LTC7803 datasheet(23/36 Pages) AD

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LTC7803

Rev 0

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Setting the Operating Frequency

The switching frequency is set using the FREQ and PLLIN/

SPREAD pins as shown in Table 1.

Table 1.

FREQ PIN

PLLIN/SPREAD PIN

FREQUENCY

375kHz

INTVCC

2.25MHz

Resistor

100kHz to 3MHz

Any of the Above

External Clock

100kHz to 3MHz

Phase Locked to

External Clock

Any of the Above

INTVCC

Spread Spectrum fOSC

modulated 0% to 20%

Tying the FREQ pin to ground selects 375kHz while tying

FREQ to INTVCC selects 2.25MHz. Placing a resistor

between FREQ and ground allows the frequency to be pro-

grammed anywhere between 100kHz and 3MHz. Choose a

FREQ pin resistor from Figure 8 or the following equation:

RFREQ (inkΩ)=

37MHz

fOSC

Switching regulators can be particularly troublesome for

applications where electromagnetic interference (EMI) is

a concern. To improve EMI, spread spectrum mode can

optionally be selected by tying the PLLIN/SPREAD pin to

INTVCC. When spread spectrum in enabled, the switch-

ing frequency varies within 0% to +20% of the frequency

selected by the FREQ pin. Spread spectrum may be used

in any operating mode selected by the MODE pin (Burst

Mode, pulse-skipping, or forced continuous mode).

Selecting the Light-Load Operating Mode

The LTC7803 can be set to enter high efficiency Burst

Mode operation, constant frequency pulse-skipping mode

or forced continuous conduction mode at light load cur-

rents. To select Burst Mode operation, tie the MODE to

ground. To select forced continuous operation, tie the

MODE pin to INTVCC. To select pulse-skipping mode, tie

the MODE pin to INTVCC through a 100k resistor. An inter-

nal 100k resistor from the MODE pin to ground selects

Burst Mode if the pin is floating. When synchronized to

an external clock through the PLLIN/SPREAD pin, the

LTC7803 operates in pulse skipping mode if it is selected,

or in forced continuous mode otherwise. Table 2 sum-

marizes the use of the MODE pin to select the light load

operating mode.

Table 2.

MODE PIN

LIGHT-LOAD

OPERATING MODE

MODE WHEN

SYNCHRONIZED

0V or Floating

Burst Mode

Forced Continuous

100k to INTVCC

Pulse-Skipping

INTVCC

Forced Continuous

In general, the requirements of each application will dictate

the appropriate choice for light-load operating mode. In

Burst Mode operation, the inductor current is not allowed

to reverse. The reverse current comparator turns off the

bottom MOSFET just before the inductor current reaches

zero, preventing it from reversing and going negative.

Thus, the regulator operates in discontinuous operation.

Figure 8. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

APPLICATIONS INFORMATION

FREQ PIN RESISTOR (Ω)

10k

100k

500k

100k

10M

7803 F08

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