LT8418 datasheet(29/50 Pages) AD

29 / 50 page

Data Sheet

LTC7892

analog.com

Rev. 0

29 of 50

Figure 39. Relationship between Maximum Duty Cycle and Operating Frequency

A further constraint on the operating frequency is the maximum duty cycle of the boost converter. The maximum

duty cycle (DCMAX) is limited, as shown in Figure 39, and can be approximated as DCMAX = (1 – VIN(MIN)/VOUT) • 100%. If the

duty cycle is greater than DCMAX, the output can lose regulation and may not maintain the constant frequency

operation. An operation frequency should be selected so that the boost converter duty cycle is less than DCMAX.

To improve EMI performance, spread spectrum mode can optionally be selected by tying the PLLIN/SPREAD pin to

INTVCC. When the spread spectrum mode is enabled, the switching frequency modulates within the frequency

selected by the FREQ pin and is +20%. Spread spectrum mode may be used in any operating mode selected by the

MODE pin (Burst Mode, PS mode, or FCM).

A PLL is also available on the LTC7892 to synchronize the internal oscillator to an external clock source connected to

the PLLIN/SPREAD pin. After the PLL locks, the BGxx1 is synchronized to the rising edge of the external clock signal,

and BGxx2 is 180° out of phase from BGxx1. See the Phase-Locked Loop and Frequency Synchronization section for

more details.

Selecting the Light-Load Operating Mode

The LTC7892 can be set to enter high-efficiency Burst Mode operation, constant frequency PS mode, or forced

continuous conduction mode at light load currents. To select the Burst Mode operation, tie the MODE pin to GND. To

select forced continuous operation, tie the MODE pin to INTVCC. To select PS mode, tie the MODE pin to INTVCC through

a 100kΩ resistor. An internal 100kΩ resistor from the MODE pin to the ground selects Burst Mode if the pin is floating.

When synchronized to an external clock through the PLLIN/SPREAD pin, the LTC7892 operates in PS mode if it is

selected. Otherwise, the LTC7892 operates in FCM. Table 5 summarizes the use of the MODE pin to select the light

load operating mode.

Table 5. Using the MODE Pin to Select Light Load Operating Mode

MODE PIN

LIGHT-LOAD OPERATING MODE

MODE WHEN SYNCHRONIZED

0V or Floating

Burst Mode

FCM

100kΩ to INTVCC

PS Mode

INTVCC

FCM

77.5% AT 2.25MHz

FREQUENCY (MHz)

0.1

100

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