LTC7803 datasheet(22/44 Pages) AD

22 / 44 page

LTC7817

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

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 skip the

top MOSFET (TG3) turn-on and keep the bottom MOSFET

(BG3) on for more than one clock cycle to achieve the

higher duty cycle at an effectively lower frequency. 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/MODE pins

as shown in Table 1.

Table 1.

FREQ PIN

PLLIN/MODE PIN

FREQUENCY

DC Voltage

380kHz

INTVCC

DC Voltage

2.25MHz

Resistor to GND

DC Voltage

100kHz to 3MHz

Any of the Above

External Clock

100kHz to 3MHz

Phase-Locked to

External Clock

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(in k� )=

37MHz

fOSC

Figure 3. Setting the Operating Frequency

FREQ PIN RESISTOR (Ohms)

10k

100k

500k

100k

10M

7817 F03b

77.5% at 2.25MHz

FREQUENCY (Hz)

100k

100

7817 F03a

3a. Relationship Between Oscillator Frequency

and Boost Channel Maximum Duty Cycle

3b. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

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