Datasheet

LTC4278
29
4278fc
APPLICATIONS INFORMATION
Selecting the Load Compensation Resistor
The expression for R
CMP
was derived in the Operation
section as:
R
CMP
= K1•
R
SENSE
• 1–DC
( )
ESR + R
DS(ON)
• R1• N
SF
Continuing the example:
K1=
V
OUT
V
IN
• Eff






=
5
48 • 90%
= 0.116
DC=
1
1+
N•V
IN(NOM)
V
OUT
=
1
1+
1
8
•
48
5
= 45.5%
If ESR+R
DS(ON)
= 8mΩ
R
CMP
= 0.116 •
33mΩ • 1− 0.455
( )
8mΩ
• 37.4kΩ •
1
3
= 3.25k
This value for R
CMP
is a good starting point, but empirical
methods are required for producing the best results.
This is because several of the required input variables
are difficult 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
IN
to V
OUT
times efficiency,
but theoretically estimating efficiency is not a simple
calculation.
The suggested empirical method is as follows:
1. Build a prototype of the desired supply including the
actual secondary components.
2. Temporarily ground the C
CMP
pin to disable the load
compensation function. Measure output voltage while
sweeping output current over the expected range.
Approximate the voltage variation as a straight line.
ΔV
OUT
/ΔI
OUT
= R
S(OUT)
.
3. Calculate a value for the K1 constant based on V
IN
, V
OUT
and the measured efficiency.
4. Compute:
R
CMP
= K1•
R
SENSE
R
S(OUT)
• R1• N
SF
5. Verify this result by connecting a resistor of this value
from the R
CMP
pin to ground.
6. Disconnect the ground short to C
CMP
and connect a 0.1µF
filter capacitor to ground. Measure the output imped-
ance R
S(OUT)
= ΔV
OUT
/ΔI
OUT
with the new compensation
in place. R
S(OUT)
should have decreased significantly.
Fine tuning is accomplished experimentally by slightly
altering R
CMP
. A revised estimate for R
CMP
is:
′
R
CMP
=R
CMP
• 1+
R
S(OUT)CMP
R
S(OUT)








where R′
CMP
is the new value for the load compensation
resistor. R
S(OUT)CMP
is the output impedance with R
CMP
in place and R
S(OUT)
is the output impedance with no
load compensation (from step 2).
Setting Frequency
The switching frequency of the LTC4278 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
C
OSC
(pF)
30
50
f
OSC
(kHz)
100
200
300
100 200
4278 F12
Figure 12. f
OSC
vs OSC Capacitor Values