LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 36

no-image

LTC4269-1

Manufacturer Part Number
LTC4269-1
Description
IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
Manufacturer
Linear Technology Corporation
Datasheet
www.DataSheet4U.com
APPLICATIONS INFORMATION
LTC4269-1
and poorer effi ciency.
The LTC4269-1 gate drives will clamp the max gate voltage
to roughly 7.5V, so you can safely use MOSFETs with
maximum V
Synchronous Gate Drive
There are several different ways to drive the synchronous
gate MOSFET. Full converter isolation requires the synchro-
nous gate drive to be isolated. This is usually accomplished
by way of a pulse transformer. Usually the pulse driver is
used to drive a buffer on the secondary, as shown in the
application on the front page of this data sheet.
However, other schemes are possible. There are gate drivers
and secondary-side synchronous controllers available
that provide the buffer function as well as additional
features.
Capacitor Selection
In a fl yback converter, the input and output current fl ows
in pulses, placing severe demands on the input and output
fi lter capacitors. The input and output fi lter capacitors
are selected based on RMS current ratings and ripple
voltage.
Select an input capacitor with a ripple current rating
greater than:
Continuing the example:
Keep input capacitor series resistance (ESR) and inductance
(ESL) small, as they affect electromagnetic interference
suppression. In some instances, high ESR can also
produce stability problems because fl yback converters
exhibit a negative input resistance characteristic. Refer
to Application Note 19 for more information.
The output capacitor is sized to handle the ripple current
and to ensure acceptable output voltage ripple. The output
36
I
I
RMS PRI
RMS PRI
(
(
)
)
GS
=
=
29 5
V
of 10V and larger.
IN MIN
41
P
.
(
IN
V
W
)
1 49 4
− 1
49 4
DC
DC
. %
MAX
. %
MAX
=
0 728
.
A
capacitor should have an RMS current rating greater
than:
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 fl yback
converter are illustrated in Figure 17.
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 ΔV. This percentage ripple changes,
depending on the requirements of the application. You can
modify the following equations.
For a 1% contribution to the total ripple voltage, the ESR
of the output capacitor is determined by:
I
I
Continuing
ESR
RMS SEC
RMS SEC
RIPPLE WAVEFORM
OUTPUT VOLTAGE
Figure 17. Typical Flyback Converter Waveforms
COUT
(
(
SECONDARY
PRIMARY
CURRENT
CURRENT
)
)
=
=
I
1
5 3
the exa a mple:
OUT
% •
.
A
ΔV
V
COUT
− 1
I
OUT
PRI
1 49 4
DC
49 4
ΔV
DC
ESR
MAX
. %
(
I
MAX
1
. %
OUT
DC
=
MAX
I
5 24
PRI
N
.
DUE TO ESL
)
RINGING
A
42691 F17
42691f

Related parts for LTC4269-1