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LTC7106

Rev A

For more information www.analog.com

OPERATION

The LTC7106 is a PMBus controlled 7-bit D/A converter

currentsource.ThroughitsPMBusinterface,the LTC7106

receives a 7-bit DAC code and converts this value to a

bidirectional analog output current through the pin IDAC.

By connecting IDAC to the feedback node of a voltage

regulator,IDACcanchangetheoutputvoltageoftheregula-

tor with the equation:

VOUT = VREF • (1 + RFB1/RFB2) – IDAC • RFB1

where VREF is the reference voltage of the voltage regula-

tor. RFB1 and RFB2 are the resistor divider for the voltage

regulator. IDAC is the programmed bidirectional current

shown in Table 2.

A typical application diagram is shown on the front page.

Therefore, the traditional pure analog designed oriented

PWM controller can be controlled by a PMBus interface.

This illustrates the flexibility of the LTC7106 providing a

PMBusinterfacetoconventionalanalogDC/DCconverters.

CHIP ENABLE (EN PIN)

The LTC7106 is activated by the EN pin. It turns on/off the

device with threshold of 1.2V. When EN is low (<1.2V),

IDAC is in high impedance (Hi-Z).

However, PMBus interface is still active when EN is

low which means users can program the device and

readback the internal register's value. The device will

execute the commands of MFR_IOUT_COMMAND,

MFR_IOUT_MARGIN_HIGH, MFR_IOUT_MARGIN_LOW

after EN goes high.

SLEW RATE CONTROL

To prevent abrupt changes in the D/A output current

and subsequently the output voltage of the DC/DC regula-

tor, an internal digital programmable slew rate control is

included. The slew rate range can be programmed with

a 6-bit register from 0.5µs/step to 3.58ms/step with a

default value of 3.58ms/step.

CURRENT RANGE SETTING AND D/A PROGRAMMING

The LTC7106 is a 7-bit bidirectional current DAC with a

1µA LSB as its default setting. The MSB determines the

current direction. When MSB is 0, IDAC is sourcing cur-

rent (reducing VOUT), which is positive current flowing

out of the pin, and when MSB is 1, IDAC is sinking cur-

rent (increasing VOUT), which is negative current flowing

into the pin. The LTC7106 also provides range high and

range low options through its digital interface to change

the LSB value to 4µA expanding the output current range

and subsequently widening the programmable output

voltage range. Alternately for higher resolution, the low

range is provided with a LSB of 0.25µA. Users have ad-

ditional flexibility of choosing the resistor divider ratio and

resistor values to meet the output specification target.

However, the design is most accurate using the nominal

range which is the recommended setting. Table 1 lists the

output current range and Table 2 lists the detailed DAC

codes vs IDAC current.

Table 1. Output Current Range

Range

LSB (μA)

IMIN (µA)

IMAX (µA)

Nominal

–64

Range High

–256

252

Range Low

0.25

–16

15.75

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