ADM1066 datasheet(21/32 Pages) AD

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ADM1066

Rev. 0 | Page 21 of 32

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, for example,

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/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 ADM1066 can perform open-loop margining for up to six

supplies. The six on-board voltage DACs (DAC1–6) 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/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/dc output voltage does not change.

Taking DACn above the feedback voltage forces current into the

feedback node, and the output of the dc/dc converter is forced

to fall to compensate for this. The dc/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/dc converter is

recommended.

OUTPUT

DC/DC

CONVERTER

FEEDBACK

GND

ATTENUATION

RESISTOR

PCB

TRACE NOISE

DECOUPLING

CAPACITOR

ADM1066

DACOUTn

VOUT

DAC

µCONTROLLER

VIN

DEVICE

CONTROLLER

(SMBus)

Figure 32. Open-Loop Margining System Using the ADM1066

The ADM1066 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 much more accurate and comprehensive method of margin-

ing is to implement a closed-loop system. The voltage on the

rail to be margined can be read back so that the rail can be

accurately margined 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 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 margining system that allows any

dc/dc or LDO supply to be set to any voltage, accurate to within

±0.5% of the target.

OUTPUT

DC/DC

CONVERTER

FEEDBACK

GND

ATTENUATION

RESISTOR, R3

PCB

TRACE NOISE

DECOUPLING

CAPACITOR

VH/VPn/VXn

ADM1066

DACOUTn

MUX

ADC

DAC

DEVICE

CONTROLLER

(SMBus)

µCONTROLLER

VIN

Figure 33. Closed-Loop Margining System Using the ADM1066

To implement closed-loop margining:

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/dc output, which is connected to

one of the VP1–4, VH, or VX1–5 pins.

If necessary, modify the DACn output code up or down to

adjust the dc/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.

Steps 1 to 3 ensure that when the DACn output buffer is turned

on it has little effect on the dc/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.

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