcherCAD
simulation tool that is available at www.linear.com. Final
optimization of output ripple must be done on a dedicated
PC board. Parasitic inductance due to poor layout can
signifi cantly impact ripple. Refer to the PC Board Layout
section for more details.
ELECTRO STATIC DISCHARGE AND SURGE
PROTECTION
The LTC4268-1 is specifi ed to operate with an absolute
maximum voltage of –90V and is designed to tolerate
brief over-voltage events. However, the pins that interface
to the outside world (primarily V
can routinely see peak voltages in excess of 10kV. To
protect the LTC4268-1, it is highly recommended that the
SMAJ58A unidirectional 58V transient voltage suppressor
be installed between the diode bridge and the LTC4268-1
(D3 in Figure 4).
OUT2
) for high frequency noise on V
OUT
OUT
. C1 may be best implemented
OUT
TM
1/4 the size of the nonfi lter
to make the second fi lter
is a terrifi c free circuit
LTC4268-1
PORTN
and V
OUT
43
PORTP
42681fa
. For
)