Datasheet

LTC4268-1
32
42681fc
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
= 35mW. Round
to the nearest available lower value, 33mW.
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)
= 8mW
R
CMP
= 0.116 •
33mW • 1− 0.455
( )
8mW
• 37.4kW •
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.
 DV
OUT
/DI
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)
= DV
OUT
/DI
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).
applicaTions inForMaTion