LTC3709_15 Datasheet, PDF(17/24 Page) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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LTC3709_15 Datasheet, PDF (17/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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LTC3709

APPLICATIO S I FOR ATIO

3.3V OR 5V

VIN

RUN/SS

RSS*

CSS

(5a)

VCC

RSS*

D2* RUN/SS

2N7002

CSS

3709 F05

*OPTIONAL TO OVERRIDE

OVERCURRENT LATCHOFF

(5b)

Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated

When RUN/SS rises 0.6V above the ON threshold, the

reference clamp is invalidated and the internal precision

reference takes over.

After the controller has been started and given adequate

time to charge the output capacitor, CSS is used as a short-

circuit timer. After the RUN/SS pin charges above 3V, and

if the output voltage falls below 67% of its regulated value,

then a short-circuit fault is assumed. A 2ÂµA current then

begins discharging CSS. If the fault condition persists until

the RUN/SS pin drops to 2.5V, then the controller turns off

both power MOSFETs, shutting down the converter per-

manently. The RUN/SS pin must be actively pulled down

to ground in order to restart operation.

The overcurrent protection timer requires that the soft-

start timing capacitor CSS be made large enough to guar-

antee that the output is in regulation by the time CSS has

reached the 3V threshold. In general, this will depend upon

the size of the output capacitance, output voltage and load

current characteristic. A minimum soft-start capacitor can

be estimated from:

CSS > COUT VOUT RSENSE (10â4 [F/VS])

Overcurrent latchoff operation is not always needed or

desired and can prove annoying during troubleshooting.

The feature can be overridden by adding a pull-up current

of >5ÂµA to the RUN/SS pin. The additional current pre-

vents the discharge of CSS during a fault and also shortens

the soft-start period. Using a resistor to VIN as shown in

Figure 5 is simple, but slightly increases shutdown cur-

rent. Any pull-up network must be able to pull RUN/SS

above the 3V threshold that arms the latchoff circuit and

overcome the 2ÂµA maximum discharge current.

Output Voltage Tracking

The feedback voltage, VFB, will follow the TRACK pin

voltage when the TRACK pin voltage is less than the

reference voltage, VREF (0.6V). When the TRACK pin

voltage is greater than VREF, the feedback voltage will

servo to VREF. When selecting components for the TRACK

pin, ensure the final steady-state voltage on the TRACK pin

is greater than VREF at the end of the tracking interval.

The LTC3709 allows the user to set up start-up sequenc-

ing among different supplies in either coincident tracking

or ratiometric tracking as shown in Figure 6. To implement

the coincident tracking, connect an extra resistor divider

VOUT1

VOUT2

TIME

(6a) Coincident Tracking

TIME

3709 F06

(6b) Ratiometric Tracking

Figure 6. Two Different Forms of Output Voltage Sequencing

3709fb

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