LT3574IMS#PBF Linear Technology, LT3574IMS#PBF Datasheet - Page 13

• Manufacturer Part Number: LT3574IMS#PBF
• Description: IC PWM FLYBCK ISO CM 16MSOP
• Manufacturer: Linear Technology
• Type: Flyback, Isolated, PWM - Current Moder

Specifications of LT3574IMS#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 60V
• Current - Output: 650mA
• Frequency - Switching: 40kHz ~ 1MHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: *
• Package / Case: 16-MSOP
• Voltage - Supply: 3 V ~ 40 V
• Frequency-max: 1MHz
• Duty Cycle: 52%
• Pwm Type: Current Mode
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

applications inForMation
Secondary Leakage Inductance
In addition to the previously described effects of leakage
inductance in general, leakage inductance on the second-
ary in particular exhibits an additional phenomenon. It
forms an inductive divider on the transformer secondary
that effectively reduces the size of the primary-referred
flyback pulse used for feedback. This will increase the
output voltage target by a similar percentage. Note that
unlike leakage spike behavior, this phenomenon is load
independent. To the extent that the secondary leakage
inductance is a constant percentage of mutual inductance
(over manufacturing variations), this can be accommodated
by adjusting the R
Figure 6. Good Snubber Diode Limits SW Pin Voltage
10V/DIV
FB
C
Figure 4. RCD Clamp
/R
REF
R
D
resistor ratio.
100ns/DIV
L
S
3574 F04
•
•
3574 F06
Winding Resistance Effects
Resistance in either the primary or secondary will reduce
overall efficiency (P
tion will be maintained independent of winding resistance
due to the boundary mode operation of the LT3574.
Bifilar Winding
A bifilar, or similar winding technique, is a good way to
minimize troublesome leakage inductances. However, re-
member that this will also increase primary-to-secondary
capacitance and limit the primary-to-secondary breakdown
voltage, so, bifilar winding is not always practical. The
Linear Technology applications group is available and
extremely qualified to assist in the selection and/or design
of the transformer.
Figure 7. Bad Snubber Diode Does Not Limit SW Pin Voltage
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
10V/DIV
< 60V
< 55V
< 50V
V
SW
t
SP
< 150ns
OUT
/P
t
OFF
IN
> 350ns
). Good output voltage regula-
100ns/DIV
3574 F05
LT3574
TIME
3574 F07

3574f