In this video we’ll examine the current consumption of Car infotainment SoC Core
and CPU rails, discuss SoC high current Power supply design and critical output measurements,
and check the CPU thermal throttling behavior at high ambient temperatures.
Hello Everyone! In this video, I’ll explain the design of the low voltage
high current rails for the CPU and GPU of this Mediatek Car infotainment evaluation board.
Let’s look at the power requirements of the Mediatek SoC.
The MT2712 SoC is available in different SKU’s: The higher power version has
6 high frequency processors. Lower power versions have less processors and run at lower frequency.
There are 3 high current rails: The Core rail for the GPU and video processing, the B_CPU
rail for the ARM CORTEX A72 processors and the LCPU rail for the ARM CORTEX A35 processors.
The current consumption of the processors in the SoC is to a large extent caused by the charging
and discharging of parasitic capacitances of the numerous switches. The higher the
switching frequency and the more switches are active, the higher the current consumption.
Here I measure the current in the MT2712 EVB CORE rail: As you can see the green current
waveform is very dynamic and has very short fast current pulses especially when screen is dynamic.
The PMIC needs to ensure the CORE voltage stays within operation tolerance window,
so tight regulation during load transients is very important.
For powering these high current rails with fast changing loads, we use the RTQ2134 PMIC.
The basic building block of this IC is a low voltage ACOT Buck converter,
which runs at 2.1MHz, has 5A current capability with remote output voltage and ground sense
and has I2C control. One of the key points of this converter is the very fast control loop:
It uses Richtek’s Advanced Constant On Time ACOT topology which includes PSR ramp for stability
and a frequency Locked loop for constant average switching frequency.
During Steady state switching, the error amplifier output voltage is compared to
a PSR ramp in order to stabilize the loop when using low ESR MLCC output capacitors.
The Frequency Locked Loop slowly adjusts the ON time to keep the switching frequency within the
2.1MHz design tolerance range. During fast rising load steps, the ON time stays fixed
and the control loop quickly reduces the OFF time. The resulting burst of ON time pulses will quickly
increase the inductor current to match the new load current value. This fast step load response
will minimize the output voltage sag and soar during fast changing loads, which is
ideal for processor power supply applications. The RTQ2134 PMIC integrates four of these
5A ACOT buck converters. The input voltage can range from 3.3V ~ 6V and the output voltage can be
programmed from 0.3V to 1.85V with high accuracy. There are three configurations versions:
2+1+1 where two 5A channels are combined to one 10A channel.2+2, where the four 5A channels are
combined into two 10A channels.4 + 0, where the four 5A channels are combined into one
20A channel. Lower power SoC versions like the MT2712H can be powered by a single RTQ2134 PMIC:
in this case the 2+2 phase version is used to power the CORE and BCPU/LCPU
rails. The SoC controls the buck converter via the I2C bus. The high power MT2712S SKU needs
the most current and is powered via two RTQ2134 PMICs: Two Dual Phase rails for CORE and BCPU
and one single phase rail for the LCPU. This configuration is implemented on the MT2712 EVB.
Let’s test the performance of the RTQ2134 PMIC by means of the RTQ2134 EVB. We will check the
combined 2-phase 10A channel. I’ll use the Richtek Wrenboard to control the I2C bus of the PMIC.
For testing the load transient capability, I’ll use the Richtek
Fast Load Transient Tool as dynamic load and a variable resistor as static load. I’ll
measure the converter load current, the output voltage, and the two phase switching waveforms.
The input voltage is 5V.
After switch-on, I’ll connect the Wrenboard to the PC and start the RTQ2134 GUI program.
The default start-up conditions are defined by the OTP programming of the RTQ2134 version.
In this version, the default start-up condition is 1.0V
output and Pulse Skip mode for enhanced efficiency at light load.
You can see that only one phase is active, and operates in pulse skip mode. When I increase
the static load, you can see that the frequency of the active phase increases. Above 0.6A load,
it operates in CCM continuous conduction mode at 2.1MHz. Above 2.8A, the other phase
also becomes active, to increase the current capability and improve load transient response.
When we zoom in, you can see that the two phases operate 180 degrees out of phase, which reduces
input and output ripple and EMI. When the load reduces, the 2nd phase is switched off again,
and at light load the converter goes in PSM mode. When I now activate the load transient tool with
load step from 1.5A to 5.8A, you can see that Phase 1 is always active, but
phase 2 only becomes active during the load pulse. The output voltage shows a large voltage dip,
because it takes time to activate the 2nd phase. For these kind of situations,
it is better to force the 2 phases active over the entire load range.
Here you see the RTQ2134 GUI tool,
and in the lower window it shows that I2C connection is successful.
The GUI lets you adjust many parameters like output voltage setting, DVS slewrate,
Softstart slewrate. Let’s set the converter operation mode from Auto PSM mode into Force
Continuous Conduction mode. As you can see the two phases are now both active all the time.
I now reduce the pulse load duty cycle to show the pulse load response more clearly. But you can see
that the pulse current rising slewrate is a bit slow. This is caused by the inductance of the
load transient tool lead wires. For ultra fast load step slew-rate, it is better to reduce the
inductance, by using thick wide conductors between load transient tool and the buck converter output.
You can see that after using the low inductance wires, the load step is much faster:
The rising edge rise time is now around 800nsec, which resembles the SoC load speed.
When we make a single shot measurement, you can clearly see the ACOT loop increasing pulse
frequency during the rising current step, and reducing the pulses during the falling load step.
The voltage sag during this fast load step is around 30mV
and the voltage soar is around 40mV, which is quite OK for SoC supply.
Voltage sag and voltage soar of ACOT converters during load steps is related to the load step
amplitude, input and output voltage, inductor value and output capacitance and the maximum
and minimum duty-cycles that the converter is able to reach during the load transitions.
Reducing the inductor value or increasing the output capacitance can reduce the voltage sag
and soar. Dual phase operation effectively halves the total inductor value, so dual phase operation
will reduce the voltage sag and soar by a factor 2 compared to single phase operation.
The SoC can dynamically change the Supply voltage during operation by means of the I2C controlled
Digital Voltage Scaling function with 5mV resolution. Lets change the voltage from 1.0V
to 1.2V: You can see that the voltage rises with a certain slewrate, which was defined by the RSP
UP setting, which was in this case 16mV/usec. The falling slewrate RSPDown setting is 4mV/usec here,
so when stepping from 1.2V back to 1.0V, you can see the voltage drops much slower.
Another cause for current consumption in SoCs is the leakage current of each switch. This
leakage current rises exponentially with temperature. In automotive environment,
high ambient temperatures should also be considered during design stage,
as a car internal temperature can become very high when the car is out in the sun during summer time.
Let’s have a look at the current consumption of one of the processor rails of the MT2712:
I have mounted a thermocouple to the MT2712 SoC and measure the average current via a
1mohm sense resistor in the B_CPU rail, so 1mV equals 1A. I’m running the ARM CORTEX A72 CPUs
with a fixed stress test load. Initially the average current is around 3.5A.
When I now blow hot air on the SoC, you can see that the average current slowly increases.
The curve does show an exponential behavior,
but at 85 degrees C there is a sudden drop in current. This is because the SoC has an internal
die temperature sensor, and it will reduce the processor clock frequency at higher temperatures,
thereby reducing SoC current consumption to avoid excessive SoC dissipation.
At high ambient temperatures, the RTQ2134 PMIC could also overheat due to the higher SoC load
and higher PCB temperature. For this reason, the RTQ2134 also includes a temperature sensor.
Let’s test this by heating up the RTQ2134 IC with hot air. We will now set the GUI in continuous
I2C register reading mode. At a 110 degrees die temperature, an interrupt will be set to tell the
SoC to read the HOT DIE register value which as you can see has changed from zero to one. The SoC
can now take measures to reduce the loading current by reducing clock frequency as well.
I hope you now have a better understanding of electronic power design for automotive
applications. Please stay tuned for more interesting videos at Richtek, your Power Partner.