ADM1069_15 datasheet(21/32 Pages) AD

Part #	ADM1069

Description	Super Sequencer with Margining Control

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ADM1069

Rev. C | 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 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 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 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 ADM1069 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 ADM1069 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 four 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.

OUTPUT

DC-TO-DC

CONVERTER

FEEDBACK

GND

ATTENUATION

RESISTOR, R3

PCB

TRACE NOISE

DECOUPLING

CAPACITOR

ADM1069

DACx

VOUT

DAC

MICROCONTROLLER

VIN

DEVICE

CONTROLLER

(SMBus)

R1

R2

Figure 32. Open-Loop Margining System Using the ADM1069

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