lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 12

• Manufacturer Part Number: lt3825
• Description: Isolated No-opto Synchronous Flyback Controller With Wide Input Supply Range
• Manufacturer: Linear Technology Corporation

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