A low-power design for relay driver with built-in power-saving circuit

Multiple relay coils can be powered by a single power supply, which must be large enough to drive all coils simultaneously. In addition, these relays are densely packed into a small area, and the power consumption of the coils must be considered during design. The pull-in voltage required for a relay coil is much higher than its holding voltage. Knowing this, it is possible to design a circuit that saves energy by reducing the coil drive current. This application note discusses a relay driver device with a built-in power saving circuit to reduce the power consumption of the entire system.

Power saving design method

The MAX4822/MAX4824 relay drivers have a power-saving feature that reduces the driver voltage after the FET is turned on for a period of time. Initially, the output driver is a fully saturated, on-FET. After an adjustable delay, the voltage drop across the FET adjusts to the register-programmed value. This delay can be set by an external capacitor (Figure 1).

The power saving feature reduces both the relay coil power consumption and the power supply power consumption. The device's output driver has two states: ON and OFF.

The ON state has two different states, called the "start-up state" and the "power-down state". In the start-up state, the output FET is fully saturated and turned on.

After a delay set by the pin capacitor, the device enters a power-saving state, where the voltage drop across the FET is regulated by the control loop.

To illustrate the power saving principle in power saving mode, the power consumption in the two ON modes can be compared. Assume that the relay coil has a DC resistance of 100 and the system uses a 5V power supply. Figure 2 shows the relay coil model composed of an ideal inductor and resistor RCOIL.

In the startup state, the maximum output resistance of the MAX4822/MAX4824 is 5 Ω. Therefore, the power dissipation can be calculated as follows:

ICOIL = 5V/105 = 47.6mA

PCOIL = ICOIL2 * RCOIL = 47.6mA2 * 100 = 0.227W

PDRIVER = ICOIL2 * RDRIVER = 47.6mA2 * 5 = 0.011W

PTOTAL_HEAT = 0.238W

The power consumption analysis for power saving state is slightly different. The power consumption of the coil must be determined first, and then the power consumption of the driver can be determined. Finally, the two are simply summed.

In the power-down state, the voltage at the FET output is regulated to a percentage of the supply voltage, which is set by an internal register. This means that the voltage VDRIVER shown in Figure 2 is regulated by the internal control loop. Returning to the previous example, assuming that VDRIVER is 50% (although the MAX4822/MAX4824 allows a range of 10% to 70%), the power dissipated in the coil is:

VCOIL = 5V- (50% * 5V) = 2.5V

ICOIL_PS = VCOIL/RCOIL = 2.5V/100 = 0.025A

PCOIL = 2.5V * 25mA = 0.0625W

To calculate the power dissipation in the driver, remember that its current is the same as the coil current:

IDRIVER_PS = 0.025A

VDRIVER = 50% * 5V = 2.5V

PDRIVER = 0.0625W

PTOTAL_PS = 0.125W

SAVINGS = 1 - PTOTAL_PS/PTOTAL_INIT

Therefore, in this example, the power saving mode reduces the power consumption by about 47.5% compared to the active state.

Here is another formula for calculating power savings. Note: Once the current is known, the information needed to calculate power savings is available.

PTOTAL_PS = VCC * ICOIL_PS

PTOTAL_INIT = VCC * IDRIVER_INIT

SAVINGS = 1 - ICOIL_PS/IDRIVER_INIT

This equation explains why power-save mode saves power: the supply voltage remains the same, but the current drawn from the supply decreases.

It is easy to create a table to illustrate the possible power savings of various settings for the MAX4822/MAX4824. In this table, VCC = 5V, RDRIVER = 5, and RCOIL = 100, the same as in the previous example.

It can be noticed that the power saving capability increases as the driver setting increases. However, it is important to note that for the highest setting, the voltage drop across the coil is only 1.5V, which may not be enough to keep the relay in the ON position.

in conclusion

The power-saving feature of the MAX4822/MAX4824 relay drivers effectively reduces the power consumption of the monostable relay in the ON state. In the example used in this article, the total power consumption was reduced by 47.5%. Tests have shown that the power saving range can be 5.5% to 68.5%, depending on the type of relay used. This power-saving feature also

This helps reduce the size and cost of the relay drive power supply, and is a way to achieve a miniaturized, inexpensive system.