LT3694 datasheet(22/34 Pages) LINER

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LT3667

3667fb

For more information www.linear.com/LT3667

APPLICATIONS INFORMATION

Figure 7. Diode D1 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output. It Also Protects the Circuit

from a Reversed Input, in Which Case the Resistor at the EN Pin

Limits the Current Drawn from That Pin. The LT3667 Runs Only

When the Input Is Present

Figure 8. Setting the Output Voltage of Each LDO

or some other supply is diode ORed with the switching

regulator’s output. If the IN1 pin is allowed to float and

the EN and UVLO1 pins are held high (either by a logic

signal or because they are tied to IN1), then the LT3667’s

internal circuitry will pull its quiescent current through the

SW pin. This is fine if the system can tolerate a few μA

in this state. If the EN pin or the UVLO1 pin is grounded,

the SW pin current will drop to 0.7μA. However, if the IN1

pin is grounded while the output is held high, regardless

of EN and UVLO1, parasitic diodes inside the LT3667 can

pull current from the output through the SW pin and the

IN1 pin. Figure 7 shows a circuit that will run only when

the input voltage is present and that protects against a

shorted or reversed input. Alternatively, the switching

regulator can be supplied by the LDO at OUT2 as shown

in the Applications Information section of the LDOs.

LDOs

FB2/FB3 Resistor Networks

Each LDO output voltage of the LT3667 is programmed

with a resistor divider between the output of that LDO and

its FB2/FB3 pin as shown in Figure 8. The pin current, IFB,

(3nA at 25°C, ±40nA at 150°C) of each FB2/FB3 pin flows

out of that pin, which results in

VOUT = 0.8V







 –IFB •R2

The value of R1 should not exceed 160k to provide a

minimum 5µA load current so that the output voltage

error, caused by the FB2/FB3 pin current, is minimized.

Rearranging for R2 gives:

R2 =

0.8V – VOUT

IFB –0.8V/R1

Note that choosing smaller resistors will increase the

quiescent current of the application circuit.

LT3667

BOOST

IN1

UVLO1

VIN

VOUT

BACKUP

3667 F07

MBRS140

FB1

GND

LT3667

GND

OUTn

FBn

INn

VOUT

3667 F08

VIN

IFB

LT3694 Datasheet(PDF) 22 Page - Linear Technology

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