ADM1066 Analog Devices, ADM1066 Datasheet - Page 22

• Manufacturer Part Number: ADM1066
• Manufacturer: Analog Devices

Specifications of ADM1066

• # Supplies Monitored: 12
• Volt Monitoring Accuracy: 1%
• # Output Drivers: 10
• Fet Drive/enable Output: Both
• Voltage Readback: 12-bit ADC
• Supply Adj/margining: 12-bit ADC+6 DACs
• Package: 40 ld LFCSP ,48 ld TQFP

ADM1066
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 ADM1066 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 ADM1066 can be commanded to margin a supply up or
down over the SMBus by updating the values on the relevant
DAC output.
CONVERTER
DC-TO-DC
FEEDBACK
VIN
OUTPUT
GND
Figure 32. Open-Loop Margining System Using the ADM1066
R1
R2
ATTENUATION
RESISTOR, R3
V
Rev. E | Page 22 of 32
OUT
PCB
TRACE NOISE
DECOUPLING
CAPACITOR
DACx
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 ADM1066 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.
2.
3.
4.
5.
6.
7.
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.
ADM1066
Disable the six DACx 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 converter output that is
connected to one of the VPx, VH, or VXx pins.
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.
Set the DAC output voltage to a value that alters the supply
output by the required amount (for example, ±5%).
Repeat Step 4 through Step 6 until the measured supply
reaches the target voltage.
DAC
MICROCONTROLLER
CONTROLLER
(SMBus)
DEVICE