EVAL-ADM1063LFEB AD [Analog Devices], EVAL-ADM1063LFEB Datasheet - Page 21

• Manufacturer Part Number: EVAL-ADM1063LFEB
• Description: Multisupply Supervisor/Sequencer with ADC and Temperature Monitoring
• Manufacturer: AD [Analog Devices]

SUPPLY MARGINING
OVERVIEW
It is often necessary for the system designer to adjust supplies,
either to optimize their level or force them away from nominal
values to characterize the system performance under these
conditions. This is a function typically performed during an
in-circuit test (ICT), such as when the manufacturer wants to
guarantee that a product under test functions correctly at
nominal supplies −10%.
OPEN-LOOP MARGINING
The simplest method of margining a supply is to implement an
open-loop technique. A popular method for this is to switch extra
resistors into the feedback node of a power module, such as a
dc-to-dc converter or low dropout regulator (LDO). The extra
resistor alters the voltage at the feedback or trim node and forces
the output voltage to margin up or down by a certain amount.
The ADM1063 can perform open-loop margining for up to six
supplies. The six on-board voltage DACs (DAC1 to DAC6) can
drive into the feedback pins of the power modules to be margined.
The simplest circuit to implement this function is an attenua-
tion resistor, which connects the DACn pin to the feedback node
of a dc-to-dc converter. When the DACn output voltage is set
equal to the feedback voltage, no current flows in the attenua-
tion resistor, and the dc-to-dc output voltage does not change.
Taking DACn above the feedback voltage forces current into the
feedback node, and the output of the dc-to-dc converter is forced
to fall to compensate for this. The dc-to-dc output can be forced
high by setting the DACn output voltage lower than the feedback
node voltage. The series resistor can be split in two, and the node
between them decoupled with a capacitor to ground. This can help
to decouple any noise picked up from the board. Decoupling to a
ground local to the dc-to-dc converter is recommended.
The ADM1063 can be commanded to margin a supply up or
down over the SMBus by updating the values on the relevant
DAC output.
CONVERTER
DC/DC
FEEDBACK
VIN
OUTPUT
Figure 32. Open-Loop Margining System Using the ADM1063
GND
ATTENUATION
RESISTOR
V
OUT
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
DACOUTn
ADM1063
DAC
µCONTROLLER
CONTROLLER
(SMBus)
DEVICE
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CLOSED-LOOP SUPPLY MARGINING
A much more accurate and comprehensive method of margining
is to implement a closed-loop system. With this technique, the
voltage of a rail is read back so that it can be accurately margined
to the target voltage. The ADM1063 incorporates all the circuits
required to do this, with the 12-bit successive approximation
ADC used to read back the level of the supervised voltages, and
the six voltage output DACs, implemented as described in the
Open-Loop Margining section, used to adjust supply levels.
These circuits can be used along with other intelligence such as
a microcontroller to implement a closed-loop margining system
that allows any dc-to-dc or LDO supply to be set to any voltage,
accurary to within ±0.5% of the target.
To implement closed-loop margining:
1.
2.
3.
4.
5.
6.
7.
Step 1 to Step 3 ensure that when the DACn output buffer is
turned on, it has little effect on the dc-to-dc output. The DAC
output buffer is designed to power up without glitching by first
powering up the buffer to follow the pin voltage. It does not
drive out onto the pin at this time. Once the output buffer is
properly enabled, the buffer input is switched over to the DAC
and the output stage of the buffer is turned on. Output glitching
is negligible.
CONVERTER
DC/DC
FEEDBACK
VIN
Disable the six DACn outputs.
Set the DAC output voltage equal to the voltage on the
feedback node.
Enable the DAC.
Read the voltage at the dc-to-dc output, which is connected
to one of the VP1 to VP4, VH, or VX1 to VX5 pins.
If necessary, modify the DACn output code up or down to
adjust the dc-to-dc output voltage; otherwise, stop because
the target voltage has been reached.
Set the DAC output voltage to a value that alters the supply
output by the required amount (for example, ±5%).
Repeat from Step 4.
OUTPUT
Figure 33. Closed-Loop Margining System Using the ADM1063
GND
R1
R2
ATTENUATION
RESISTOR, R3
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
VH/VPn/VXn
DACOUTn
ADM1063
MUX
ADC
DAC
ADM1063
µCONTROLLER
CONTROLLER
(SMBus)
DEVICE