Example calculation for the power loss

Figure 3: Power loss as a function of the load resistance RLOAD at an output current of 20 mA.

© Analog Devices

In examples 1 and 2, the current output is selected and outputs 20 mA. The load is 250 Ω, the A/D converter is activated in its basic configuration and converts at 20 samples/s.

Example 1 (without PPC):

Output power of the AD74115H = (AUDD = 24V) × 20 mA = 480 mW

Input power of the AD74115H = quiescent current of the AD74115H (206 mW) + power consumption of the A/D converter (30 mW) + 480mW = 716 mW

Input power of the module = 716 mW + power consumption of the ADP1034 (132 mW) = 848 mW

Power consumption of the load = (20 mA)² × 250 Ω = 100 mW

Total power consumption of the module = input power of the module - power consumption of the load = 748 mW

Example 2 shows that the power dissipation of the module drops to 348 mW when the PPC function is activated to bring AUDD to the required voltage of (20 mA × 250 Ω) + 3.6 V reserve = 8.6 V.

Example 2 (PPC activated):

Output power of the AD74115H = (AUDD = 8.6 V) × 20 mA = 172 mW

Input power of the AD74115H = quiescent current of the AD74115H (136 mW) + power consumption of the A/D converter (30 mW) + 172 mW = 338 mW

Input power of the module = 338 mW + power consumption of the ADP1034 (100 mW) = 438mW

Power consumption of the load = (20 mA)² × 250 Ω = 100 mW

Total power consumption of the module = input power of the module - power consumption of the load = 338mW

Table 1: Typical applications for the AD74115H I/O component and corresponding power dissipation with PPC.

© Analog Devices

Figure 2 illustrates the measured power dissipation on the board with the AD74115H at 25 °C. The measurement shows that the power loss is slightly lower than the calculated power loss. This will vary slightly from component to component.

Figure 3 shows the power dissipation curve of the module (ADP1034 and AD74115) with PPC (optimized AUDD was programmed for each load resistance value) over the load resistance. Two different voltages (15 and 24 V) were applied to the UINP connection of the ADP1034 to show the efficiency of the ADP1034. The measurements were carried out at 25 °C.

Figure 4 shows the power dissipation with PPC (optimized AUDD was programmed for each load resistance value) as a function of the load resistance at different temperatures.

Digital output use case

Figure 4: Power loss as a function of the load resistance RLOAD at different temperatures.

© Analog Devices

In industrial applications, the digital output is the application with the highest power consumption. The AD74115H I/O component supports internal and external sourcing and sinking digital outputs. The ADP1034 power management unit can supply sufficient current for the internal digital output function, which can source or sink a continuous current of up to 100 mA. In this case, the supply voltage of the digital output circuit DO_UDD is connected directly to AUDD. The external digital output function is available for currents above 100 mA. It requires an additional power supply connected to DO_UDD.

Timeouts for the internal digital output use case

Figure 5: The system supply is 24 V, the voltage DO_UDD = 24 V.

© Analog Devices

To charge capacitive loads at switch-on, a higher short-circuit current limit (~280 mA) can be activated for a programmable time period T1 while the internal digital output use case is used. A second short-circuit limit value (~140 mA) is used once the T1 time has elapsed. This is a lower current limit which is active for a programmable time period T2.

As the system requires more current during these short-circuit conditions, care must be taken to ensure that the output voltage UOUT1 at the power management unit ADP1034 does not collapse. To avoid voltage dips, a system supply voltage of 24 V should be applied to the ADP1034 if DO_UDD of 24 V is required. This is a typical voltage for a 24 V relay. For a 12 V relay, a minimum system supply voltage (ADP1034 UINP) of 18 V is recommended. This ensures that the system can supply sufficient current to the load.

Fig. 5 and Fig. 6 show the voltage DO_UDD to the short-circuit limit values T1 and T2. This demonstrates how stable the ADP1034 power management unit can deliver high currents.

Data isolation and size

Figure 6: The system supply is 24 V, the voltage DO_UDD = 12V.

© Analog Devices

The ADP1034 power management unit uses Analog Devices' patented iCoupler technology. The component contains three isolated power supply channels, one SPI interface and three isolated GPIO channels in a 7 mm × 9 mm housing. Due to this high integration density, the component requires little space on the PCB and at the same time fulfills the requirements for channel isolation. Power consumption is also reduced.

On the controller side of the ADP1034, the other SPI isolation channels are set to a low-power state when the channels are not in use. This means that the channels are only active when required. The three isolated GPIO channels are used to isolate the RESET, ALERT and ADC_RDY pins of the AD74115H. This fulfills all the isolation requirements of the AD74115H without the additional cost of another isolator IC.

Simplified development at system level

The author: Valerie Hamilton is a Product Applications Engineer at Analog Devices in Ireland

© Analog Devices

Developing an isolated channel-to-channel I/O solution with low power consumption and a small form factor can challenge even experienced developers. The solution presented here, using the ADP1034 and AD74115H devices, offers high integration density and simplifies system-level development. Since a single IC provides three isolated power supplies from a single system supply and integrated data isolation, component costs are significantly reduced. Combined with the flexibility of the AD74115H, the system design meets most industrial I/O applications.