LTC1698IGN datasheet(13/24 Pages) LINER

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LTC1698

1698f

supply must cross the isolation boundary. Coupling this

signal requires an element that will withstand the isolation

potentials and still transfer the loop error signal.

Optocouplers are widely used for this function due to their

ability to couple DC signals. To properly apply them, a

number of factors must be considered. The gain, or

current transfer ratio (CTR) through an optocoupler is

loosely specified and is a strong function of the input

current through the diode. It changes considerably as a

function of time (aging) and temperature. The amount of

aging accelerates with higher operating current. This

variation directly affects the overall loop gain of the sys-

tem. To be an effective optical detector, the output transis-

tor of the optocoupler must have a large base area to

collect the light energy. This gives it a large collector to

base capacitance which can introduce a pole into the

feedback loop. This pole varies considerably with the

current and interacts with the overall loop frequency

compensation network.

The common collector optocoupler configuration removes

the miller effect due to the parasitic capacitance and

increases the frequency response. Figure 7 shows the

optocoupler feedback circuitry using the common collec-

tor approach. Note that the terms RD, CTR, CDE and rπ vary

from part to part. They also change with bias current. The

dominant pole of the opto feedback is due to RF and CF. The

feedforward capacitor CK at the optocoupler creates a low

frequency zero. This zero should be chosen to provide a

phase boost at the loop crossover frequency. The parallel

combination of RK and RD form a high frequency pole with

CK. For most optocouplers, RD is 50Ω at a DC bias of 1mA,

and 25

Ω at a DC bias of 2mA. The CTR term is the small

signal AC current transfer ratio. For the QT Optoelectron-

ics MOC207 optocoupler used here, the AC CTR is around

1, even though the DC CTR is much lower when biased at

1mA or 2mA. The first denominator term in the VC/VOUT

equation has been simplified and assumes that CFB<<CC.

The actual term is:

CFB

•• (

)•

•









APPLICATIO S I FOR ATIO

s R

CTR

s r

s R

OUT

















–

(•

•

)

( ••)• (

•

)

••

(•

•

)

••

•

(•

•

)

•

(•

•

)

where

Optocoupler diode equivalent small signal resis

CTR

Optocoupler current transfer ratio

Optocoupler nonlinear capacitor across base to emitter

Optocoupler small signal resis

ce across the base emitter

tan

=−

20k

VREF

VCOMP

CFB

VREF

VOUT

LTC1698

VFB

1698 F07

100k

OPTODRV

MOC207

–

VREF

LT3781

VFB

VCC

–

Figure 7. Error Signal Feedback

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