LT1725IS Linear Technology, LT1725IS Datasheet - Page 15

• Manufacturer Part Number: LT1725IS
• Description: IC CNTRLR ISOLATD FLYBACK 16SOIC
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1725IS

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Voltage - Output: -
• Voltage - Input: -
• Power - Output: -

APPLICATIO S I FOR ATIO
to transformer leakage inductance. This spike is not in-
dicative of actual output voltage (see Figure 4B). Delaying
the enabling of the feedback amplifier allows this system
to effectively ignore most or all of the voltage spike and
maintain proper output voltage regulation. The enable
delay time should therefore be set to the maximum ex-
pected duration of the leakage spike. This may have
implications regarding output voltage regulation at mini-
mum load (see Minimum Load Considerations).
A second benefit of the enable delay time function occurs
at light load. Under such conditions the amount of energy
stored in the transformer is small. The flyback waveform
becomes “lazy” and some time elapses before it indicates
the actual secondary output voltage (see Figure 4C). So
the enable delay time should also be set long enough to
ignore the “irrelevant” portion of the flyback waveform at
light load.
Additionally, there are cases wherein the gate output is
called upon to drive a large geometry MOSFET such that
the turnoff transition is slowed significantly. Under such
circumstances, the enable delay time may be increased to
accommodate for the lengthy transition.
A
B
C
ENABLE DELAY
TIME NEEDED
NEEDED
ENABLE
DELAY
TIME
U
Figure 4
U
DISCONTINUOUS
RINGING
MODE
W
MOSFET GATE DRIVE
IDEALIZED FLYBACK
WAVEFORM
FLYBACK WAVEFORM
WITH LARGE LEAKAGE
SPIKE AT HEAVY LOAD
“SLOW” FLYBACK
WAVEFORM AT
LIGHT LOAD
U
1725 F04
Minimum Enable Time
This function sets a minimum duration for the expected
flyback pulse. Its primary purpose is to provide a mini-
mum source current at the V
problems.
Minimum enable time can also have implications at light
load (see Minimum Load Considerations). The temptation
is to set the minimum enable time to be fairly short, as this
is the least restrictive in terms of minimum load behavior.
However, to provide a “reliable” minimum start-up current
of say, nominally 1µA, the user should set the minimum
enable time at no less that 2% of the switching period
(= 1/switching frequency).
CURRENT SENSE RESISTOR CONSIDERATIONS
The external current sense resistor allows the user to
optimize the current limit behavior for the particular appli-
cation under consideration. As the current sense resistor
is varied from several ohms down to tens of milliohms,
peak switch current goes from a fraction of an ampere to
tens of amperes. Care must be taken to ensure proper
circuit operation, especially with small current sense
resistor values.
For example, a peak switch current of 10A requires a sense
resistor of 0.025Ω. Note that the instantaneous peak power
in the sense resistor is 2.5W, and it must be rated accord-
ingly. The LT1725 has only a single sense line to this re-
sistor. Therefore, any parasitic resistance in the ground side
connection of the sense resistor will increase its apparent
value. In the case of a 0.025Ω sense resistor, one milliohm
of parasitic resistance will cause a 4% reduction in peak
switch current. So resistance of printed circuit copper
traces and vias cannot necessarily be ignored.
An additional consideration is parasitic inductance. Induc-
tance in series with the current sense resistor will accen-
tuate the high frequency components of the current
waveform. In particular, the gate switching spike and
multimegahertz ringing at the MOSFET can be considerably
Minimum Enable Time
Average “start-up” V
Switching Frequency
C
current =
•
I
SRC
C
node to avoid start-up
LT1725
15
1725fa