Datasheet

LTC4269-1
27
42691fc
APPLICATIONS INFORMATION
in an abrupt increase in inductor ripple current and,
consequently, output voltage ripple. Do not allow the core
to saturate! The maximum peak primary current occurs
at minimum V
IN
:
I
PK
=
P
IN
V
IN(MIN)
•DC
MAX
•1+
X
MIN
2
⎛
⎝
⎜
⎞
⎠
⎟
now :
DC
MAX
=
1
1+
N•V
IN MIN
()
V
OUT
=
1
1+
1
8
•
41
5
= 49.4%
X
MIN
=
V
IN(MIN)
•DC
MAX
()
2
f
OSC
•L
P
•P
IN
=
41• 49.4%
()
2
200kHz • 260µH • 29.5W
= 0.267
Using the example numbers leads to:
I
PK
=
29.5W
41• 0.494
•1+
0.267
2
⎛
⎝
⎜
⎞
⎠
⎟
= 1.65A
Multiple Outputs
One advantage that the fl yback topology offers is that
additional output voltages can be obtained simply by adding
windings. Designing a transformer for such a situation is
beyond the scope of this document. For multiple windings,
realize that the fl yback winding signal is a combination of
activity on all the secondary windings. Thus load regulation
is affected by each winding’s load. Take care to minimize
cross regulation effects.
Setting Feedback Resistive Divider
The expression for V
OUT
developed in the Operation section
is rearranged to yield the following expression for the
feedback resistors:
R1=R2
V
OUT
+I
SEC
•ESR+R
DS(ON)
()
⎡
⎣
⎤
⎦
V
FB
•N
SF
−1
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
Continuing the example, if ESR + R
DS(ON)
= 8mΩ, R2 =
3.32k, then:
R1= 3.32k
5+5.3 • 0.008
1.237 • 1/ 3
−1
⎛
⎝
⎜
⎞
⎠
⎟
= 37.28k
choose 37.4k.
It is recommended that the Thevenin impedance of the
resistive divider (R1||R2) is roughly 3k for bias current
cancellation and other reasons.
Current Sense Resistor Considerations
The external current sense resistor is used to control peak
primary switch current, which controls a number of key
converter characteristics including maximum power and
external component ratings. Use a noninductive current
sense resistor (no wire-wound resistors). Mounting the
resistor directly above an unbroken ground plane connected
with wide and short traces keeps stray resistance and
inductance low.
The dual sense pins allow for a full Kelvin connection. Make
sure that SENSE+ and SENSE– are isolated and connect
close to the sense resistor.
Peak current occurs at 100mV of sense voltage V
SENSE
. So
the nominal sense resistor is V
SENSE
/I
PK
. For example, a
peak switch current of 10A requires a nominal sense resistor
of 0.010Ω Note that the instantaneous peak power in the
sense resistor is 1W, and that it is rated accordingly. The
use of parallel resistors can help achieve low resistance,
low parasitic inductance and increased power capability.
Size R
SENSE
using worst-case conditions, minimum L
P
,
V
SENSE
and maximum V
IN
. Continuing the example, let us
assume that our worst-case conditions yield an I
PK
of 40%
above nominal, so I
PK
= 2.3A. If there is a 10% tolerance
on R
SENSE
and minimum V
SENSE
= 88mV, then R
SENSE
•
110% = 88mV/2.3A and nominal R
SENSE
= 35mΩ. Round
to the nearest available lower value, 33mΩ.