adm1069ast-reel Analog Devices, Inc., adm1069ast-reel Datasheet - Page 23

• Manufacturer Part Number: adm1069ast-reel
• Description: Super Sequencer With Margining Control
• Manufacturer: Analog Devices, Inc.

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 the manufacturer wants to guarantee
that the product under test functions correctly at nominal
supplies minus 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 ADM1069 can perform open-loop margining for up to four
supplies. The four on-board voltage DACs (DAC1 to DAC4)
can drive into the feedback pins of the power modules to be
margined. The simplest circuit to implement this function is an
attenuation 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 attenuation 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 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 ADM1069 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 31. Open-Loop Margining System Using the ADM1069
GND
ATTENUATION
RESISTOR
V
OUT
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
DACOUTn
ADM1069
DAC
μCONTROLLER
CONTROLLER
(SMBus)
DEVICE
Rev. A | Page 23 of 36
CLOSED-LOOP SUPPLY MARGINING
A more accurate and comprehensive method of margining is to
implement a closed-loop system. The voltage on the rail to be
margined can be read back to accurately margin the rail to the
target voltage. The ADM1069 incorporates all the circuits
required to do this, with its 12-bit successive approximation
ADC used to read back the level of supervised voltages, and its
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 some other intelligence
such as a microcontroller to implement a closed-loop margin-
ing 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.
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 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.
CONVERTER
DC/DC
FEEDBACK
VIN
Disable the four 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 that is connected to
one of the VPn, VH, or VXn 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 32. Closed-Loop Margining System Using the ADM1069
GND
ATTENUATION
RESISTOR
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
VH/VPn/VXn
DACOUTn
ADM1069
MUX
ADC
DAC
ADM1069
μCONTROLLER
CONTROLLER
(SMBus)
DEVICE