LT8311_15 Datasheet, PDF (27/48 Pages) Linear Technology – Synchronous Rectifier Controller with Opto-Coupler Driver for Forward Converters

LT8311
APPLICATIONS INFORMATION
sourced into the opto-coupler by the LT8311’s OPTO
pin; 4.5mA is the typical V_IN current of the LT8311;
T_A is the ambient temperature.
2. In the second configuration, the V_IN pin’s bias supply
drives the INTV_CC pin through a direct connection,
bypassing the internal LDO. This configuration re-
duces power dissipation inside the IC by not having to
incur any power loss within the INTV_CC LDO. Use this
optional configuration for V_IN voltages that are below
16V, allowing sufficient margin for INTV_CC to stay below
its OVLO(+) voltage of 16.5V. Ensure that V_IN, during
transients, does not exceed INTV_CC’s abs max voltage
of 18V.
When an external supply or auxiliary winding is available,
use this configuration (tying V_IN and INTV_CC together)
to deliver power to the IC. This configuration is most
applicable when using the LT3752 as a primary-side IC.
The LT3752’s housekeeping supply can be connected
to the LT8311’s V_IN and INTV_CC through an auxiliary
winding, as shown in Figure 21 in the Typical Applica-
tions section.
INTV_CC should be bypassed with a minimum of 4.7µF
ceramic capacitor to ground for all three configurations.
Place the capacitor close to the INTV_CC pin, ensuring that
the ground terminal of the capacitor has the shortest pos-
sible return path to the LT8311’s ground (exposed pad).
LT8311 OPTO CONTROL FUNDAMENTALS
Setting Output Voltage
Figure 15 shows how to program the forward converter’s
output voltage with a resistor divider feedback network.
Connect the top of R_FB1 to V_OUT, the tap point of R_FB1/
R_FB2 to the FB pin, and the bottom of R_FB2 to ground. The
ground return of R_FB2 must be kept as close as possible to
the ground of the LT8311, and must be kept away from the
forward converter’s power path. The power path contains
switching currents, and possibly large value currents (de-
pending upon the load) which may introduce unintended
noise, or I • R drops into the FB resistor divider path. The
FB pin regulates to 1.227V and has a typical input pin
bias current of 120nA flowing out of the pin. The output
voltage is set by the formula:
V_OUT
=
1.227
•


1+
R_FB1 
R_FB2 
− 120nA
• R_FB1
LT8311
FB
GND
V_OUT
120nA
R_FB1
R_FB2
8311 F15
Figure 15. Setting Output Voltage of Forward Converter
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8311f
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