LT1725 Datasheet, PDF (18/28 Pages) Linear Technology – General Purpose Isolated Flyback Controller

LT1725
APPLICATIO S I FOR ATIO
Capacitor C1 should then be made large enough to avoid
the relaxation oscillatory behavior described above. This
is complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended. (Use of the optional
soft-start function will lengthen the power-up timing and
require a correspondingly larger value for C1.)
A further note—certain users may wish to utilize the
general functionality of the LT1725, but may have an
available input voltage significantly lower than, say, 48V.
If this input voltage is within the allowable V_CC range, i.e.,
perhaps 20V maximum, the internal wide hysteresis range
UVLO function becomes counterproductive. In such cases
it is simply better to operate the LT1725 directly from the
available DC input supply. The LT1737 is identical to the
LT1725, with the exception that it lacks the internal wide
hysteresis UVLO function. It is therefore designed to
operate directly from DC input supplies in the range of
4.5V to 20V. See the LT1737 data sheet for further
information.
FREQUENCY COMPENSATION
Loop frequency compensation is performed by connect-
ing a capacitor from the output of the error amplifier (V_C
pin) to ground. An additional series resistor, often re-
quired in traditional current mode switcher controllers, is
usually not required and can even prove detrimental. The
phase margin improvement traditionally offered by this
extra resistor will usually be already accomplished by the
nonzero secondary circuit impedance, which adds a “zero”
to the loop response.
In further contrast to traditional current mode switchers,
V_C pin ripple is generally not an issue with the LT1725. The
dynamic nature of the clamped feedback amplifier forms
an effective track/hold type response, whereby the V_C
voltage changes during the flyback pulse, but is then “held”
during the subsequent “switch on” portion of the next
cycle. This action naturally holds the V_C voltage stable
during the current comparator sense action (current mode
switching).
OUTPUT VOLTAGE ERROR SOURCES
Conventional nonisolated switching power supply ICs
typically have only two substantial sources of output
voltage error: the internal or external resistor divider
network that connects to V_OUT and the internal IC refer-
ence. The LT1725, which senses the output voltage in both
a dynamic and an isolated manner, exhibits additional
potential error sources to contend with. Some of these
errors are proportional to output voltage, others are fixed
in an absolute millivolt sense. Here is a list of possible
error sources and their effective contribution.
Internal Voltage Reference
The internal bandgap voltage reference is, of course,
imperfect. Its error, both at 25°C and over temperature is
already included in the specifications.
User Programming Resistors
Output voltage is controlled by the user-supplied feedback
resistor divider ratio. To the extent that the resistor ratio
differs from the ideal value, the output voltage will be
proportionally affected. Highest accuracy systems will
demand 1% components.
Schottky Diode Drop
The LT1725 senses the output voltage from the trans-
former primary side during the flyback portion of the cycle.
This sensed voltage therefore includes the forward drop,
V_F, of the rectifier (usually a Schottky diode). The nominal
V_F of this diode should therefore be included in feedback
resistor divider calculations. Lot to lot and ambient tem-
perature variations will show up as output voltage shift/
drift.
Secondary Leakage Inductance
Leakage inductance on the transformer secondary re-
duces the effective secondary-to-third winding turns ratio
(N_S/N_T) from its ideal value. This will increase the output
voltage target by a similar percentage. To the extent that
secondary leakage inductance is constant from part to
part, this can be accommodated by adjusting the feedback
resistor ratio.
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