LTC3602EUF datasheet(12/20 Pages) LINER

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LTC3602

3602fb

APPLICATIONS INFORMATION

Topside MOSFET Driver Supply (BOOST Pin)

The LTC3602 uses a bootstrapped supply to power the

gate of the internal topside MOSFET (Figure 3). When the

topside MOSFET is off and the SW pin is low, diode DBST

charges capacitor CBST to the voltage on the INTVCC sup-

ply. In order to turn on the topside MOSFET, the voltage on

the BOOST pin is then applied to its gate. As the topside

MOSFET turns on, the SW pin rises to the PVIN voltage

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).

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 cap,

resulting in a soft-start ramp time given by:

SS = CSS •

0.6

1.2µA

When the LTC3602 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:

OS,TRK = (1.2µA) •

TARTA

TA + RTB

•

A + RB

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

LTC3602

3602 F04

PVIN

RUN

LTC3602

3.3V OR 5V

RUN

Figure 4. RUN Pin Interfacing

DBST

LTC3602

CBST

3602 F03

BOOST

INTVCC

CINTVCC

Figure 3. Topside MOSFET Supply

Run and Soft-Start/Tracking Functions

The LTC3602 has a low power shutdown mode which is

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

puts the LTC3602 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

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