Datasheet

LT3825
12
3525fe
OPERATION
node. This effectively increases the voltage required at the
top of the R1/R2 feedback divider to achieve equilibrium.
The average primary-side switch current increases to main-
tain output voltage regulation as output loading increases.
The increase in average current increases the R
CMP
resistor
current which affects a corresponding increase in sensed
output voltage, compensating for the IR drops.
Assuming a relatively fixed power supply
efficiency, Eff,
power balance gives:
P
OUT
= Eff • P
IN
V
OUT
• I
OUT
= Eff • V
IN
• I
IN
T1
•
•
•
MP
R
CMPF
50k
V
IN
V
FLBK
R2
LOAD
COMP I
R1
FB
V
FB
Q1 Q2
R
CMP
C
CMP
R
SENSE
SENSE
+
3825 F01
Q3
–
+
A1
8
14 13
12
Figure 1. Load Compensation Diagram
Average primary-side current is expressed in terms of
output current as follows:
I
IN
= K1 • I
OUT
where :
K1 =
V
OUT
V
IN
• Eff
So the effective change in V
OUT
target is:
∆V
OUT
= K1• ∆I
OUT
•
R
SENSE
R
CMP
• R1• N
SF
thus :
∆V
OUT
∆I
OUT
= K1•
R
SENSE
R
CMP
• R1• N
SF
where:
K1 = dimensionless variable related to V
IN
, V
OUT
and
efficiency as explained above
R
SENSE
= external sense resistor
Nominal output impedance cancellation is obtained by
equating this expression with R
S(OUT)
:
K1•
R
SENSE
R
CMP
• R1• N
SF
=
ESR + R
DS(ON)
1– DC
Solving for RCMP gives:
R
CMP
= K1•
R
SENSE
• 1– DC
( )
ESR + R
DS(ON)
• R1• N
SF
The practical aspects of applying this equation to determine
an appropriate value for the R
CMP
resistor are found in the
Applications Information.