Datasheet

LT3837
17
3837fd
APPLICATIONS INFORMATION
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 the
requisite 0.1µF filter capacitor to ground. Measure
the output impedance 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 LT3837 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 3 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 LT3837 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 period. 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. 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
feedback loop. The LT3837 does not employ cycle skipping
at light loads. Therefore, minimum on-time along with
synchronous rectification sets the switch over in 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.
Figure 3. f
OSC
vs OSC Capacitor Values
C
OSCAP
(pF)
30
50
f
OSC
(kHz)
100
200
300
100 200
3837 F03