Data Sheet

LT3077

analog.com

Rev. 0

27 of 38

Figure 66. REF Overdriven by a Voltage Source Followed by a Resistor-Divider

When using an external voltage source with resistor-dividers, choose REXT1 and REXT2 such that REXT2 value is at most

10% of nominal RINT value to assure accuracy across process variations in RINT.

Example: Setting REF to be 1.025V with a 1.25V external reference and using resistor-dividers.

Set the VOX pins such that LT3077 selects the 1V output setting (VO0 and VO2 are Hi-Z and VO1 is GND).

For 1V setting:

������������������������(������������������������������������������) = 1������/100������������ = 10������Ω

Select:

������������������������2 = ������������������������/10 = 1������Ω

Using these values, the equation is:

������������������������2||������������������������ = 1������Ω||10������Ω = 909.0909Ω

Substituting in the above equation for VREF, the equation is:

1.025������ = 1.25������ × (

909.0909Ω

909.0909٠+ ������������������������1

) + 100µ������ × (909.0909Ω||������������������������1)

Solving for REXT1 gives REXT1 = 218.978Ω. The closest 1% resistor is 221Ω.

If overdriving the REF pin, VOUT thresholds for power good mentioned in Table 1 can no longer be guaranteed.

Enable Function – Turning ON and OFF

The EN pin enables/disables the reference, the output transistor, and disables auxiliary functions. Pulling EN low

places the regulator into Nap mode. In Nap mode, the quiescent current decreases to less than 10μA. The LT3077 has

an accurate 1.26V turn-on threshold on the EN pin with 80mV of hysteresis. This threshold can be used with a resistor-

divider from the bias supply to define an accurate UVLO threshold for the regulator. The EN pin current (IEN) at the

threshold shown in Table 1 must be considered when calculating the resistor-divider network as follows:

������������������������������(������������������������) = 1.26������ × (1 +

������������������2

������������������1

) + ������������������/������������ × ������������������2

Where:

REN1 and REN2 are the resistors from the EN pin to GND and the EN pin to BIAS pin respectively. IEN can be ignored if

REN1 is less than 100kΩ. If unused, tie the EN pin to BIAS.

51kΩ

2.2µF

4.7µF

REXT2

RINT

IINT

VEXT

OUT

BIASF

LT3077

VO2

VO1

VO0

VBIAS

IN

EN

GND

SENSE

ILIM

POWER

GOOD

BIAS

PG

REF

VIN

47µF

22µF

REXT1

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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