HCPL260L datasheet(13/14 Pages) HP

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Figure 13. Recommended drive circuit for High-CMR.

0.01 F

350

Ω

74LS04

OR ANY TOTEM-POLE

OUTPUT LOGIC GATE

VCC+

SHIELD

HCPL-260L

GND

GND2

220

Ω

VCC

220

Ω

* HIGHER CMR MAY BE OBTAINABLE BY CONNECTING PINS 1, 4 TO INPUT GROUND (GND1).

GND1

Application Information

Common-Mode Rejection for

HCPL-260L Families:

Figure 13 shows the recom-

mended drive circuit for optimal

common-mode rejection

performance. Two main points to

note are:

1. The enable pin is tied to V CC

rather than floating (this

applies to single-channel parts

only).

2. Two LED-current setting

resistors are used instead of

one. This is to balance ILED

variation during common-

mode transients.

If the enable pin is left floating, it

is possible for common-mode

transients to couple to the enable

pin, resulting in common-mode

failure. This failure mechanism

only occurs when the LED is on

and the output is in the Low

State. It is identified as occurring

when the transient output voltage

rises above 0.8 V. Therefore, the

enable pin should be connected

to either VCC or logic-level high

for best common-mode

performance with the output low

(CMRL). This failure mechanism

is only present in single-channel

parts which have the enable

function.

Also, common-mode transients

can capacitively couple from the

LED anode (or cathode) to the

output-side ground causing

current to be shunted away from

the LED (which can be bad if the

LED is on) or conversely cause

current to be injected into the

LED (bad if the LED is meant to

be off). Figure 14 shows the

parasitic capacitances which

exists between LED

anode/cathode and output ground

(CLA and CLC). Also shown in

Figure 14 on the input side is an

AC-equivalent circuit.

Figure 14. AC equivalent circuit.

350

Ω

1/2 RLED

VCC+

15 pF

VCM

SHIELD

CLA

GND

0.01 F

1/2 RLED

CLC

ILN

ILP

–

For transients occurring when the

LED is on, common-mode rejec-

tion (CMRL, since the output is in

the “low” state) depends upon the

amount of LED current drive (I F).

For conditions where IF is close

to the switching threshold (ITH),

CMRL also depends on the extent

which ILP and ILN balance each

other. In other words, any

condition where common-mode

transients cause a momentary

decrease in IF will cause

common-mode failure for

transients which are fast enough.

Likewise for common-mode

transients which occur when the

LED is off (i.e. CMR H, since the

output is “high”), if an imbalance

between ILP and ILN results in a

transient IF equal to or greater

than the switching threshold of

the optocoupler, the transient

“signal” may cause the output to

spike below 2 V (which consti-

tutes a CMRH failure).

By using the recommended

circuit in Figure 13, good CMR

can be achieved. The balanced

ILED-setting resistors help equalize

ILP and ILN to reduce the amount

by which ILED is modulated from

transient coupling through CLA

and CLC

6章104-117(PDF用)

01.5.24, 4:24 PM

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