ADM1266ACPZ-R7 datasheet(20/62 Pages) AD

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ADM1266

Data Sheet

Rev. A | Page 20 of 62

SUPPLY MARGINING

OVERVIEW

Due to tolerances of circuit components, input voltage ranges,

and variations in reference voltages, load, and temperature, for

example, the output voltage of the dc-to-dc converter deviates

from the nominal setpoint value. The worst case conditions

need to be simulated on the power supply during manufacturing

and production, and the corner conditions can be measured to

check for an out-of-limit condition. Additionally, the accuracy

of the output voltage is also a critical factor for some applications

and must be tightly maintained when the tolerance of the output

voltage resistive divider is large (see Figure 12).

The procedure of ensuring this output voltage regulation is

called margining (or voltage margining). This voltage margining is

accomplished by the use of an on-chip DAC that pulls up/down

the feedback node of the error amplifier of the power controller.

A typical application circuit for margining is shown in Figure 12.

Using nodal analysis and basic circuit theory, modifying the

feedback node changes the output voltage and, typically, there is

an inversely proportional relationship between the output of the

DAC and the output voltage.

Because the ADM1266 has nine internal DACs, margining is

possible on nine rails.

OUTPUT

DC-TO-DC

CONVERTER

FEEDBACK

GND

ATTENUATION

RESISTOR

VHx/VPx

ADM1266

DACx

MUX

ADC

DAC

CONTROL

BLOCK

Figure 12. Typical Application Circuit for Margining

Margining can be performed two ways: open-loop margining

and closed-loop margining.

The margining is actuated by a DAC and a series resistor that are

connected to the feedback node of the power supply controller

(see Figure 12). The equivalent change in output voltage can be

determined by the following equations:

OUT

DAC

−

(1)

OUT

(2)

Subtracting the two equations yields

(

)

DAC

OUT

−

∆

Table 8. DAC_CODE_CONFIGURATION[3:1], Register 0xEB,

DAC Ranges

Bits[3:1]

Midcode

Voltage (V)

Minimum

Voltage

Output (V)

Maximum

Voltage

Output (V)

0x00= 3’b000

0.506

0.202

0.808

0x01= 3’b001

0.607

0.303

0.909

0x02= 3’b010

0.809

0.505

1.111

0x03= 3’b011

1.011

0.707

1.313

0x04= 3’b100

1.263

0.959

1.565

Open-Loop Margining

In open-loop margining, the user has direct access to the

internal DACs. The DAC forces a voltage on the feedback node

of the power controller, which causes a deviation in the output

voltage. Typical values for this test are ±1%, ±2.5%, ±5%, ±7.5%,

and ±10% of the nominal output voltage. The user can program

up to 16 preset values, and can use a pointer command to instruct

the device regarding the value that must be loaded into the DAC.

Both the preset values and the value of the pointer can be saved

into the memory. At power-up, the device downloads the

settings and configures the DAC automatically.

Closed-Loop Margining

Closed-loop margining is the preferred method of margining. It

determines the ability of the power supply to regulate the output

under extreme corner conditions. It is recommended to use the

Power Studio software because it provides all related calculations

for resistors and parameters for this feature.

The ADM1266 uses the PMBus Power System Management

Protocol Specification (Revision 1.2, September 6, 2010)

command set that offers the margining commands through the

following commands:

•

OPERATION (Register 0x01)

•

VOUT_MARGIN_HIGH (Register 0x25)

•

VOUT_MARGIN_LOW (Register 0x26)

•

VOUT_SCALE_LOOP (Register 0x29)

•

VOUT_COMMAND (Register 0x21)

•

VOUT_MARGIN_LOOP (Register 0xDA)

•

MARGIN_CONFIGURATION (Register 0xDB)

These commands enable margining, position the output voltage

at either the high or low value, monitor the feedback node, and

set the ratio of R1 and R3.

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