LTC7802EUFDM datasheet(17/34 Pages) AD

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LTC7802

Rev. 0

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Setting the Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing gate charge and transition losses, but requires

larger inductance values and/or more output capacitance

to maintain low output ripple voltage.

In higher voltage applications transition losses contrib-

ute more significantly to power loss, and a good balance

between size and efficiency is generally achieved with a

switching frequency between 300kHz and 900kHz. Lower

voltage applications benefit from lower switching losses

and can therefore more readily operate at higher switch-

ing frequencies up to 3MHz if desired. The switching fre-

quency is set using the FREQ and PLLIN/SPREAD pins

as shown in Table 1.

Table 1.

FREQ PIN

PLLIN/SPREAD PIN

FREQUENCY

350kHz

INTVCC

2.25MHz

Resistor to GND

100kHz to 3MHz

Any of the Above

External Clock 100kHz

to 3MHz

Phase-Locked to

External lock

Any of the Above

INTVCC

Spread Spectrum

Modulated

Tying the FREQ pin to ground selects 350kHz 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 3 or the following equation:

RFREQ (in kΩ)=

37MHz

fOSC

To improve electromagnetic interference (EMI) perfor-

mance, spread spectrum mode can optionally be selected

by tying the PLLIN/SPREAD pin to INTVCC. When spread

spectrum is enabled, the switching frequency modulates

within –12% to +15% 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-skip-

ping, or forced continuous mode).

APPLICATIONS INFORMATION

Figure 3. Relationship Between Oscillator Frequency and

Resistor Value at the FREQ Pin

FREQ PIN RESISTOR ( )

10k

100k

500k

100k

10M

7802 F03

A phase-locked loop (PLL) is also available on the

LTC7802 to synchronize the internal oscillator to an exter-

nal clock source connected to the PLLIN/SPREAD pin.

After the PLL locks, TG1 is synchronized to the rising edge

of the external clock signal, and TG2 is 180° out of phase

from TG1. See the Phase-Locked Loop and Frequency

Synchronization section for details.

Selecting the Light-Load Operating Mode

The LTC7802 can be set to enter high efficiency Burst Mode

operation, constant frequency pulse-skipping mode or

forced continuous conduction mode at light load currents.

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 internal 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 LTC7802 operates in

pulse-skipping mode if it is selected, or in forced contin-

uous mode otherwise. Table 2 summarizes 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

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