lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 17

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

APPLICATIONS INFORMATION
4. Compute:
5. Verify this result by connecting a resistor of this value
6. Disconnect the ground short to C
Setting Frequency
The switching frequency of the LT3825 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF , yielding switching frequencies between 50kHz and
250kHz. Figure 2 shows the nominal relationship between
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
trace length and area to minimize stray capacitance and
potential noise pickup.
You can synchronize the oscillator frequency to an external
frequency. This is done with a signal on the SYNC pin. Set
the LT3825 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 initi-
ates 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
from the R
fi lter capacitor to ground. Measure the output imped-
ance R
in place. R
Fine tuning is accomplished experimentally by slightly
altering R
where Rʹ
resistor, R
in place and R
load compensation (from step 2).
R
R
CMP
CMP
S(OUT)
= 1
= R
CMP
K
CMP
S(OUT)CMP
CMP
CMP
S(OUT)
•
= ΔV
is the new value for the load compensation
R
R
. A revised estimate for R
S(OUT)
S OUT
SENSE
pin to ground.
• 1+
(
OUT
should have decreased signifi cantly.
is the output impedance with R
)
/ΔI
R
is the output impedance with no
•
S(OUT)CMP
R
R N
OUT
S(OUT)
1
•
with the new compensation
SF
CMP
and connect a 0.1μF
CMP
is:
CMP
occur with slope compensation and system stability. Keep
the sync pulse width greater than 500ns.
Selecting Timing Resistors
There are three internal “one-shot” times that are pro-
grammed 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 On Time (t
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 there is always a signal to close the loop.
The LT3825 does not employ cycle skipping at light loads.
Therefore, minimum on time along with synchronous
rectifi cation sets the switch over to forced continuous
mode operation.
300
200
100
50
Figure 2. f
30
OSC
ON(MIN)
vs OSC Capacitor Values
C
OSC
(pF)
)
100
3825 F02
200
LT3825
17
3825fa