LT1425CS Linear Technology, LT1425CS Datasheet - Page 16

• Manufacturer Part Number: LT1425CS
• Description: IC SW REG ISOLATED FLYBCK 16SOIC
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1425CS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 200mA
• Frequency - Switching: 285kHz
• Voltage - Input: 2.8 ~ 20 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Power - Output: 6W
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -

APPLICATIONS
LT1425
Average supply current (including driver current) is:
Switch power dissipation is given by:
Total power dissipation of the die is the sum of supply
current times supply voltage plus switch power:
FREQUENCY COMPENSATION
Loop frequency compensation is performed by connect-
ing a capacitor from the output of the error amplifier (V
pin) to ground. An additional series resistor, often
required in traditional current mode switcher controllers
is usually not required, and can even prove detrimental.
The phase margin improvement traditionally offered by
this extra resistor will usually be already accomplished by
the nonzero secondary circuit impedance, which adds a
“zero” to the loop response.
In further contrast to traditional current mode switchers,
V
dynamic nature of the clamped feedback amplifier forms
an effective track/hold type response, whereby the V
voltage changes during the flyback pulse, but is then
“held” during the subsequent “switch ON” portion of the
16
C
P
R
P
DC = On switch duty cycle
pin ripple is generally not an issue with the LT1425. The
I
where,
I
IN
SW
SW
D(TOTAL)
SW
= 7mA + DC
= Switch current
= (I
= Output switch ON resistance
SW
= (I
)
2
(R
IN
SW
• V
U
I
)(DC)
35
SW
IN
) + P
INFORMATION
U
SW
W
U
C
C
V
next cycle. This action naturally holds the V
during the current comparator sense action (current mode
switching).
PCB LAYOUT CONSIDERATIONS
For maximum efficiency, switch rise and fall times are
made as short as practical. To prevent radiation and high
frequency resonance problems, proper layout of the com-
ponents connected to the IC is essential, especially the
power paths (primary and secondary). B field (magnetic)
radiation is minimized by keeping output diode, switch pin
and output bypass capacitor leads as short as possible. E
field radiation is kept low by minimizing the length and
area of all traces connected to the switch pin. A ground
plane should always be used under the switcher circuitry
to prevent interplane coupling.
The high speed switching current paths are shown sche-
matically in Figure 2. Minimum lead length in these paths
are essential to ensure clean switching and minimal EMI.
The path containing the input capacitor, transformer pri-
mary, output switch, the path containing the transformer
secondary, output diode and output capacitor are the only
ones containing nanosecond rise and fall times. Keep
these paths as short as possible.
IN
CIRCULATING
FREQUENCY
HIGH
PATH
•
Figure 2
•
CIRCULATING
FREQUENCY
PATH
HIGH
C
voltage stable
ISOLATED
LOAD
F
1425 F02
V
OUT