LTC3603 datasheet(13/22 Pages) LINER

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LTC3603

3603fa

APPLICATIONS INFORMATION

and the BOOST pin rises to PVIN + INTVCC, thereby keep-

ing the MOSFET fully enhanced. For most applications, a

0.22μF ceramic capacitor is appropriate for CBST. Schottky

diode DBST should have a reverse breakdown voltage that

is greater than PVIN(MAX).

When the LTC3603 detects a fault condition (either

undervoltage lockout or overtemperature), the TRACK/SS

pin is quickly pulled to ground and the internal soft-start

timer is also reset. This ensures an orderly restart when

using an external soft-start capacitor.

To implement tracking, a resistor divider is placed between

an external supply (VX) and the TRACK/SS pin as shown

in Figure 5a. This technique can be used to cause VOUT to

ratiometrically track the VX supply (Figure 5b), according

to the following:

OUT

•

For coincident tracking, as shown in Figure 5c, (VOUT =

VX during start-up),

RTA = RA, RTB = RB

Note that the 1.2μA current that is sourced from the

TRACK/SS pin will cause a slight offset in the voltage seen

on the TRACK/SS pin and consequently on the VOUT volt-

age during tracking. This VOUT offset due to the TRACK/SS

current is given by:

VµA

OS TRK

TA TA

() •

•

For most applications, this offset is small and has minimal

effect on tracking performance. For improved tracking ac-

curacy, reduce the parallel impedance of RTA and RTB.

4.7MΩ

LTC3603

3603 F04

PVIN

RUN

LTC3603

3.3V OR 5V

RUN

LTC3603

3603 F05a

TRACK/SS

VFB

VOUT

RTB

RTA

Figure 4. RUN Pin Interfacing

Figure 5a. Using the TRACK/SS Pin to Track VX

DBST

LTC3603

CBST

3603 F03

BOOST

INTVCC

CINTVCC

Figure 3. Topside MOSFET Supply

Run and Soft-Start/Tracking Functions

The LTC3603 has a low power shutdown mode which is

controlled by the RUN pin. Pulling the RUN pin below 0.7V

puts the LTC3603 into a low quiescent current shutdown

mode (IQ < 1μA). When the RUN pin is greater than 0.7V,

the controller is enabled. The RUN pin can be driven directly

from logic as shown in Figure 4.

Soft-start and tracking are implemented by limiting the

effective reference voltage as seen by the error ampliﬁer.

Ramping up the effective reference into the error amp in

turn causes a smooth and controlled ramp on the output

voltage of the converter. To use the default, internal 1ms

soft-start ramp, leave the TRACK/SS pin ﬂoating. Do not

tie the TRACK/SS pin to INTVCC or to PVIN. To increase the

soft-start time above 1ms, place a cap on the TRACK/SS pin.

A 1.2μA internal pull-up current will charge this capacitor,

resulting in a soft-start ramp time given by:

µA

•

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