Datasheet

ADM1062
Rev. C | Page 22 of 36
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 condi-
tions. 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 ADM1062 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 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 ADM1062 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 ADM1062 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
Supply 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 converter or LDO supply to be set to
any voltage, accurate to within ±0.5% of the target.
To implement closed-loop margining
1.
Disable the six DACx outputs.
2.
Set the DAC output voltage equal to the voltage on the
feedback node.
3.
Enable the DAC.
4.
Read the voltage at the dc-to-dc converter output that is
connected to one of the VPx, VH, or VXx pins.
5.
If necessary, modify the DACx output code up or down to
adjust the dc-to-dc converter output voltage. Otherwise,
stop because the target voltage has been reached.
6.
Set the DAC output voltage to a value that alters the supply
output by the required amount (for example, ±5%).
7.
Repeat Step 4 through Step 6 until the measured supply
reaches the target voltage.
Step 1 to Step 3 ensure that when the DACx output buffer is
turned on, it has little effect on the dc-to-dc converter 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.
04433-067
OUTPUT
DC-TO-DC
CONVERTER
FEEDBACK
GND
ATTENUATION
RESISTOR
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
ADM1062
DACx
V
OUT
DAC
MICROCONTROLLER
VIN
DEVICE
CONTROLLER
(SMBus)
Figure 32. Open-Loop Margining System Using the ADM1062