Texas Instruments | DACx3401 Delivers Holistic Solution for Power-Supply Margining and AVS | Application notes | Texas Instruments DACx3401 Delivers Holistic Solution for Power-Supply Margining and AVS Application notes

Texas Instruments DACx3401 Delivers Holistic Solution for Power-Supply Margining and AVS Application notes
Uttama Sahu, Anand Warrier
DACx3401 Delivers Holistic Solution for Power-supply
Margining and AVS
Power-supply margining and control and AVS circuits
require good accuracy, smooth step-response, design
simplicity, high-density, standard interfaces, and
robustness. The DACx3401 family of devices provide
a competitive feature set specific to this application
while keeping the total cost of the design very low.
L
VIN
IN
PH
VOUT
BOO T
SMPS
CB
SENSE
VDD
R1
CL
R3
DACx340 1
VFB
GND
R2
Introduction
L
VIN
IN
PH
SMPS
SENSE
R3
DPOT
VFB
GND
R2
Figure 2. Power-supply Control Using DPOT
L
VIN
IN
VOUT
PH
SMPS
VREF
BOO T
CL
CB
SENSE
R1
R3
RFIL TE R
R2
CFIL TE R
VIO
VREF
PWM
source
RDIV
VFB
GND
VREF
RDIV (VREF /2)
Figure 3. Power-supply Control Using PWM
L
VIN
IN
PH
SMPS
VOUT
BOO T
CB
SENSE
GND
Table 1 lists the key requirements of power-supply
control.
CL
VDD
R1
Curren t-output
DAC
VFB
R2
RSET
Figure 4. Power-supply Control Using IDAC
PARAMETER
VALUE
Accuracy
±1% to ±5%
Start-up mode for control device
Hi-Z
Step response
Slew-rate control
Using a DPOT
Interfaces
I2C, PMBus™
Others
Small-size, robustness, low-cost
SLAA887 – August 2019
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R1
CL
CB
Key Requirements of Power-supply Control
Table 1. Key Requirements
VOUT
BOO T
±
Power supplies, such as low-dropout regulators (LDO)
and DC-DC switch-mode power supplies (SMPS)
provide a feedback or sense input that is used to set
the desired output. This pin is typically used to sense
the output voltage using a resistive divider. This
feedback network can be made programmable using
either a voltage-output digital-to-analog converter
(DAC), a current-output DAC (IDAC), a digital
potentiometer (DPOT), or a pulse-width modulation
(PWM). The DACx3401 family of DACs provide a rich
feature set to address key requirements of powersupply control circuits.
Figure 1. Power-supply Control Using the
DACx3401
+
A power-supply margining or control circuit is used for
testing the accuracy of a system at the boundary
conditions of the power supply, adjusting the offset
and drift of the power-supply output, or programing the
power-supply output to different values, depending on
the system requirement. High-end processors and
computing systems often need to dynamically scale
the voltages for power management. This is referred to
as adaptive voltage scaling (AVS). Typical applications
of power-supply control and AVS include test and
measurement, communications equipment, enterprise
systems, and general-purpose power-supply modules.
A simple circuit for power-supply control can be
achieved using a DPOT in the feedback path of the
power supply as shown in Figure 2. The series
resistor, R3, protects the power-supply from going in to
saturation when the DPOT goes to 0 Ω in a failure
condition. DPOTs have an initial accuracy of around
±20% and code-to-code glitch much higher than that of
DACx3401 Delivers Holistic Solution for Power-supply Margining and AVS
Copyright © 2019, Texas Instruments Incorporated
1
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a DAC. Because of this, a DPOT-based design suffers
both the control accuracy and the step-response. Also,
DPOTs do not have a Hi-Z output mode. These
drawbacks limit the use of DPOTs in these
applications.
Using PWM
Although PWMs are easier to implement in software,
substantial analog signal conditioning is required in
order to meet these application requirements. PWMs
get generated in the digital domain and the high and
low levels are defined by the IO voltage of the
processor. These IO supplies typically have an
accuracy of around ±10% excluding the noise and
ripple from the power supply. In order to meet the
accuracy and filter the noise and ripple, you need a
comparator circuit driven by a precision reference as
shown in Figure 3. In addition, the PWM switching
frequency must be filtered before feeding it to the
power-supply. A complete removal of the switching
noise needs a high-order active filter. Figure 5 shows a
response with an RC filter. Note that the filtered PWM
output does not completely follow the commanded
pattern. Finally, the op amp must have a shutdown pin
to power up to Hi-Z. Thus, a PWM approach, even
though it is simpler in software, requires significant
signal conditioning.
reference with ±1% initial accuracy and the DAC has a
total unadjusted error (TUE) of ±0.7%. It can work with
both I2C and PMBus, enabling seamless integration
with existing power-management software. Due to the
Hi-Z power-down mode, the DACx3401 can be
powered down whenever it is not in use, without
impacting the nominal functionality of the power
supply. When the DAC is powered up with the output
voltage equal to VFB, the nominal condition remains
undisturbed. The DAC can then be programmed for
margin-high or margin-low using specific commands
either through I2C or PMBus. As the current from the
DAC is defined by both the DAC voltage and the value
of the resistor R3, the safe current range and accuracy
can be easily selected.
Using an IDAC
Another way of controlling the power supply is by
sourcing or sinking current into the feedback pin using
a current-output DAC (IDAC). Figure 4 shows a powersupply control circuit using an IDAC. Most IDAC
implementations do not provide any protection against
a failure mode when the DAC pulls the feedback node
down either due to a design bug or due to a system
failure. Pulling the VFB node down pushes the power
supply output to saturation, thereby damaging the load
circuits. Some IDACs provide programmable clamping
of the current output, but they do not take care of all
failure conditions. For example, when an IDAC has a
current output range of ±50 µA and the application
requirement is ±10 µA, the only way to protect the
circuit is by changing the resistors in the feedback
network. Assuming a ±10% margining design, the
current through the feedback resistors now needs to
be 500 µA, which otherwise would have been only 100
µA. With this approach, additional current is consumed
in the system, increasing the power budget.
Power-supply Control Using the DACx3401
The DACx3401 family of voltage-output DACs are
equipped to address all the requirements of powersupply control. Figure 1 shows a power-supply control
circuit using the DACx3401. The DACx3401 has a HiZ power-down mode that is selected by default. It has
programmable slew-rate control that varies from a few
microseconds to a couple of milliseconds. The
programmable slew-rate helps achieve a desired stepresponse in the design. The DACx3401 has an internal
2
Figure 5. Responses with PWM-based and
DACx3401-based Margining
The DACx3401 Family of DACs
The 10-bit DAC53401 and 8-bit DAC43401
(DACx3401) are pin-compatible families of buffered
voltage-output DACs. These devices have NVM,
internal reference, and PMBus-compatible I2C
interface. The DACx3401 operates with either an
internal reference or the power supply as a reference,
and provides full-scale output of 1.8 V to 5.5 V. These
devices communicate through the I2C interface. These
devices support I2C standard mode (100 kbps), fast
mode (400 kbps), and fast mode plus (1 Mbps). The
DACx3401 is available in a tiny 2x2 package.
Conclusion
The DACx3401 is a family of DACs that have been
designed with a feature set specifically targeted for
power-supply margining and control, and AVS. It
delivers highly differentiating performance as
compared to the alternative technologies and
competitive voltage-output DACs. With the tiny
package and ultra-low price point, this DAC is the best
choice for precision, high-density, and cost-sensitive
applications.
DACx3401 Delivers Holistic Solution for Power-supply Margining and AVS
Copyright © 2019, Texas Instruments Incorporated
SLAA887 – August 2019
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