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LT8603

8603f

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pin is connected to a switching regulator channel it will

improve efficiency, reduce on-chip dissipation and lower

the sleep current. The INTVCC may be used to configure

other inputs and supply pull-ups related to the LT8603. It

is not recommended to draw more than 1mA or connect

INTVCC to any components not related to the LT8603 to

avoid unexpected interactions.

BOOST CONTROLLER

Functional Description

Channel 4 is a set of functional elements that are con-

nected to external components to form a boost converter.

The Block Diagram in Figure 2 shows these elements and

how they are connected internally.

The error amplifier compares a fractional part of the out-

put voltage to an 800mV reference. The output, ILIM4,

goes to the current comparator to set the peak current.

The current comparator senses the inductor current using

a small sense resistor across the ISP4 and ISN4 pins. The

trip level is set by the error amplifier output, ILIM4, and

the slope compensation signal, SLOPE4. The maximum

The internal oscillator of the LT8603 can be synchronized

to an external clock of 250kHz up to 2.2MHz applied to

the SYNC pin. The RT value should be chosen such that

the frequency set by RT is close to the anticipated external

clock frequency.

Mode Selection and Synchronization

To select low ripple Burst Mode operation, the SYNC

pin should be connected to a voltage below 0.3V such

as ground. To select pulse-skipping operation, connect

the SYNC pin to an available voltage above 1.2V such

as INTVCC.

To synchronize the LT8603 to an external frequency, drive

the SYNC pin with a pulse train with a high voltage above

1.2V and a low voltage below 0.3V. The minimum pulse

width is 120ns for a high pulse and 90ns for a low pulse.

The LT8603 will operate in pulse-skipping mode while

synchronized to an external clock. The LT8603 may be

synchronized over a 250kHz to 2.2MHz range. The RT

resistor should be chosen to set the LT8603 switching

frequency close to the synchronization input.

For some applications it is desirable for the LT8603 to

operate in pulse-skipping mode, offering two major differ-

ences from Burst Mode operation. First, in pulse-skipping

mode the clock stays awake at all times and all switching

cycles are aligned to the clock. Second, full frequency

switching is reached at a lower output load in pulse-skip-

ping than Burst Mode operation. These two differences

come at the expense of increased quiescent current for

pulse-skipping. To enable pulse-skipping mode, the SYNC

pin is tied high either to a logic output or to the INTVCC

pin. Do not leave the SYNC pin floating.

INTVCC Regulator

The INTVCC supplies the common circuitry such as the

oscillator and reference but its main current demand

comes from the gate drivers for the high voltage buck

converters. The current draw will depend on the operat-

ing frequency, the higher the switching frequency, the

greater the current drawn from INTVCC. The regulator is

supplied by VIN at start-up, but it will draw its supply

current from BIAS if BIAS is at least 3.1V. If the BIAS

APPLICATIONS INFORMATION

Figure 2. Boost Controller Block Diagram

8603 F02

GATE4

LOGIC 4

–

ISN4

DRIVER

ISP4

CLK4

SS4

0.8V

FB4

ILIM4

ITRIP

–

LOOP

COMPENSATION

0.736V

PG4

BIAS

VIN

FSEL4B

FSEL4A

BOOST DIVIDER

AND SLOPE GEN

INTVCC4

REGULATOR

–

INTVCC4

CURRENT

COMPARATOR

ERROR

AMPLIFIER

CLK2

SLOPE4

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