Datasheet

LT3825
17
3825fe
4. Compute:
R
CMP
= K1•
R
SENSE
R
S(OUT)
• R1• N
SF
5. Verify this result by connecting a resistor of this value
from the R
CMP
pin to ground.
6. Disconnect the ground short to C
CMP
and connect a 0.1µF
filter capacitor to ground. Measure the output imped-
ance R
S(OUT)
= ∆V
OUT
/∆I
OUT
with the new compensation
in place. R
S(OUT)
should have decreased significantly.
Fine tuning is accomplished experimentally by slightly
altering R
CMP
. A revised estimate for R
CMP
is:
′
R
CMP
= R
CMP
• 1+
R
S(OUT)CMP
R
S(OUT)








where R′
CMP
is the new value for the load compensation
resistor, R
S(OUT)CMP
is the output impedance with R
CMP
in place and R
S(OUT)
is the output impedance with no
load compensation (from step 2).
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 initiates an OSC capacitor discharge forcing
primary MOSFET off (PG voltage goes low). If the oscilla-
tor frequency is much different from the sync frequency,
problems may 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 flyback 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
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 flyback 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
rectification sets the switch over to forced continuous
mode operation.
APPLICATIONS INFORMATION
C
OSC
(pF)
30
50
f
OSC
(kHz)
100
200
300
100 200
3825 F02
Figure 2. f
OSC
vs OSC Capacitor Values