LTC1531 Datasheet, PDF (8/16 Pages) Linear Technology – Self-Powered Isolated Comparator

LTC1531
APPLICATIONS INFORMATION
The zero-cross comparator inputs, ZCPOS and ZCNEG,
have an input common mode range that allows signal
swings near the positive supply rails. The ZCPOS and
ZCNEG inputs contain ESD diode protection devices which
will clamp input signals that go below GND. The current
into the diode should be limited to less than 5mA. The
Isolated Thermistor Temperature Controller shows an
example phase shift network with attenuation that satis-
fies these conditions. The positive input voltage should
not exceed the 12V maximum rating or the 5mA input
current to the ESD diode clamp.
ISOLATION dV/dt
The maximum continuous dV/dt across the isolation bar-
rier that will still allow the isolated comparator to operate
is 50V/µs. Continuous rates of dV/dt greater than this
cause the isolated side to not detect when its power cycle
has stopped and a comparison should begin. Figure 4
shows the maximum continuous rate trade-off of fre-
quency vs voltage of a sine wave:
SR = (π)(f)(V_P-P)
where SR = slew rate, V_P-P = the peak-to-peak voltage and
f = frequency. Noise immunity to intermittent dV/dt rates
greater than 50V/µs can be rejected by the LTC1531.
AC Noise Rejection and Things to Avoid
Minimizing AC noise pickup at the isolated comparator
should follow the following guidelines.
1. Allow the isolated side ground to float. The isolated side
should only be common with the isolated circuit.
2. Use hysteresis to decrease sensitivity by using CMPOUT.
3. Use lower impedance circuits if powered by V_REG.
Avoid large capacitance tied directly to V_REG output,
since this output does not go Hi-Z (high impedance)
during the off time.
PC Board Layout
The PC board layout should not have copper near the lead
frame isolation capacitors. The copper reduces the power
coupling and power delivery to the isolated side (see
Figure 6).
V_CC 1
SHDN 2
ZCNEG 3
ZCPOS 4
TOP VIEW
28 GND
27 ZCDATA
26 DATA
25 VALID
ETCH COPPER
FROM THE
SHADED AREA
V_PW 11
CMPOUT 12
V_REG 13
ISOGND 14
18 V1
17 V2
16 V3
15 V4
SW PACKAGE
28-LEAD PLASTIC SO (ISO)
1531 F06
Figure 6. PC Board Layout Consideration
TYPICAL APPLICATIONS DESCRIPTION
The Isolated Thermistor Temperature Controller (front
page) uses a simple AC power rectifier and Zener to
provide 5.6V of DC power to the LTC1531. To avoid power
dissipation in the 3k, 3W resistor, DC power can be
provided with a charge pump circuit similar to the Isolated
Switch Control. In this circuit, a thermistor half bridge is
used with the 4-input comparator connected to provide
the other half of the bridge, V4 = 2.5V, V3 = 0V, giving
(V4 + V3)/2 = 1.25V. With the 50k pot set to about 30k, the
trip point is 25°C with a hysteresis of ±0.5°C. Here, V_REG
turns on and powers the half-bridge while the comparator
samples the result. The zero-cross comparator, ZCPOS
and ZCNEG, is used to trigger a triac at the 10° phase point.
The circuit, R1, R2 and C1, provides the phase shift, θ, as
determined by:
R2C1 ≅ tan(θ)/2π60Hz
and where the attenuation ≅ R2/R1. In this example,
R1 = 680k, R2 = 47k and C1 = 0.01µF, provide a 7V peak
input signal with 10° of phase lag.
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