Datasheet

LTC4268-1
28
42681fc
applicaTions inForMaTion
lowpass filtered by the internal 50k resistor R
CMPF
and
the external capacitor on C
CMP
. This voltage is impressed
across the external R
CMP
resistor by op amp A1 and
transistor Q3 producing a current at the collector of Q3 that
is subtracted from the 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
maintain output voltage regulation as output loading
increases. The increase in average current increases R
CMP
resistor current which affects a corresponding increase
in sensed output voltage, compensating for the IR drops.
Assuming relatively fixed power supply efficiency, Eff,
power balance gives:
P
OUT
= Eff • P
IN
V
OUT
• I
OUT
= Eff • V
IN
• I
IN
Average primary side current is expressed in terms of
output current as follow:
I
IN
=K1• I
OUT
where:
K1=
V
OUT
V
IN
• Eff
So the effective change in V
OUT
target is:
DV
OUT
=K1•
R
SENSE
R
CMP
• R1• N
SF
thus:
DV
OUT
DI
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 R
CMP
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.
Transformer Design
Transformer design/specification is the most critical part of
a successful application of the LTC4268-1. The following
sections provide basic information about designing the
transformer and potential trade-offs. If you need help, the
LT C Applications group is available to assist in the choice
and/or design of the transformer.
Turns Ratios
The design of the transformer starts with determining
duty cycle (DC). DC impacts the current and voltage stress
on the power switches, input and output capacitor RMS
currents and transformer utilization (size vs power). The
ideal turns ratio is:
N
DEAL
=
V
OUT
V
IN
•
1−DC
DC
Avoid extreme duty cycles as they, in general, increase
current stresses. A reasonable target for duty cycle is 50%
at nominal input voltage.
For instance, if we wanted a 48V to 5V converter at 50%
DC then:
N
DEAL
=
5
48
•
1− 0.5
0.5
=
1
9.6