Datasheet

LT3825
20
3525fe
Switching action commences and the converter begins to
deliver power to the output. Initially the output voltage is low
and the flyback voltage is also low, so C
TR
supplies most
of the LT3825 current (only a fraction comes from R
TR
.)
V
CC
voltage continues to drop until after some time, typi-
cally tens of milliseconds, the output voltage approaches
its desired value. The flyback winding then
provides the
LT3825 supply current and the V
CC
voltage stabilizes.
If C
TR
is undersized, V
CC
reaches the V
CC
turn-off threshold
before stabilization and the LT3825 turns off. The V
CC
node then begins to charge back up via R
TR
to the turn-
on threshold, where the part again turns on. Depending
upon the circuit, this may result in either several on-off
cycles
before proper operation is reached, or permanent
relaxation oscillation at the V
CC
node.
R
TR
is selected to yield a worst-case minimum charging
current greater than the maximum rated LT3825 start-up
current, and a worst-case maximum charging current less
than the minimum rated LT3825 supply current.
R
TR(MAX)
<
V
IN(MIN)
– V
CC(ON _ MAX)
I
CC(ST _ MAX)
and
R
TR(MIN)
>
V
IN(MAX)
– V
CC(ON _ MIN)
I
CC(MIN)
Make C
TR
large enough to avoid the relaxation oscillatory
behavior described above. This is complicated to deter-
mine theoretically as it depends on the particulars of the
secondary circuit and load behavior. Empirical testing is
recommended. Note that the use of the optional soft-start
function lengthens the power-up timing and requires a
correspondingly larger value for C
TR
.
If you have an available input voltage
within the V
CC
range, the internal wide hysteresis range UVLO function
becomes counterproductive. In such cases it is better to
operate the LT3825 directly from the available supply. In
this case, use the LT3837 which is identical to the LT3825
except that it lacks the internal V
CC
undervoltage lockout
function. It is designed to operate directly from supplies
in the range of 4.5V to 19V
. See the LT3837 data sheet
for further information.
The LT3825 has an internal clamp on V
CC
of approximately
19.5V. This provides some protection for the part in the
event that the switcher is off (UVLO low) and the V
CC
node
is pulled high. If R
TR
is sized correctly the part should
never attain this clamp voltage.
Control Loop Compensation
Loop frequency compensation is performed
by connecting
a capacitor network from the output of the feedback ampli-
fier (V
C
pin) to ground as shown in Figure 5. Because of
the sampling behavior of the feedback amplifier, compen-
sation is different from traditional current mode switcher
controllers. Normally only C
VC
is required. R
VC
can be
used to add a “zero” but the phase margin improvement
traditionally offered by this extra resistor is usually
already
accomplished by the nonzero secondary circuit impedance.
C
VC2
can be used to add an additional high frequency pole
and is usually sized at 0.1 times C
VC
.
In further contrast to traditional current mode switchers,
V
C
pin ripple is generally not an issue with the LT3825.
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
APPLICATIONS INFORMATION
9
R
VC
V
C
C
VC
3825 F05
C
VC2
Figure 5. V
C
Compensation Network