LTC4269-1
29
42691fc
APPLICATIONS INFORMATION
trace length and area to minimize stray capacitance and
potential noise pick-up.
You can synchronize the oscillator frequency to an
external frequency. This is done with a signal on the SYNC
pin. Set the LTC4269-1 frequency 10% slower than the
desired external frequency using the OSC pin capacitor,
then use a pulse on the SYNC pin of amplitude greater
than 2V and with the desired frequency. The rising edge
of the SYNC signal initiates an OSC capacitor discharge
forcing primary MOSFET off (PG voltage goes low). If
the oscillator frequency is much different from the sync
frequency, problems may occur with slope compensation
and system stability. Also, keep the sync pulse width
greater than 500ns.
Selecting Timing Resistors
There are three internal “one-shot” times that are
programmed by external application resistors: minimum
on-time, enable delay time and primary MOSFET turn-on
delay. These are all part of the isolated fl yback control
technique, and their functions are previously outlined in
the Theory of Operation section. The following information
should help in selecting and/or optimizing these timing
values.
Minimum Output Switch On-Time (t
ON(MIN)
)
Minimum on-time is the programmable period during which
current limit is blanked (ignored) after the turn-on of the
primary-side switch. This improves regulator performance
by eliminating false tripping on the leading edge spike in
the switch, especially at light loads. This spike is due to
both the gate/source charging current and the discharge
of drain capacitance. The isolated fl yback sensing requires
a pulse to sense the output. Minimum on-time ensures
that the output switch is always on a minimum time and
that there is always a signal to close the loop.
The LTC4269-1 does not employ cycle skipping at light
loads. Therefore, minimum on-time along with synchro-
nous rectifi cation sets the switch over to forced continuous
mode operation.
The t
ON(MIN)
resistor is set with the following equation
R
tON(MIN)
kΩ
()
=
t
ON(MIN)
ns
()
−104
1.063
Keep R
tON(MIN)
greater than 70k. A good starting value
is 160k.
Enable Delay Time (ENDLY)
Enable delay time provides a programmable delay between
turn-off of the primary gate drive node and the subsequent
enabling of the feedback amplifi er. As discussed earlier,
this delay allows the feedback amplifi er to ignore the
leakage inductance voltage spike on the primary side.
The worst-case leakage spike pulse width is at maximum
load conditions. So, set the enable delay time at these
conditions.
While the typical applications for this part use forced
continuous operation, it is conceivable that a secondary-
side controller might cause discontinuous operation at
light loads. Under such conditions, the amount of energy
stored in the transformer is small. The fl yback waveform
becomes “lazy” and some time elapses before it indicates
the actual secondary output voltage. The enable delay time
should be made long enough to ignore the “irrelevant”
portion of the fl yback waveform at light loads.
Even though the LTC4269-1 has a robust gate drive, the gate
transition time slows with very large MOSFETs. Increase
delay time as required when using such MOSFETs.
The enable delay resistor is set with the following
equation:
R
ENDLY
kΩ
()
=
t
ENDLY
ns
()
−30
2.616
Keep R
ENDLY
greater than 40k. A good starting point is
56k.
