LTC1698IGN datasheet(14/24 Pages) LINER

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LTC1698

1698f

A series RC network can be added in parallel with R2

(Figure 7) to provide a zero for the feedback loop fre-

quency compensation.

The opto driver will drive a capacitive load up to 0.1

µF. For

optocouplers with a base pin, switching signal noise can

get into this high impedance node. Connect a large resis-

tor, 1M or 2M between the base and the emitter. This

increases the diode current and the overall feedback

bandwidth slightly, and decreases the optocoupler gain.

When designing the resistor in series with the optocoupler

diode, it is important to consider the part to part variations

in the current transfer ratio and its reduction over tem-

perature and aging. The bigger the biasing current, the

faster the aging. The LTC1698 opto driver is designed to

source up to 10mA of current and swing between 0.4V to

(VDD – 2.5V). This should meet the design consideration

of most optocouplers.

Besides the voltage feedback function, the LTC1698 opto

driver couples fault signals to the primary controller and

prevents catastrophic damage to the circuit. Upon current

limit or an overvoltage fault, the ILIM or OVP comparator

overrides the error amplifier output and forces the

OPTODRV pin high. This sources maximum current into

the external optodiode and reduces the forward converter

duty cycle.

Average Current Limit

The secondary current limit function is implemented by

measuring the negative voltage across the current sense

resistor RSECSEN. The current limit transconductance

amplifier ILIM has a – 25mV threshold. As shown in

Figure 8, if the secondary current is small, the ICOMP pin

goes low and the transistor MILIM shuts off. The potential

at VCOMP determines the OPTODRV output. If the second-

ary current is large, ICOMP pulls high and forces the tran-

sistor MILIM to turn on hard. Thus the current limit circuit

overrides the voltage feedback and forces OPTODRV high

and injects maximum current into the external optocoupler.

The RILIM resistor provides a linear relationship between

the current sensed and the OPTODRV output.

The ISNS and ISNSGND pins allow a true Kelvin current

sense measurement and offer true differential measure-

ment across the sense resistor. A differential lowpass

filter formed by R6 and C2 removes the pulse-to-pulse

inductor current ripple and generates the average sec-

ondary current which is equal to the load current. The

lowpass corner frequency is typically set to 1 to 2 orders

of magnitude below the switching frequency and follows

the relationship:

SECSEN

LMAX

••

•

where:

RSECSEN = Secondary current sense resistor

ILMAX = Maximum allowed secondary current

fSW = Forward converter switching frequency

APPLICATIO S I FOR ATIO

–

OPTO

ILIM

OPTODRV

VOUT

RCILM

CCILM

VREF

VCOMP

LTC1698

ISNSGND

ISNS

DRIVE

25mV

20k

100k

MILIM

RILIM

ICOMP

RSECSEN

1698 F08

RDIV

(OPTIONAL)

Figure 8. Secondary Average Current Limit

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