LTC3805 Linear Technology, LTC3805 Datasheet - Page 14

• Manufacturer Part Number: LTC3805
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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LTC3805
APPLICATIONS INFORMATION
Feedback in Isolated Applications
Isolated applications do not use the FB pin and error ampli-
fi er but control the I
driven on the other side of the isolation barrier as shown
in Figure 5. A detailed version is shown in Figure 9. For
isolated converters, the FB pin is grounded which pro-
vides pull-up on the I
to properly bias the optoisolator which is typically biased
using a resistor to V
optoisolator bias current, a diode is required to block
it from the I
ensure that the optoisolator is able to pull I
its lower clamp.
Oscillator Synchronization
The oscillator may be synchronized to an external clock
by connecting the synchronization signal to the SYNC
pin. The LTC3805 oscillator and turn-on of the switch are
synchronized to the rising edge of the external clock. The
frequency of the external sync signal must be ±33% with
respect to f
value of f
Current Sense Resistor Considerations
The external current sense resistor (R
allows the user to optimize the current limit behavior for
the particular application. 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 several amperes. Care must be taken to ensure proper
circuit operation, especially with small current sense
resistor values.
14
SYNC
OSC
Figure 5. Circuit for Isolated Feedback
TH
must be between 70kHz and 700kHz.
(as programmed by R
pin. A Schottky diode should be used to
LTC3805
TH
CC
GND
V
I
TH
CC
TH
FB
. Since the I
pin directly using an optoisolator
pin. This pull-up is not enough
3805 F05
TH
ISOLATION
BARRIER
FS
pin cannot sink the
SENSE
). Additionally, the
in Figure 2)
TH
down to
For example, with the peak current sense voltage of 100mV
on the I
a sense resistor of 0.020Ω. Note that the instantaneous
peak power in the sense resistor is 0.5W and it must be
rated accordingly. The LTC3805 has only a single sense
line to this resistor. Therefore, any parasitic resistance
in the ground side connection of the sense resistor will
increase its apparent value. In the case of a 0.020Ω sense
resistor, one milliohm of parasitic resistance will cause a
5% reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
Programmable Slope Compensation
The LTC3805 injects a ramping current through its I
pin into an external slope compensation resistor R
This current ramp starts at zero right after the GATE pin
has been high for the LTC3805’s minimum duty cycle of
6%. The current rises linearly towards a peak of 10μA at
the maximum duty cycle of 80%, shutting off once the
GATE pin goes low. A series resistor R
I
ramping voltage drop. From the perspective of the I
pin, this ramping voltage adds to the voltage across the
sense resistor, effectively reducing the current comparator
threshold in proportion to duty cycle. This stabilizes the
control loop against subharmonic oscillation. The amount
of reduction in the current comparator threshold (ΔV
can be calculated using the following equation:
Note: LTC3805 enforces 6% < Duty Cycle < 80%. A good
starting value for R
in current comparator threshold at 80% duty cycle.
Designs that do not operate at greater than 50% duty cycle
do not need slope compensation and may replace R
with a direct connection.
SENSE
ΔV
SENSE
pin to the current sense resistor R
SENSE
=
pin, a peak switch current of 5A requires
DutyCycle − 6%
SLOPE
80%
is 3k, which gives a 30mV drop
10µA • R
SLOPE
SENSE
connecting the
SLOPE
develops a
SENSE
SLOPE
SENSE
SENSE
SLOPE
3805fb
)
.