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Data Sheet

LT3074

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Rev 0.

31 of 62

Table 7. Description of Data Formats

DATA

FORMAT

DATA FORMAT

NAME

DESCRIPTION

L11

Linear_5s_11s

PMBus Data field b[15:0]

Value = Y x 2N

where, N = b[15:11] is a 5-bit 2’s complement integer and Y = b[10:0] is a 11-

bit 2’s complement integer.

Example:

For b[15:0] = 0xCAC0 = ‘b1100_1010_1100_0000 (Y = 010_1100_0000; N =

11001)

Value = 704 x 2−7 = 5.5

L16

Linear_16u

PMBus Data Field b[15:0]

Value = Y x 2N

where, Y = b[15:0] is an unsigned integer and N = VOUT_MODE[4:0] is a 5-bit

2’s complement signed integer, hardwired to -13 decimal.

Example:

For b[15:0] = 0xA000 = ‘b1010_0000_0000_0000

Value = 40960 x 2−13 = 5

Reg

PMBus Data Field b[15:0] or b[7:0]

Bit field meaning is defined in detail in the PMBus Command Details section.

ASC

Ascii Format

A variable length string of text characters conforming to ISO/IEC 9959-1

standard.

Output Voltage

The LT3074 incorporates a 100µA precision current source flowing out of the SETRES pin, which also connects to the

noninverting input of the error amplifier through an RC filter network (see the Output Noise and Fast Start sections

for more details). Connecting a resistor from the SETRES pin to ground generates a reference voltage for the error

amplifier. This reference voltage is the product of the SETRES pin current and the SETRES pin resistance. The unity-

gain configuration of the error amplifier produces a low-impedance version of this voltage at the SENSE pin, which

is the inverting input of the error amplifier. The SENSE pin is externally connected to the OUT pin.

The benefit of using current reference compared to the typical voltage reference used in conventional regulators is

that the regulator always operates in unity-gain configuration, independent of the programmed output voltage. This

configuration allows the LT3074 to have loop gain, frequency response, and bandwidth independent of the output

voltage. As a result, noise, PSRR, and transient performance do not change with the output voltage. Moreover,

because none of the error amplifier gain is needed to amplify the SETRES pin voltage to a higher output voltage,

output load regulation is more tightly specified in the hundreds of microvolts range and not as a fixed percentage of

the output voltage.

Because the zero temperature-coefficient current source is highly accurate, the SETRES pin resistor can become a

limiting factor in achieving high accuracy. Therefore, the SETRES pin resistor must be a precision resistor. Table 8

lists many common output voltages and their corresponding 1% SETRES pin resistance values. Additionally, any

leakage paths to or from the SETRES (or SETCAP) pin create errors in the output voltage. Leakages on the SETCAP

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