LT3825EFE-PBF LINER [Linear Technology], LT3825EFE-PBF Datasheet - Page 20

• Manufacturer Part Number: LT3825EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller with Wide Input Supply Range
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3825
Switching action commences and the converter begins to
deliver power to the output. Initially the output voltage is low
and the fl yback voltage is also low, so C
of the LT3825 current (only a fraction comes from R
V
cally tens of milliseconds, the output voltage approaches
its desired value. The fl yback winding then provides the
LT3825 supply current and the V
If C
old before stabilization and the LT3825 turns off. The V
node then begins to charge back up via R
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
RTR 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.
Make C
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
If you have an available input voltage within the V
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
20
and
CC
R
R
TR
voltage continues to drop until after some time, typi-
TR MAX
TR MIN
is undersized, V
(
(
TR
large enough to avoid the relaxation oscillatory
)
)
>
<
V
V
IN MAX
IN MIN
(
(
I
CC ST MAX
CC
)
)
I
CC MIN
(
–
–
reaches the V
(
V
V
CC ON MAX
_
CC ON MIN
CC
(
(
)
node.
)
CC
_
_
TR
voltage stabilizes.
.
)
CC
)
TR
turn-off thresh-
TR
supplies most
to the turn-
TR
CC
CC
.)
this case, use the LT3837 which is identical to the LT3825
except that it lacks the internal V
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
19.5V. This provides some protection for the part in the
event that the switcher is off (UVLO low) and the V
is pulled high. If R
never attain this clamp voltage.
Control Loop Compensation
Loop frequency compensation is performed by connect-
ing a capacitor network from the output of the feedback
amplifi er (V
cause of the sampling behavior of the feedback amplifi er,
compensation is different from traditional current mode
switcher controllers. Normally only C
can be used to add a “zero” but the phase margin improve-
ment traditionally offered by this extra resistor is usually
already accomplished by the nonzero secondary circuit
impedance. C
frequency pole and is usually sized at 0.1 times C
In further contrast to traditional current mode switchers,
V
The dynamic nature of the clamped feedback amplifi er
forms an effective track/hold type response, whereby the
V
“held” during the subsequent “switch on” portion of the
C
C
voltage changes during the fl yback pulse, but is then
pin ripple is generally not an issue with the LT3825.
C
Figure 5. V
VC2
pin) to ground as shown in Figure 5. Be-
can be used to add an additional high
TR
V
9
C
is sized correctly the part should
C
Compensation Network
3825 F05
C
VC2
CC
R
VC
C
VC
undervoltage lockout
CC
VC
of approximately
is required. R
CC
VC
node
.
3525fa
VC