Data Sheet

LT3077

analog.com

Rev. 0

26 of 38

REF – Voltage Reference

The REF pin is the voltage output of the internal current reference feeding into a resistor DAC. A 4.7µF REF capacitor

to GND decreases reference voltage noise, and soft starts OUT at enable. Soft-start time is determined by the value

of the REF capacitor used.

The REF pin must not be DC loaded by anything except for applications that parallel other LT3077 regulators for

higher output currents. See the Paralleling Devices for Higher Output Current section for further details.

Overdriving the REF Pin

The REF pin can be overdriven by an external source for applications that need to set the Output voltage to a value

other than those programmable using the VO0, VO1, and VO2 pins. The LT3077 uses a current source with a typical value

of 100µA into a resistor DAC. The resistor DAC and the current source are inversely related and may vary up to ±15%

such that the IR product is constant. This variation in the internal current and resistor needs to be accounted for

when externally driving the REF pin.

It is recommended that the VOX pin configuration be set to select a REF pin voltage lower than the required

overdriven REF voltage to ensure the fast start current is shut off when LT3077 regulates the output. As shown in

Figure 65, the REF can be overdriven directly by an external voltage source, or as shown in Figure 66, a voltage source

followed by a resistor-divider. In case where the REF pin is driven by an external voltage source followed by a resistor-

divider, the external voltage source can be a fixed voltage source or can be varied using a servo loop to achieve higher

accuracy.

When using an external voltage source with a resistor-divider, the resulting REF pin voltage can be calculated using

superposition principle as shown in the following equation:

������������������������ = ������������������������ × (

(������������������������||������������������������2)

(������������������������||������������������������2) + ������������������������1

) + ������������������������ × (������������������������||������������������������1||������������������������2)

Where IINT is the internal 100µA current reference, RINT is the nominal resistor value for the corresponding VOX setting,

REXT1 and REXT2 are the external resistors forming the resistor-divider and VEXT is the external voltage source overdriving

the REF pin.

Figure 65. REF Overdriven Directly by an External Voltage Source

51kΩ

2.2µF

4.7µF

RINT

IINT

VEXT

OUT

BIASF

LT3077

VO2

VO1

VO0

VBIAS

IN

EN

GND

SENSE

ILIM

POWER

GOOD

BIAS

PG

REF

VIN

47µF

22µF

806Ω

VOUT

0.5V TO 4.2V

10mA TO 3A

A N A L O G D E V I C E S

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