OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT1425IS#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
10
LT1425
OPERATION
U
may be minimized by the use of the load compensation
function.
To implement the load compensation function, a voltage is
developed that is proportional to average output switch
current. This voltage is then impressed across the external
R
OCOMP
resistor and the resulting current is then sub-
tracted from the R
FB
node. As output loading increases,
average switch current increases to maintain rough output
voltage regulation. This causes an increase in R
OCOMP
resistor current subtracted from the R
FB
node, through
which feedback loop action causes a corresponding
increase in target output voltage.
Assuming a relatively fixed power supply efficiency, Eff,
Power Out = (Eff)(Power In)
(V
OUT
)(I
OUT
) = (Eff)(V
IN
)(I
IN
)
Average primary side current may be expressed in terms
of output current as follows:
I
IN
= I
OUT
V
OUT
(V
IN
)(Eff)
)
)
Combining the efficiency and voltage terms in a single
variable,
I
IN
= K1(I
OUT
)
where,
K1
=
V
OUT
(V
IN
)(Eff)
)
)
Switch current is converted to voltage by a sense resistor
and amplified by the current sense amplifier with associ-
ated gain G. This voltage is then impressed across the
external R
OCOMP
resistor to form a current that is
subtracted from the R
FB
node. So the effective change in
V
OUT
target is:
∆V
OUT
= K1(∆I
OUT
) R
FB
(R
SENSE
)(G)
R
OCOMP
)
)
Expressing the product of R
SENSE
and G as the data sheet
value of ∆V
RCCOMP
/∆I
SW
,
∆V
RCCOMP
∆I
SW
)
)
∆V
RCCOMP
∆I
SW
)
)
R
FB
R
OCOMP
)
)
R
OUT
= K1 and,
R
FB
R
OUT
)
)
R
OCOMP
= K1
∆V
RCCOMP
∆I
SW
)
)
= Data sheet value for R
CCOMP
pin
action vs switch current
where,
K1 = Dimensionless variable related to V
IN
, 
V
OUT
and efficiency as above
R
FB
= External “feedback” resistor value
R
OUT
= Uncompensated output impedance
∆V
RCCOMP
∆I
SW
∆V
OUT
∆I
OUT
)
)
R
FB
R
OCOMP
)
)
= K1
Nominal output impedance cancellation is obtained by
equating this expression with R
OUT
. The practical aspects
of applying this equation to determine an appropriate
value for the R
OCOMP
resistor are found in the Applications
Information section.
11
LT1425
APPLICATIONS INFORMATION
WUU
U
R
OCOMP
, the external resistor value required for its nomi-
nal compensation:
1
1 – DC
)
)
R
OUT
= ESR
∆V
RCCOMP
∆I
SW
)
)
R
FB
R
OUT
)
)
R
OCOMP
= K1
While the value for R
OCOMP
may therefore be theoretically
determined, it is usually better in practice to employ
empirical methods. This is because several of the required
input variables are difficult to estimate precisely. For
instance, the ESR term above includes that of the trans-
former secondary, but its effective ESR value depends on
high frequency behavior, not simply DC winding resis-
tance. Similarly, K1 appears to be a simple ratio of V
IN
to
V
OUT
times (differential) efficiency, but theoretically esti-
mating efficiency is not a simple calculation. The sug-
gested empirical method is as follows:
Build a prototype of the desired supply using the
eventual secondary components. Temporarily ground
the R
CCOMP
pin to disable the load compensation func-
tion. Operate the supply over the expected range of
output current loading while measuring the output
voltage deviation. Approximate this variation as a single
value of R
OUT
(straight line approximation). Calculate a
value for the K1 constant based on V
IN
, V
OUT
and the
measured (differential) efficiency. They are then com-
bined with the data sheet typical value for (∆V
RCCOMP
/
∆I
SW
) to yield a value for R
OCOMP
.
Verify this result by connecting a resistor of roughly this
value from the R
OCOMP
pin to ground. (Disconnect the
ground short to R
CCOMP
and connect the requisite
0.1µF filter capacitor to ground.) Measure the output
impedance with the new compensation in place. Modify
the original R
OCOMP
value if necessary to increase or
decrease the effective compensation.
Once the proper load compensation resistor has been
chosen, it may be necessary to adjust the value of the
R
FB
resistor. This is because the load compensation
system exhibits some nonlinearity. In particular, the
circuit can shift the reference current by a noticeable
SELECTING R
FB
AND R
REF
RESISTOR VALUES
The expression for V
OUT
developed in the Operation
section can be rearranged to yield the following expres-
sion for R
FB
:
V
OUT
+ V
F
+ I
SEC
(ESR)
V
BG
))
))
R
FB
= R
REF
α
N
SP
The unknown parameter α, which represents the fraction
of R
FB
current flowing into the R
REF
node, can be repre-
sented instead by specified data sheet values as follows:
V
BG
(I
REF
)(3k)
)
)
(I
REF
)(α)(3k) = V
BG
α =
Allowing the expression for R
FB
to be rewritten as:
V
OUT
+ V
F
+ I
SEC
(ESR)
I
REF
(3k)N
SP
)
)
R
FB
= R
REF
where,
V
OUT
= Desired output voltage
V
F
= Switching diode forward voltage
(I
SEC
)(ESR) = Secondary resistive losses
I
REF
= Data sheet reference current value
N
SP
= Effective secondary-to-primary turns ratio
Strictly speaking, the above equation defines R
FB
not as an
absolute value, but as a ratio of R
REF
. So the next question
is, “What is the proper value for R
REF
?” The answer is that
R
REF
should be approximately 3k. This is because the
LT1425 is trimmed and specified using this value of R
REF
.
If the impedance of R
REF
varies considerably from 3k,
additional errors will result. However, a variation in R
REF
of several percent or so is perfectly acceptable. This yields
a bit of freedom in selecting standard 1% resistor values
to yield nominal R
FB
/R
REF
ratios.
SELECTING R
OCOMP
RESISTOR VALUE
The Operation section previously derived the following
expressions for R
OUT
, i.e., effective output impedance and
12
LT1425
APPLICATIONS INFORMATION
WUU
U
amount when output switch current is zero. Please refer
to Figure 1 which shows nominal reference current shift
at zero load for a range of R
OCOMP
values. Example: for
a load compensation resistor of 12k, the graph indi-
cates a 1.0% shift in reference current. The R
FB
resistor
value should be adjusted down by about 1.0% to
restore the original target output voltage.
integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which
yield more freedom in setting total turns and mutual
inductance. Turns ratio can then be chosen on the basis of
desired duty cycle. However, remember that the input
supply voltage plus the secondary-to-primary referred
version of the flyback pulse (including leakage spike) must
not exceed the allowed output switch breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after output switch turn-off.
This is increasingly prominent at higher load currents
where more stored energy must be dissipated. In many
cases a “snubber” circuit will be required to avoid over-
voltage breakdown at the output switch node. LTC’s
Application Note 19 is a good reference on snubber
design.
In situations where the flyback pulse extends beyond the
enable delay time, the output voltage regulation will be
affected to some degree. It is important to realize that the
feedback system has a deliberately limited input range,
roughly ±50mV referred to the R
REF
node, and this works
to the user’s advantage in rejecting large, i.e., higher
voltage leakage spikes. In other words, once a leakage
spike is several volts in amplitude, a further increase in
amplitude has little effect on the feedback system. So the
user is generally advised to arrange the snubber circuit to
clamp at as high a voltage as comfortably possible,
observing switch breakdown, such that leakage spike
duration is as short as possible.
As a rough guide, total leakage inductances of several
percent (of mutual inductance) or less may require a
snubber, but exhibit little to no regulation error due to
leakage spike behavior. Inductances from several percent
up to perhaps ten percent cause increasing regulation
error.
Severe leakage inductances in the double digit percentage
range should be avoided if at all possible as there is a
potential for abrupt loss of control at high load current.
This curious condition potentially occurs when the leak-
age spike becomes such a large portion of the flyback
waveform that the processing circuitry is fooled into
thinking that the leakage spike itself is the real flyback
In less critical applications, or when output current
remains relatively constant, the load compensation func-
tion may be deemed unnecessary. In such cases, a
reduced component solution may be obtained as follows:
Leave the R
OCOMP
node open (R
OCOMP
= ∞), and replace
the filter capacitor normally on the R
CCOMP
node with a
short to ground.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification and design is perhaps the most
critical part of applying the LT1425 successfully. In addi-
tion to the usual list of caveats dealing with high frequency
isolated power supply transformer design, the following
information should prove useful.
Turns Ratio
Note that due to the use of an R
FB
/R
REF
resistor ratio to set
output voltage, the user has relative freedom in selecting
transformer turns ratio to suit a given application. In other
words, “screwball” turns ratios like “1.736:1.0” can scru-
pulously be avoided! In contrast, simpler ratios of small
R
OCOMP
(kΩ)
1
∆I
REF
(%)
2
10 100 1000
1425 F01
1
0
Figure 1
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT1425IS#PBF

Mfr. #:
Buy LT1425IS#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Isolated Flyback DC/DC Converter
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet