Texas Instruments | Octal Programmable Comparator Using DAC53608 | Application notes | Texas Instruments Octal Programmable Comparator Using DAC53608 Application notes

Texas Instruments Octal Programmable Comparator Using DAC53608 Application notes
Power Supply Supervision Using Programmable Window
Comparators With DACx3608
Protection circuits are a necessary overhead in
products designed for robustness. Achieving such
protection with least amount of trade-off is always
the goal of a product developer. The DACx3608
family of DACs offer a solution to one such
challenge of circuit protection by enabling efficient
power supply voltage and current supervision.
___________________________________________
Power supply supervision is a circuit that is present in
almost all applications that need power supply rails
shared across boards and modules. This circuit is
often seen as an overhead as it ensures the protection
of various components in case of a fault, rather than
directly aiding the application. Therefore, implementing
a robust power supply supervision circuit in a cost
effective way is a challenge for any designer. The
application space for this circuit includes
Communications equipment, Battery Test System,
Automated Test Equipment, and others. In this
application note, we will discuss methods of
implementing a robust power supply supervision circuit
using precision DACs.
Power Supply Supervision with Precision DACs
Figure 1 provides a high-level block diagram of how
power supply voltage and current supervision is
implemented. A precision DAC sets the threshold
voltages for a window comparator and the measured
voltage or current is compared with these thresholds.
The comparator subsequently triggers a processor in
case the measured values move outside the
programmed band. We will focus on only voltage
supervision in the rest of the document.
+
DAC
+
DAC
1.2V_CURRENT_ALM
±
±
+
±
+
±
+
±
+
DAC
5V_RAIL
3.3V_RAIL
1.8V
1.8V_RAIL
1.2V
1.2V_RAIL
+
GND
5V_ALA RM
DAC
±
+
3.3V_ALA RM
DAC
±
+
1.8V_ALA RM
DAC
VIO
VCC
DAC536 08
DAC536 08
RP
VTH-HI
+
VALAR M-HI
VTH-LO
±
RA
VIO
VIN
RB
RP
GND
+
VALAR M-LO
±
GND
Figure 2. Window Comparator for Control Loop
Supervision for Fault Indication (Open Loop)
1.8V_CURRENT_ALM
±
+
Power Sup ply
3.3V_CURRENT_ALM
±
+
3.3V
A common approach towards power supply
supervision is to detect the direction of the failure so
that the power supply source can be regulated
accordingly. This topology requires two trigger outputs
from every supervisory circuit as shown in Figure 2.
Two DAC channels are used to generate the high- and
low-threshold voltages independently. The resistors RA
and RB bring the nominal value of monitored voltage
(VIN) in the range of the DAC. Usually open-drain
comparators are preferred in order to generate trigger
signals at the IO voltage level of the processing circuit.
When the attenuated input voltage increases beyond
VTH-HI, the output VALARM-HI goes LOW. VALARM-LO goes
LOW in a similar manner when the voltage decreases
below VTH-LO. The outputs are pulled HIGH otherwise.
Figure 3 shows the waveforms at different nodes of
this circuit. TLV1701 dual comparator was used for
simulating the circuit.
±
DAC
5V
5V_CURRENT_ALM
Power Supply Supervision Inside A Control Loop
The circuit in Figure 2 is very useful but requires two
trigger pins per monitoring channel. In applications
where only fault indication is required, this circuit can
be further simplified. Figure 4 shows a method to
generate a single trigger output by combining the
open-drain comparator outputs. This trigger output
goes LOW in case at least one of the comparator
output is LOW. Note that this circuit cannot be used
inside a control loop as the output only conveys a fault
condition, not the type of fault.
±
+
1.2V_ALA RM
DAC
±
Figure 1. Block Diagram of Power Supply
Supervision
SLAA854 – June 2018
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Power Supply Supervision Using Programmable Window Comparators With
DACx3608
Copyright © 2018, Texas Instruments Incorporated
1
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DAC is required only for programming the nominal
voltage. For such applications, the number of DAC
channels required can be reduced by half. Figure 6
depicts such a topology. The DAC sets the highthreshold voltage while the low-threshold voltage is
defined by the resistor ratio as shown by the equation.
VIO
VCC
DAC536 08
RP
VTH-HI
+
VALAR M
±
Figure 3. Dual Output Waveform
R1
Figure 5 shows the corresponding waveform for the
fault indication circuit. It can be seen that trigger output
is LOW whenever there is a fault.
§ R2 ·
VTH-LO =VTH-HI × ¨
¸
© R1 +R 2 ¹
VTH-LO
RA
VIN
R2
RB
GND
+
±
VIO
GND
GND
VCC
DAC536 08
DAC536 08
Figure 6. Resource Optimized Fault Indication
RP
VTH-HI
+
VALAR M
VTH-LO
±
RA
VIN
RB
GND
+
±
GND
Figure 4. Window Comparator for Fault Indication
DACx3608 Family of Precision DACs
The DACx3608 family of DACs is an 8-channel
buffered voltage output DAC with a tiny 3x3 QFN
package. It has a single supply operation and comes
in 8-bit and 10-bit pin-compatible versions. The DAC
provides an I2C interface whose device address can
be configured to four different values using a single
hardware pin. This will allow use of 32 channels
without using any I2C buffer. All these features
combined with tiny footprint makes DAC53608 an
excellent choice for power supply supervision.
Discussion
Power supply supervision is a necessary overhead in
many applications. In this application note, we
discussed how this overhead circuit can be made
simpler and robust using a precision DAC53608.
When used with the circuits described above,
DAC53608 family can provide both closed-loop and
open-loop circuit functionality for both voltage and
current supervision.
Figure 5. Fault Indication Waveform
Resource Optimized Open Loop Supervision
It can be observed in circuits shown in Figure 2 and
Figure 4 that two DAC channels are required for every
monitoring channel in order to provide full
programmability over the threshold voltage levels.
However, in some applications the ratio between the
high- and low-threshold voltages can be fixed and a
2
Related End Equipment
•
•
•
•
Communication Equipment
Enterprise Systems
Memory and Semiconductor Test
Battery Test
Power Supply Supervision Using Programmable Window Comparators With
DACx3608
Copyright © 2018, Texas Instruments Incorporated
SLAA854 – June 2018
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