ACPL-K376-000E Datasheet, PDF (12/13 Pages) AVAGO TECHNOLOGIES LIMITED – Isolated Voltage/Current Detector

Electrical Considerations
The ACPL-K370/K376 optocouplers have internally tem-
perature compensated, predictable voltage and current
threshold points. This allows a single external resistor, R_X,
to determine larger external threshold voltage levels. For
a desired external threshold voltage, V_±, the approximate
R_x value is shown in Figure 12. Equation 1 can be used to
calculate Rx.
V₊ and V_– voltage threshold levels can be simultaneously
set with two resistors, R_X and R_P, as shown in Figure 13 and
determined by Equations 4 and 5.
R_X can provide over-current transient protection by
limiting input current during a transient condition. For
monitoring contacts of a relay or switch, the ACPL-K370/
K376 in combination with R_X and R_P can be used to allow a
specific current to be conducted through the contacts for
cleaning purposes (wetting current).
The choice of which input voltage clamp level to choose
depends upon the application of this device (see Figure 4).
It is recommended that the low clamp condition be used
when possible. The low clamp condition in conjunction
with the low input current feature will ensure extremely
low input power dissipation.
In applications where dV_CM/dt may be extremely large
(such as with a static discharge), a series resistor, R_CC,
should be connected in series with V_CC and pin 8 to protect
the detector IC from destructive high surge currents. The
recommended value for R_CC is 240 : per volt of allowable
drop in V_CC (between Pin 8 and V_CC) with a minimum value
of 240 :. In addition, it is recommended that a ceramic
disc bypass capacitor of 0.01 PF be placed between pins 5
and 8 to reduce the effect of power supply noise.
For interfacing ac signals to TTL systems, output low pass
filtering can be performed with a pull-up resistor of 1.5 k:
and 20 PF capacitor. This application requires a Schmitt
trigger gate to avoid slow rise time chatter problems.
For AC input applications, a filter capacitor can be placed
across the DC input terminals for either signal or transient
filtering.
ISOLATION
BARRIER
RX
ITH±
1 AC1
V_CC 8
VCC
V±
VTH± _{2 DC+}
NC 7
RL
RP
3 DC–
V_O 6
VO
CL
4 AC2
GND 5
Either AC (pins 1 and 4) or DC (pins 2 and 3) input can be
used to determine external threshold levels. For single
specifically selected external threshold voltage level V₊ or
V_–, R_X can be determined without use of R_P via:
V_+(–) – V_TH+(–)
R_X =
I_TH+(–)
Equation 1
For dual specifically selected external threshold voltage
levels, V₊ and V_–, the use of R_X and R_P will permit this
selection. Two equations can be written:
V_TH+
V₊ = R_x ( I_TH+ +
) + V_TH+
R_P
Equation 2
V_TH–
V_– = R_x ( I_TH– +
) + V_TH–
R_P
Equation 3
Solving these equations for R_X and R_P yields the following
two expressions:
V_TH– (V₊) – V_TH+ (V_–)
R_X =
I_TH+ (V_TH–) – I_TH– (V_TH+)
Equation 4
V_TH– (V₊) – V_TH+ (V_–)
R_P =
I_TH+ (V_– – V_TH–) + I_TH– (V_TH+ – V₊)
Equation 5
where
V₊ and V_– are the desired external voltage threshold
levels, and values for V_TH± and I_TH± are found from the
data sheet.
Equations 4 and 5 are valid only if the conditions of
Equations 6 or 7 are met. With the V_TH± and I_TH± values,
the denominator of Equation 4 is checked to see if it is
positive or negative. If it is positive, then the following
ratios must be met:
V_{+ ≥} V_{TH+ and} V₊ – V_{TH+ <} I_TH+
V– V_TH–
V_– – V_TH– I_TH–
Equation 6
Conversely, if the denominator of Equation 4 is negative,
then the following ratios must hold:
V_{+ ≤} V_{TH+ and} V₊ – V_{TH+ >} I_TH+
V– V_TH–
V_– – V_TH– I_TH–
Equation 7
Refer to Application Note 1004 for more application infor-
mation and worked out examples.
GND
Figure 13. External threshold voltage level selection.
12