LT3837EFE-PBF LINER [Linear Technology], LT3837EFE-PBF Datasheet - Page 24

• Manufacturer Part Number: LT3837EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3837
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, reli-
ability 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 8 shows an example of the
fi lter.
The design of the fi lter is beyond the scope of this data
sheet. However, as a starting point, use these general
guide lines. Start with a C
solution. Make C1 1/4 of C
pole independent of C
implemented 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
For those interested in more details refer to “Second-Stage
LC Filter Design,” Ridley, Switching Power Magazine, July
2000, p8-10.
24
C
ESR
C
OUT
OUT
SECONDARY
COUT
≥
≥
WINDING
1% •
1
FROM
% % • . •
≤
1
% •
3 3 200
V
I
OUT
OUT
10
OUT2
3 3
A
C1
47μF
.
•
0.1μH
3
OUT
L1
f
V
OSC
) for high frequency noise on V
Figure 8
kHz
• –
OUT
. The smaller C1 may be best
(
OUT
10
1 52 4
=
1/4 the size of the nonfi lter
A
C
470μF
OUT
1515
to make the second fi lter
. %
μ
V
)
F
OUT
C
1μF
=
OUT2
1 6
3837 F08
.
m
R
Ω
LOAD
OUT
.
Circuit simulation is a way to optimize output capacitance
and fi lters, just make sure to include the component
parasitics. LTC SwitcherCAD™ is a terrifi c free circuit
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.
IC Thermal Considerations
Take care to ensure that the LT3837 junction temperature
does not exceed 125°C. Power is computed from the aver-
age supply current, the sum of quiescent supply current
(I
The primary gate drive current is computed as:
where Q
the gate charge curve) and f is the switching frequency.
Since the synchronous driver is usually driving a capacitive
load, the power dissipation is:
where C
pin max voltage.
So total IC dissipation is computed as:
V
Junction temperature is computed as:
where:
SwitcherCAD is a trademark of Linear Technology Corporation.
CC
CC
f
f
P
T
T
θ
impedance (40°C/W).
OSC
OSC
A
J
JA
D(TOTAL)
is the worst-case LT3837 supply voltage.
in the specifi cations) plus gate drive currents.
= T
is the ambient temperature
is the FE16 package junction-to-ambient thermal
• Q
• C
G
S
A
is the total gate charge at max V
is the SG capacitive load and V
+ P
S
G
• V
= V
D
SGMAX
• θ
CC
JA
• (I
CC
+ f •(Q
GPRI
+ C
GS
S
SGMAX
• V
(obtained from
SGMAX
is the SG
))
3837fa