IC drives up to four single-coil latching relays

Single-coil latching relays are often used in applications such as signal routing, audio, and automotive systems. This type of design is challenging because coil current must flow in both directions through the same coil (Figure 1). The relay is latched into the reset state when current flows from pin 8 to pin 1, and the relay is latched into the set state when current flows from pin 1 to pin 8. The relay remains latched even when the coil current is removed. Removing current after the relay is latched can save a lot of power.


Figure 1. Current in a single coil latches the corresponding relay into the SET or RESET state . A simple circuit

(Figure 2) that can drive up to four single-coil latching relays includes a parallel interface relay driver (U1) with open-drain outputs (Figure 3) and inductive kickback protection . Any of the four relays can be latched into the set or reset state by turning on the corresponding output (OUTX). When /CS is driven high, the digital address is loaded to pins A2 to A0 to select the corresponding output. Toggling /CS activates the output (Figure 4). Figure 2. This circuit can easily drive four single-coil latching relays Figure 3. This is two of the eight open-drain outputs of the circuit shown in Figure 2 Figure 4. Interface timing for output activation of the circuit shown in Figure 2 Current flows into the enabled open-drain outputs and latches the relays in the set or reset state depending on the direction of the coil current. Once the relays latch, /RESET should be driven low to turn off all enabled open-drain outputs and save power. The timing of the set/reset operation is shown in Figure 4. Allow enough time (tSET/RESET) before pulling /RESET low. Wait tSET/RESET after the last /CS toggle to ensure that the relays can properly latch into the specified state. The clamping diodes at each OUTX pin can clamp the transient high voltage when the coil current is interrupted. As shown in Figure 2, those diodes clamp the voltage at the OUTX terminal to VCC - 0.7V. A similar article appeared in the February 2004 issue of EET.