LTC3857 datasheet(21/40 Pages) LINER

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LTC3890-3

38903f

For more information www.linear.com/3890-3

applicaTions inForMaTion

(7a) Coincident Tracking

(7b) Ratiometric Tracking

Figure 8. Using the TRACK/SS Pin for Tracking

Figure 7. Two Different Modes of Output Voltage Tracking

TIME

VX(MASTER)

VOUT(SLAVE)

38903 F07a

TIME

38903 F07b

VX(MASTER)

VOUT(SLAVE)

1/2 LTC3890-3

VOUT

VFB

TRACK/SS

38903 F08

RTRACKA

RTRACKB

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

RA = RTRACKA

RB = RTRACKB

INTVCC Regulators

The LTC3890-3 features two separate internal P-channel

low dropout linear regulators (LDO) that supply power at

theINTVCCpinfromeithertheVINsupplypinortheEXTVCC

pindependingontheconnectionoftheEXTVCCpin.INTVCC

powers the gate drivers and much of the LTC3890-3’s

internalcircuitry.TheVINLDOandtheEXTVCCLDOregulate

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 IC pins is

highlyrecommended.Goodbypassingisneededtosupply

the high transient currents required by the MOSFET gate

drivers and to prevent interaction between the channels.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC3890-3 to be

exceeded. The INTVCC current, which is dominated by the

gate charge current, may be supplied by either the VIN

LDO or the EXTVCC LDO. When the voltage on the EXTVCC

pin is less than 4.7V, the VIN LDO is enabled. Power dis-

sipation for the IC in this case is highest and is equal to

VIN • IINTVCC. The gate charge current is dependent on

operating frequency as discussed in the Efficiency Con-

siderations section. The junction temperature can be

estimated by using the equations given in Note 3 of the

Electrical Characteristics. For example, the LTC3890E-3

INTVCC current is limited to less than 15mA from a 40V

supply when not using the EXTVCC supply at 70°C ambi-

ent temperature:

TJ = 70°C + (15mA)(40V)(90°C/W) = 125°C

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