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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

Opto-Coupler Design Guidance

An opto-coupler’s CTR degradation affects a forward

converter’s voltage feedback loop in two ways:

1. Large Signal Effect: A drop in CTR means that to sus-

tain the same output current from the opto-coupler,

theinputcurrentoftheopto-couplerwillhavetoincrease.

The input current of the opto-coupler is sourced by the

LT8311’s OPTO pin. The opto-feedback loop should

be designed such that, at the lowest CTR possible, the

LT8311’s OPTO pin is not current limited. The maxi-

mum current that the LT8311’s internal opto-driver can

source out of the OPTO pin is 10mA. Design the system

so that, nominally, the OPTO pin is sourcing 2mA to

3mA maximum current into the opto-coupler’s input.

2. Small Signal Effect: A reduction in CTR by 2x will

cause the DC gain and crossover frequency of the for-

ward converter’s voltage feedback loop to drop by 2x,

assuming all other parameters are constant. Likewise,

an increase in CTR by 2x, assuming no change in other

parameters, will cause the DC gain and the crossover

frequency of the voltage feedback loop to increase

by 2x. The voltage feedback loop must be designed

ensuring that at CTR(MAX) (maximum CTR of the opto-

coupler), the crossover frequency of the feedback loop

stays well within the Nyquist frequency of the system

(= switching frequency/2). A good rule of thumb is to

design the voltage feedback loop’s crossover at about

1/10 of the switching frequency for an opto-coupler at

the nominal value of CTR.

As explained earlier, improvements in opto-coupler

technology have allowed CTR changes over the operating

lifetime of an opto-coupler to become significantly smaller

andwellcontrolled.However,themorechallengingdesign

aspect of an opto-coupler is the absolute variation in its

CTR over a large sample size and operating temperature

range. It is this spread in CTR that must be accounted for

when designing an opto-coupler based voltage feedback

loop. Picking an opto-coupler whose CTR variation is no

more than 2x its nominal value, is typically a good starting

point (see Table 1 for a list of opto-couplers with small

CTR spreads at room temperature).

The following guidelines help calculate initial values for

the input and output resistors of the opto-coupler (RD and

RE, respectively) for a generic application. The final values

for RD and RE should be determined after bread-boarding

a system. Use Figure 16 as a reference when reading the

following guidelines:

Step 1: Pick resistors, R1 and R2, that set the inverting

gain of the primary-side IC’s error amplifier. A typical

starting value for R1 would be 22k on the LT3752, and

100k on the LT8310. A typical starting value for R2 would

be 33k on the LT3752, and 150k on the LT8310.

Step 2: Calculate the maximum voltage required at the

emitteroftheopto-coupler'soutputtransistor(VX_MAX)to

drive the primary-side IC’s COMP or VC pin to the voltage

needed to command zero inductor current (referred to as

VC_LOW in the following equation):

VX_MAX = VREF 1+









− VC_LOW •

VC_LOW is approximately 0.7V for the LT8310, and 1.25V

for the LT3752. VREF is 1.6V for the LT8310 and 1.25V

for the LT3752.

Step 3: Pick a maximum opto-coupler output current

(IOPTO_OUT_HIGH) in the range of 1mA to 10mA. A typical

choice for IOPTO_OUT_HIGH might be 2.5mA. Now calculate

RE to be:

RE =

VX−MAX

IOPTO_OUT_HIGH

Step 4: Estimate the maximum input current (IF_HIGH)

needed to be sourced into opto-coupler by the

LT8311’s OPTO pin, at the opto-coupler’s minimum CTR

(CTRMIN):

IF_HIGH =

IOPTO_OUT_HIGH

CTRMIN

Ensure that IF_HIGH is well within the 10mA limit that the

LT8311’s OPTO pin can source.

Step 5: Estimate the RD value needed for the OPTO pin

to source the IF_HIGH current at the maximum OPTO pin

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