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ADM1166
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 a manufacturer wants to guarantee that a product
under test functions correctly at nominal supplies minus 10%.
OPEN-LOOP SUPPLY MARGINING
The simplest method of margining a supply is to implement an
open-loop technique (see Figure 32). A popular way to do this
is to switch extra resistors into the feedback node of a power
module, such as a dc-to-dc converter or 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 ADM1166 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 attenuation
resistor that connects the DACx pin to the feedback node of a
dc-to-dc converter. When the DACx output voltage is set equal
to the feedback voltage, no current flows into the attenuation
resistor, and the dc-to-dc converter output voltage does not change.
Taking DACx above the feedback voltage forces current into the
feedback node, and the output of the dc-to-dc converter is forced to
VIN
V OUT
fall to compensate for this. The dc-to-dc converter output can
be forced high by setting the DACx output voltage lower than
the feedback node voltage. The series resistor can be split in two,
and the node between them can be 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 ADM1166 can be commanded to margin a supply up or down
over the SMBus by updating the values on the relevant DAC output.
CLOSED-LOOP SUPPLY MARGINING
A more accurate and comprehensive method of margining is to
implement a closed-loop system (see Figure 33). The voltage on
the rail to be margined can be read back to accurately margin the
rail to the target voltage. The ADM1166 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-
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 converter or LDO supply to be set to any voltage,
accurate to within ±0.5% of the target.
MICROCONTROLLER
ADM1166
OUTPUT
DC-TO-DC
CONVERTER
FEEDBACK
R1
ATTENUATION
RESISTOR, R3
DACx
DAC
DEVICE
CONTROLLER
(SMBus)
GND
R2
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
Figure 32. Open-Loop Margining System Using the ADM1166
MICROCONTROLLER
VIN
ADM1166
DC-TO-DC
VH/VPx/VXx
CONVERTER
OUTPUT
ATTENUATION
RESISTOR, R3
MUX
ADC
FEEDBACK
GND
R1
R2
DACx
PCB
TRACE NOISE
DAC
DEVICE
CONTROLLER
(SMBus)
DECOUPLING
CAPACITOR
Figure 33. Closed-Loop Margining System Using the ADM1166
Rev. 0 | Page 22 of 32
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