LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 29

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Selecting the Load Compensation Resistor
The expression for R
section as:
Continuing the example:
This value for R
methods are required for producing the best results.
This is because several of the required input variables
are diffi cult to estimate precisely. For instance, the ESR
term above includes that of the transformer secondary,
but its effective ESR value depends on high frequency
behavior, not simply DC winding resistance. Similarly, K1
appears as a simple ratio of V
but theoretically estimating effi ciency is not a simple
calculation.
The suggested empirical method is as follows:
1. Build a prototype of the desired supply including the
2. Temporarily ground the C
3. Calculate a value for the K1 constant based on V
actual secondary components.
compensation function. Measure output voltage while
sweeping output current over the expected range.
Approximate the voltage variation as a straight line.
and the measured effi ciency.
R
If
CMP
ESR R
K1=
DC=
R
ΔV
CMP
OUT
=
⎛
⎝ ⎜
+
1+
K
V
=
=
/ΔI
1
V
IN
D
N•V
0 116
3 25
OUT
•
S S ON
OUT
.
•
.
R
(
CMP
Eff
V
ESR R
1 1
IN(NOM)
SENSE
OUT
k
)
= R
⎞
⎠ ⎟
is a good starting point, but empirical
=
•
=
+
33
8
CMP
S(OUT)
48 90
m
•
m
(
DS ON
=
Ω
1
Ω
•
−
was derived in the Operation
(
1
5
+
•
8
DC
CMP
.
(
m
IN
%
)
8
1 0 455
1
1
)
Ω
−
•
to V
•
=
48
pin to disable the load
R N
5
0 116
.
1
.
OUT
•
=
45 5
)
SF
times effi ciency,
•
. %
3
7 7 4
.
k
Ω
IN
•
, V
3
1
OUT
4. Compute:
5. Verify this result by connecting a resistor of this value
6. Disconnect the ground short to C
Setting Frequency
The switching frequency of the LTC4269-1 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF , yielding switching frequencies between 50kHz and
250kHz. Figure 12 shows the nominal relationship between
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
from the R
fi lter capacitor to ground. Measure the output imped-
ance R
in place. R
Fine tuning is accomplished experimentally by slightly
altering R
where R′
resistor. R
in place and R
load compensation (from step 2).
R
R
CMP
′
CMP
S(OUT)
C MP
Figure 12. f
300
200
100
= 1
=
CMP
S(OUT)CMP
50
CMP
S(OUT)
K
R
is the new value for the load compensation
= ΔV
CMP
. A revised estimate for R
•
S(OUT)
30
R
pin to ground.
R
S OUT
SENSE
OUT
•
should have decreased signifi cantly.
(
OSC
⎛
⎜
⎝
1
is the output impedance with R
/ΔI
+
is the output impedance with no
vs OSC Capacitor Values
)
R
OUT
•
C
OSC
S OUT CMP
R N
R
(
1
S OUT
(pF)
with the new compensation
•
(
100
)
SF
CMP
)
LTC4269-1
and connect a 0.1μF
⎞
⎟
⎠
CMP
42691 F12
200
is:
29
42691f
CMP