LT3825EFE#TRPBF Linear Technology, LT3825EFE#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LT3825EFE#TRPBF
• Description: IC CNTRLR SYNC 16-TSSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT3825EFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 12 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Power - Output: 60W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -

APPLICATIONS INFORMATION
LT3825
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
of the LT3825 current (only a fraction comes from R
V
cally tens of milliseconds, the output voltage approaches
its desired value. The flyback 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
R
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
TR
R
R
TR
voltage continues to drop until after some time, typi-
is selected to yield a worst-case minimum charging
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
amplifier (V
cause of the sampling behavior of the feedback amplifier,
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 amplifier
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 flyback 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
.
3525fc
VC