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LT3694/LT3694-1

36941fa

If the current feeding the collector of the NPN through the

sense resistor comes from a supply that is not connected

to BIAS, the current limit cannot be used and the LIM pin

must be connected to BIAS to disable the current limit.

Tracking and Soft-Start

The output of the LT3694 regulates to the lowest voltage

present at either the TRK/SS pin or an internal 0.75V

reference. A capacitor from the TRK/SS pin to ground is

charged by an internal 3μA current source resulting in a

linear output ramp from 0V to the regulated output whose

duration is given by:

RAMP =

TRKSS •0.75V

3µA

At power-up or at any shutdown event, the TRK/SS pins

are internally pulled to ground through 100Ω to insure

the soft-start capacitors are discharged. The pins clamp

at 1.3V.

Ratiometric tracking is achieved by tying the TRK/SS pins

tied together and connecting to a single capacitor. The

charge current is multiplied by the number of TRK/SS

pins connected.

Coincident tracking is accomplished by adding an addi-

tional resistor divider to the master regulator output and

connecting it to the TRK/SS pin of the slave regulator. The

resistor divider should be equal to the slave’s feedback

divider. Keep in mind that the LDO pass transistor VCE(SAT)

will limit how well the LDO output can coincidentally track

the switching regulator output.

The TRK/SS pin has a low voltage detect that insures

the regulator is shut off when TRK/SS is pulled low. The

threshold low voltage is nominally 50mV. This allows

independent on/off control of the LDOs using the TRK/SS

pins. The logic drive should be open collector or have

series resistance because the TRK/SS pins are internally

pulled to ground during any shutdown event.

Shorted and Reversed Input Protection

If an inductor is chosen that will not saturate excessively,

an LT3694 buck regulator will tolerate a shorted output.

There is another situation to consider in systems where

the output will be held high when the input to the LT3694

is absent. This may occur in battery charging applications

or in battery backup systems where a battery or some

other supply is diode ORed with the LT3694’s output. If

the VIN pin is allowed to float and the EN/UVLO pin is held

high (either by a logic signal or because it is tied to VIN),

then the LT3694’s internal circuitry will pull its quiescent

current through its SW pin. This is fine if the system

can tolerate a few mA in this state. If the EN/UVLO pin

is grounded, the SW pin current will drop to essentially

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT3694 can

pull large currents from the output through the SW pin

and the VIN pin. The circuit in Figure 8 runs only when the

input voltage is present—and protects against a shorted

or reversed input.

APPLICATIONS INFORMATION

Figure 8. Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also

Protects the Circuit from a Reversed Input. The LT3694

Runs Only When the Input Is Present

VIN

BST

GND FB

EN/UVLO

VIN

LT3694

36941 F08

VOUT

BACKUP

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