LTC3736-2 Datasheet, PDF(18/28 Page) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking

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LTC3736-2 Datasheet, PDF (18/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Output Tracking

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LTC3736-2

APPLICATIO S I FOR ATIO

VOUT

1/2 LTC3736-2

VFB

CFF

37362 F05

Figure 5. Setting Output Voltage

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

optional external soft-start function and a means to shut

down the LTC3736-2.

Pulling the RUN/SS pin below 0.65V puts the LTC3736-2

into a low quiescent current shutdown mode (IQ = 9ÂµA). If

RUN/SS has been pulled all the way to ground, there will

be a delay before the LTC3736-2 comes out of shutdown

and is given by:

tDELAY

0.65V

â¢

CSS

0.7ÂµA

0.93s/ÂµF

â¢

CSS

This pin can be driven directly from logic as shown in

Figure 6. Diode D1 in Figure 6 reduces the start delay but

allows CSS to ramp up slowly providing the soft-start

function. This diode (and capacitor) can be deleted if the

external soft-start is not needed.

During soft-start, the start-up of VOUT1 is controlled by

slowly ramping the positive reference to the error amplifier

from 0V to 0.6V, allowing VOUT1 to rise smoothly from 0V

to its final value. The default internal soft-start time is 1ms.

3.3V OR 5V

RUN/SS

CSS

RUN/SS

CSS

VDD â¤ VIN

RUN/SS

(INTERNAL SOFT-START)

This can be increased by placing a capacitor between the

RUN/SS pin and SGND. In this case, the soft-start time will

be approximately:

tSS1

CSS

â¢

600mV

0.7ÂµA

Tracking

The start-up of VOUT2 is controlled by the voltage on the

TRACK pin. Normally this pin is used to allow the start-up

of VOUT2 to track that of VOUT1 as shown qualitatively in

Figures 7a and 7b. When the voltage on the TRACK pin is

less than the internal 0.6V reference, the LTC3736-2

regulates the VFB2 voltage to the TRACK pin voltage

instead of 0.6V. The start-up of VOUT2 may ratiometrically

track that of VOUT1, according to a ratio set by a resistor

divider (Figure 7c):

VOUT1 = R2A â¢ RTRACKA + RTRACKB

VOUT2 RTRACKA

R2B + R2A

For coincident tracking (VOUT1 = VOUT2 during start-up),

R2A = RTRACKA

R2B = RTRACKB

The ramp time for VOUT2 to rise from 0V to its final value

is:

tSS2

tSS1 â¢

RTRACKA

R1A

â¢

R1A + R1B

RTRACKA + RTRACKB

VOUT1

VOUT2

RTRACKB

RTRACKA

R1B LTC3736-2

R2B

VFB1 VFB2

R1A

R2A

TRACK

37362 F07a

Figure 7a. Using the TRACK Pin

37362 F06

Figure 6. RUN/SS Pin Interfacing

37362fa

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