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LT8603

8603f

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APPLICATIONS INFORMATION

Buck: Burst Mode Operation

With the SYNC pin held low to select Burst Mode opera-

tion, the LT8603 will automatically transition to Burst

Mode at light load for best efficiency. In Burst Mode

operation, most of the circuits are shut down between

switch-on bursts to minimize power loss. If at least one

channel remains full frequency, the oscillator remains

on and all bursts are synchronized to the appropriate

phase of the oscillator. If all channels go into Burst Mode

operation, the oscillator will also shut off between bursts

with a further savings in power. Because the channels

of the LT8603 may have different loads, channels can

have different switching frequencies when in Burst Mode

operation.

GENERAL FUNCTIONS

General: Power Good Comparators

Each LT8603 channel has a power good comparator with

an open drain output pin, PGx. For the buck channels,

each PG pin is pulled low when the corresponding feed-

back voltage is either above or below its reference voltage

by more than 8%. The boost channel’s PG pin is pulled

low when its feedback voltage is below its reference volt-

age by more than 8%. See the Electrical Characteristics

table for more information on each channel’s power good

thresholds. Note, the PG outputs are not valid until INTVCC

rises above 2.7V.

General: Power-On Reset Timer

The LT8603 provides a programmable reset timer. The

POREN pin is the enable for the reset timer and includes a

1μA internal pull-up. Once enabled, the reset timer begins

an internal clock counter that terminates after 64 cycles.

Upon counter termination, the RST open-drain pull-down

releases allowing the pin to transition high. The RST out-

put includes a weak, 100k, internal pull-up resistor to

approximately 2V.

The power-on reset timeout period, tRST, can be pro-

grammed by connecting a capacitor, CPOR, between the

CPOR pin and ground. The value of tRST is calculated by:

tRST = 35.2 •CPOR

where CPOR is in pF and tRST is in microseconds. For exam-

ple, using a capacitor value of 8.2nF gives a 289ms reset

timeout period. The accuracy of tRST will be determined

Figure 10. Tracking Output Waveforms

TRKSS2

VOUT1

LT8603

COINCIDENT TRACKING VOUT1> VOUT2

R1=R2• VOUT2 –1

(

)

TIME

(10a) Coincident Tracking

VOUT1

VOUT2

TIME

8603 F10

(10b) Ratiometric Tracking

VOUT1

VOUT2

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