LT1105IN8 Linear Technology, LT1105IN8 Datasheet - Page 23

• Manufacturer Part Number: LT1105IN8
• Description: IC OFFLINE SWIT CM HV 8DIP
• Manufacturer: Linear Technology

Specifications of LT1105IN8

• Output Isolation: Isolated
• Frequency Range: 20 ~ 200kHz
• Voltage - Input: 7 ~ 30 V
• Voltage - Output: 20V
• Power (watts): 100W
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
time is the most accurate representation of the output
voltage, the answer given by the bias winding voltage is
still off from the “true” answer by the amount I•R
The sampling error amplifier of the LT1103/LT1105 pro-
vides solutions to the errors associated with the bias
winding flyback voltage. The error amplifier is comprised
of a leakage inductance spike blanking circuit, a slew rate
limited tracking amplifier, a level detector, a sample-and-
hold, an output g
tion circuitry. This all seems complicated at first glance,
but its operation is straightforward and transparent to the
user of the IC. When viewed from a system or block level,
the sampling error amplifier behaves like a simple transcon-
ductance amplifier. Here’s how it works.
The sampling error amplifier takes advantage of the fact
that the voltage across the bias winding during at least a
portion of switch off time is proportional to the DC output
voltage of the secondary winding. The feedback network
used to sense the bias winding voltage is no longer
comprised of a traditional peak detector in conjunction
with a resistor divider network. The feedback network
consists of a diode in series with the bias winding feeding
the resistor divider network directly. The resultant error
signal is then fed into the input of the error amplifier. The
purpose of the diode in series with the bias winding is now
not to peak detect, but to prevent the FB pin (input of the
error amplifier) from being pulled negative and forward
biasing the substrate of the IC when the bias winding
changes polarity with “switch turn-on.”
The primary winding leakage inductance spike effects are
first eliminated with an internal blanking circuit in the
LT1103/LT1105 which suppresses the input of the FB pin
for 1.5 s at the start of “switch off” time. This prevents the
primary leakage inductance spike from being propagated
through the error amplifier and affecting the regulated
output voltage.
With the effects of the leakage inductance spike elimi-
nated, the effects of decreasing bias winding flyback
voltage can be addressed. With the traditional diode/
capacitor peak detector circuitry eliminated from the feed-
back network, the tracking amplifier of the LT1103/LT1105
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stage and load regulation compensa-
INFORMATION
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follows the flyback waveform as it changes with time and
amplifies the difference between the flyback signal and the
internal 4.5V reference. Tracking is maintained until the
point in time where the bias winding voltage collapses as
a result of all transformer energy being depleted (discon-
tinuous mode) or the switch turning on again (continuous
mode). The level detector circuit senses the fact that the
bias winding flyback voltage is no longer a representation
of the output voltage and activates an internal peak detec-
tor. This effectively saves the most accurate representa-
tion of the output voltage which is then buffered to the
second stage of the error amplifier.
The second stage of the error amplifier consists of a
sample-and-hold. When the switch turns on, the sample-
and-hold samples the buffered error voltage for 1 s and
then holds for the remainder of the switch cycle. This held
voltage is then processed by the output g
converted into a control signal at the output of the error
amplifier, the V
The final adjustment in regulation is provided by the load
regulation compensation circuitry. As stated earlier, out-
put regulation degrades with increasing load current (out-
put power). The effect is traced to secondary leakage
inductance and parasitic secondary winding, diode and
output capacitor resistances. Even though the tracking
amplifier has obtained the most accurate representation of
the output voltage, its answer is still flawed by the amount
of the voltage drop across the secondary parasitic lumped
sum equivalent impedance which is coupled to the bias
winding voltage. This error increases with increasing load
current. Therefore, a technique for sensing load current
conditions has been added to the LT1103/LT1105. The
switch current is proportional to the load current by the
turns ratio of the transformer. A small current proportional
to switch current is generated in the LT1103/LT1105 and
fed back to the FB pin. This allows the input bias current of
the sampling error amplifier to be a function of load
current. A resistor in series with the FB pin generates a
linear increase in the effective reference voltage with
increasing load current. This translates to a linear increase
in output voltage with increasing load current. By adjust-
ing the value of the series resistor, the slope of the load
C
pin.
LT1103/LT1105
m
stage and
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