Datasheet

LM1815
SNOSBU8F –SEPTEMBER 2000–REVISED MARCH 2013 www.ti.com
APPLICATION HINTS
Figure 18. LM1815 Oscillograms
INPUT VOLTAGE CLAMP
The signal input voltage at pin 3 is internally clamped. Current limit for the Input pin is provided by an external
resistor which should be selected to allow a peak current of ±3 mA in normal operation. Positive inputs are
clamped by a 1kΩ resistor and series diode (see R4 and Q12 in the internal schematic diagram), while an active
clamp limits pin 3 to typically 350mV below Ground for negative inputs (see R2, R3, Q10, and Q11 in the internal
schematic diagram). Thus for input signal transitions that are more than 350mV below Ground, the input pin
current (up to 3mA) will be pulled from the V+ supply. If the V+ pin is not adequately bypassed the resulting
voltage ripple at the V+ pin will disrupt normal device operation. Likewise, for input signal transitions that are
more than 500mV above Ground, the input pin current will be dumped to Ground through device pin 2. Slight
shifts in the Ground potential at device pin 2, due to poor grounding techniques relative to the input signal
ground, can cause unreliable operation. As always, adequate device grounding, and V+ bypassing, needs to be
considered across the entire input voltage and frequency range for the intended application.
INPUT CURRENT LIMITING
As stated earlier, current limiting for the Input pin is provided by a user supplied external resistor. For purposes of
selecting the appropriate resistor value the Input pin should be considered to be a zero ohm connection to
ground. For applications where the input voltage signal is not symmetrical with relationship to Ground the worst
case voltage peak should be used.
Minimum Rext = [(Vin peak)/3mA]
In the application example shown in Figure 17 (Rext = 18kΩ) the recommended maximum input signal voltage is
±54V (i.e. 108Vp-p).
OPERATION OF ZERO CROSSING DETECTOR
The LM1815 is designed to operate as a zero crossing detector, triggering an internal one shot on the negative-
going edge of the input signal. Unlike other zero crossing detectors, the LM1815 cannot be triggered until the
input signal has crossed an "arming" threshold on the positive-going portion of the waveform. The arming circuit
is reset when the chip is triggered, and subsequent zero crossings are ignored until the arming threshold is
exceeded again. This threshold varies depending on the connection at pin 5. Three different modes of operation
are possible:
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