LTC3891 datasheet(22/40 Pages) LINER

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LTC3890-2

38902f

APPLICATIONS INFORMATION

Alternatively, the TRACK/SS pin can be used to track two

(or more) supplies during start-up, as shown qualitatively

in Figures 7a and 7b. To do this, a resistor divider should

be connected from the master supply (VX) to the TRACK/

SS pin of the slave supply (VOUT), as shown in Figure 8.

During start-up VOUT will track VX according to the ratio

set by the resistor divider:

VOUT

RTRACKA

•

RTRACKA + RTRACKB

RA + RB

For coincident tracking (VOUT = VX during start-up):

RA = RTRACKA

RB = RTRACKB

INTVCC Regulators

The LTC3890-2 features two separate internal P-channel

low dropout linear regulators (LDO) that supply power at

the INTVCC pin from either the VIN supply pin or the EXT-

VCC pin depending on the connection of the EXTVCC pin.

INTVCC powers the gate drivers and much of the LTC3890-

2’s internal circuitry. The VIN LDO and the EXTVCC LDO

regulate INTVCC to 5.1V. Each of these can supply a peak

current of 50mA and must be bypassed to ground with a

minimum of 4.7μF ceramic capacitor. No matter what type of

bulk capacitor is used, an additional 1μF ceramic capacitor

placed directly adjacent to the INTVCC and PGND pins is

highly recommended. Good bypassing is needed to supply

the high transient currents required by the MOSFET gate

drivers and to prevent interaction between the channels.

TIME

VX(MASTER)

VOUT(SLAVE)

38902 F07a

TIME

38902 F07b

VX(MASTER)

VOUT(SLAVE)

1/2 LTC3890-2

VOUT

VFB

TRACK/SS

38902 F08

RTRACKA

RTRACKB

(7a) Coincident Tracking

(7b) Ratiometric Tracking

Figure 8. Using the TRACK/SS Pin for Tracking

1/2 LTC3890-2

TRACK/SS

CSS

SGND

38902 F06

Figure 6. Using the TRACK/SS Pin to Program Soft-Start

Figure 7. Two Different Modes of Output Voltage Tracking

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