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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

sourced into the opto-coupler by the LT8311’s OPTO

pin; 4.5mA is the typical VIN current of the LT8311;

TA is the ambient temperature.

2. In the second configuration, the VIN pin’s bias supply

drives the INTVCC 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 INTVCC LDO. Use this

optional configuration for VIN voltages that are below

16V,allowingsufficientmarginforINTVCCtostaybelow

its OVLO(+) voltage of 16.5V. Ensure that VIN, during

transients, does not exceed INTVCC’s abs max voltage

of 18V.

Whenanexternalsupplyorauxiliarywindingisavailable,

use this configuration (tying VIN and INTVCC 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 VIN and INTVCC through an auxiliary

winding, as shown in Figure 21 in the Typical Applica-

tions section.

INTVCC should be bypassed with a minimum of 4.7µF

ceramic capacitor to ground for all three configurations.

Place the capacitor close to the INTVCC 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 RFB1 to VOUT, the tap point of RFB1/

RFB2 to the FB pin, and the bottom of RFB2 to ground. The

ground return of RFB2 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:

VOUT = 1.227 • 1+

RFB1

RFB2









− 120nA •RFB1

Figure 15. Setting Output Voltage of Forward Converter

8311 F15

GND

LT8311

VOUT

120nA

RFB2

RFB1

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