Apollo MxFE Technology

Abstract

Apollo MxFE Technology | Qualifying products for aerospace or defense can be challenging. Specific procedures must be followed to guarantee zero failures during qualification and proper power supply to a mixed-signal front end. Adherence to these procedures significantly reduces the likelihood of failures during the qualification process. One use case is the Apollo MxFE™ device, which is one of the most advanced products designed by ADI. Many challenges arise due to the multiple power domains for Apollo MxFE technology. Some of the power domains are in the negative voltage region. Others need an excess current of more than 17 A. All the power domains need appropriate power turn-on sequencing while fed to an Apollo MxFE device. The Sonoma system on which the Apollo MxFE system is qualified has no –1.0 V domain. This article discusses how the –1.0 V voltage is generated from a positive voltage power supply domain. While an analog-to-digital converter (ADC) cannot measure a negative voltage, this article discusses how the negative voltage is converted to a positive voltage so that the ADC can measure it.

Introduction

The Apollo MxFE platform has 11 power domains, including a –1.0 V supply. To ensure that an Apollo MxFE device is booted properly during qualification, it is imperative that the power sequencing is done correctly both during the turn-on of the device and during the turn-off time. The Apollo MxFE system draws close to 17 A on the 0.8 V digital domain. To supply 18 A of current separately to each of the eight Apollo MxFE devices on a single board is a challenge. Designing the boards is difficult due to the power domain’s complexity and layout. This article outlines how to overcome each challenge and how the qualification for Apollo MxFE technology is being run successfully.

Currently, the Sonoma1 system is used to qualify Apollo MxFE devices. However, the Sonoma system does not have a negative power rail. Apollo MxFE devices use a –1.0 V supply. The qualification board uses Analog Devices’ ADP5074, a high performance DC-to-DC inverting regulator that generates negative supply rails and can source –1.0 V of supply. The input voltage ranges from 2.85 V to 15 V, which supports various applications. The integrated main switch enables the generation of an adjustable negative output voltage down to around 39 V below the input voltage. However, in the case of Apollo MxFE devices, it only needs to downconvert to a –1.0 V supply.

Figure 1 shows a schematic capture of the ADP5074. The 12 V of supply is part of the Sonoma system, and the output is –1.0 V.

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Thus, the boundary conditions of the device are met. Sourcing 12 V of power from the Sonoma system, and using the configuration shown, can generate –1.0 V of supply for the Apollo MxFE device with the correct current rating.

Figure 1. Schematic of the ADP5074 to generate –1.0 V of supply going to the Apollo MxFE device; the input voltage is at 12 V, derived from the Sonoma system.

For the ADP5074, the VOUT value is calculated by the formula in Equation 1.
Where VOUT = the negative voltage output at the OUT pin VFB = Feedback reference voltage
RFT = R2 is the feedback resistor from VOUT to FB, the feedback pin
RFB = R1 is the feedback resistor from the feedback pin, FB to the VREF pin or voltage reference
VREF = the reference voltage at the VREF pin
The desired values for R1 and R2 to get the necessary VOUT are given in Table 1.

Table 1. Value for R1 and R2 to Give a Desired Output Voltage (V)

The simulation file (Figure 2) shows –1.0 V at the output or OUT.

Thus, one of the main problems of generating a –1.0 V supply for the Apollo MxFE device is solved. Note that the ADP5074 can generate 600 mA of current, as per Figure 3. The Apollo MxFE front end requires 300 mA from the –1.0 V supply, so having a DC-to-DC converter with double the current capacity is ideal.

Figure 2. Simulation files showing that the ADP5074 reaches –1.0 V within 4 ms.
Figure 3. Maximum output current fSW = 1.2 MHz, TA = 25°C, based on target of 70% ILIM (MIN).

Figure 3 shows that the ADP5074 can generate 800 mA of current for a 12 V input if an inductor value of 5.6 μH is used.

Another challenge is measuring the voltage from a supply that gives –1.0 V. The voltage and current are measured in situ during qualification. However, the issue is that an analog-to-digital converter (ADC) cannot measure a negative voltage. The only way to measure a negative voltage is to convert it to a positive voltage, and then the ADC can measure it. The question is, how can a negative voltage be converted to a positive supply? This can be done by using the AD8671.

The AD8671 is an operational amplifier that combines low noise, wide bandwidth, and high precision (Figure 4). It is an excellent choice for applications requiring alternating current (AC) and precision direct current (DC) performance. For Apollo MxFE devices, both DC and AC performance are important.

Figure 4. Schematic showing how to generate 1.0 V supply from a –1.0 V supply using the AD8671.

The AD8671 can convert a –1.0 V supply to a 1 V supply, as shown in the simulation file in Figure 5.

Figure 5. Simulation file showing 1.0 V generated at the output of the AD8671.

For the operational amplifier to work, at least –3.5 V is needed to drive it. However, the Sonoma system has no negative supply, so where do we get the negative (–3.5 V) supply? To solve this problem, ADI’s MAX889 can be used—a charge pump that can generate –3.5 V from a 5 V supply and that is available on the Sonoma system. The Sonoma system supplies the 5 V supply and can source up to 1 A of current. Since there are eight sites for the device under test (DUT), this 5 V can quickly source a current of 150 mA for each of the sites.

The MAX889 is a higher frequency regulated 200 mA inverting charge pump. It operates with inputs from 2.7 V to 5.5 V and produces an adjustable regulated output voltage from –2.5 V to -VIN.

Here VOUT = –VREF × R2/R1
VREF = VIN = 5 V
R3 = R2 is the feedback resistor of 140 kΩ from feedback pin 7 to output pin 5
R3 = 140 kΩ

Figure 6 illustrates the above in terms of R2 and R3 values.

Figure 6. The MAX889 generates –3.3 V from a 5 V positive supply.

Now, the –1.0 V can be converted to a 1 V supply, which the ADC can measure.
With a 12-bit ADC in the microcontroller, the ADC converts it into
5/4096 = 1.2 mV per unit.
So, the voltage measured by the 12-bit microcontroller is:

Then, multiply by –1 in the MAX889SEVKIT software to give the negative voltage that the Apollo MxFE device sees on its –1.0 V domain.

The value is read by connecting it to the A0 pin in the microcontroller, and, in the software, one may write the following:

Thus, the voltage that goes to the Analog MxFE front end can be read. So far, the solution is presented as how to measure –1.0 V. The preceding methodology effectively explains how –1.0 V can be measured continuously when running the qualification on an Apollo MxFE device.

Next, the focus is on how to measure the current that an Apollo MxFE device draws when –1.0 V is supplied. It is crucial to monitor the current during in-situ monitoring to detect overcurrent conditions. Prompt detection allows for the removal of the unit for further analysis, preventing potential damage to other blocks in the Apollo MxFE front end during the aging process.

To measure the current from the –1.0 V domain going to the Apollo MxFE device, one of the power traces must pass through a precision shunt resistor of 0.1 Ω. The two sides of the shunt resistor are then fed to an operational amplifier (preferably an instrumentation amplifier) with a gain of 20 V/V. The output is fed to an ADC, which can then precisely measure the current. One such instrumentation amplifier is the AD8418A, a high voltage, high resolution current shunt amplifier with a gain of 20 V/V in unidirectional operation.

Figure 7. Operational amplifier schematic to measure the current.

The output at VOUT is a representative of the current and is shown in Figure 8.

Figure 8. This output signal can be fed to a 12-bit or 18-bit ADC, and the current can be measured.

If the signal is given as shown in Figure 9, and the shunt resistor is being fed the same signal, then the output is as shown, and the ADC can measure the output voltage and translate it to the current being drawn.

Figure 9. Connecting the instrumentation amplifier.

One of the other requirements for an Apollo MxFE device is that the –1.0 be turned on last among the 11 power supplies. One way to do this is to use the enable pin and attach a resistor and capacitor, which induces a 2 ms lag time when the –1.0 V is supplied.

Figure 10. R5 and C6 can introduce a 4 ms delay before the supply is given to the Apollo MxFE device.

Figure 10 shows R5 connected to VIN and C6 applied across it. This induces a delay of 4 ms before –1.0 V is given to the Apollo MxFE device, as shown in the simulation file in Figure 11.

Figure 11. VOUT is at –1.0 V after a delay of 4 ms.

Filtering circuit for the –1.0 V converter: To ensure proper filtering is done at the output of the –1.0 V converter, a pi filter is added to ensure the voltage ripple is less than 20 mV for sensitive devices like Apollo MxFE devices during the qualification process.

The LC filter is added to the output of the –1.0 V DC-to-DC converter to reduce the ripple. The cutoff frequency (Fc) is set at 58 kHz since the converter’s switching frequency is 580 kHz. The LC filter is shown in Figure 12.

Figure 12. LC or pi filter for the –1.0 V regulator to reduce the ripple to less than 20 mV peak to peak.

It is essential to select the correct values during component selection. For L, the values are as follows: L1 is rated to 330 nH and can manage up to 800 mA, and RDC is at 160 Ω. Why is component selection so crucial? Since a DC-to-DC converter can give 400 mA to Apollo MxFE devices, the inductor’s maximum current capacity needs to be rated to 800 mA, or the inductor will get very hot or may get damaged. The capacitors are rated up to 4 V so that even if there is a voltage transient above 1 V, the capacitor in the pi filter is protected, and adequate protection is provided against ripples in the voltage coming out of the DC-to-DC converter.

The 12 V supply to the –1.0 V DC-to-DC converter comes from the Sonoma system. However, it is unknown how protected the 12 V supply is. The Sonoma system has a 110 V main converted to 48 V using a transformer. The 12 V is extracted from this 48 V. To ensure there is no surge in the 12 V during the average high temperature operating life (HTOL) process during qualification, a TVS diode is added to the 12 V supply to ensure the voltage is clamped to 12 V. This way, the input boundary condition for the ADP5074 is met, which downconverts the 12 V input signal to –1.0 V.

Conclusion

The Apollo MxFE device is an innovative technology with 11 power domains. One such power domain is the -1 V supply. In the Sonoma system where the Apollo MxFE device is being qualified, the system has no negative power supply. This article presents how to generate a –1.0 V supply from a 12 V positive supply. It is imperative to monitor the voltage and current for the –1.0 V supply. A conventional 12-bit ADC cannot measure a negative voltage. A novel methodology showed how to convert a negative voltage to a positive voltage, which an ADC can measure. Also, the inverting operational amplifier, which converts a negative voltage to a positive voltage, needs a negative supply. A charge pump integrated circuit chip is used to overcome this, which can generate the required –3.5 V supply from a 5 V supply. Despite the various technical challenges presented by the Sonoma system, –1.0 V is generated and monitored for voltage and current.

Reference

1Sonoma Test System. AEHR Test Systems.

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