LTC7802EUFDM-3.3 datasheet(18/34 Pages) AD

18 / 34 page

LTC7802-3.3

Rev. 0

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The maximum power loss in R1 is related to duty cycle and

occurs in continuous mode at the maximum input voltage:

PLOSS R1=

(VIN(MAX) � VOUT) • VOUT

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing

or sense resistors. Light load power loss can be mod-

estly higher with a DCR network than with a sense resis-

tor, due to the extra switching losses incurred through

R1. However, DCR sensing eliminates a sense resistor,

reduces conduction losses and provides higher efficiency

at heavy loads. Peak efficiency is about the same with

either method.

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:

(in k)

FREQ

37MHz

fOSC

APPLICATIONS INFORMATION

Figure 3. Relationship Between Oscillator Frequency and

Resistor Value at the FREQ Pin

FREQ PIN RESISTOR (Ohms)

10k

100k

500k

100k

10M

780233 F03b

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 operat-

ing mode selected by the MODE pin (Burst Mode, pulse-

skipping, or forced continuous mode).

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

LTC7802-3.3 to synchronize the internal oscillator to an

external clock source connected to the PLLIN/SPREAD

pin. After the PLL locks, TG1 is synchronized to the ris-

ing 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.

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