LTC7803 datasheet(22/42 Pages) AD

22 / 42 page

LTC7811

Rev. 0

For more information www.analog.com

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 switching

frequencies up to 3MHz if desired.

A further constraint on the operating frequency is due

to the maximum duty cycle of the boost channel. The

maximum duty cycle, which can be approximated as

DCMAX≈(1-VIN(MIN)/VOUT3)*100%, is limited as shown

in Figure 3a. At low frequencies, the output will dropout

if the required duty cycle is higher than 93%. At high

frequencies, the maximum duty cycle available to main-

tain constant frequency operation is reduced further. In

this region, if a higher duty cycle is required to keep the

output voltage in regulation, the controller will keep the

bottom MOSFET (BG3) on for more than one clock cycle

to achieve the higher duty cycle at an effectively lower fre-

quency. Choose a frequency that limits the maximum duty

cycle to a value lower than the curve shown in Figure 3a.

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

380kHz

INTVCC

2.25MHz

Resistor to GND

100kHz to 3MHz

Any of the Above

External Clock

100kHz to 3MHz

Phase-Locked to

External Clock

Any of the Above

INTVCC

Spread Spectrum

Modulated

Tying the FREQ pin to ground selects 380kHz while

tying FREQ to INTVCC selects 2.25MHz. Placing a resis-

tor between FREQ and ground allows the frequency to

be programmed anywhere between 100kHz and 3MHz.

Choose a FREQ pin resistor from Figure 3b or the follow-

ing equation:

RFREQ(inkΩ)=

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

−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-skipping,

or forced continuous mode).

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

LTC7811 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.

APPLICATIONS INFORMATION

LTC7803 Datasheet(PDF) 22 Page - Analog Devices

LTC7803 Datasheet(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Datasheet Download

Link URL