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LTM4619

4619f

Mode Selections and Phase-Locked Loop

The LTM4619 can be enabled to enter high efﬁciency

Burst Mode operation, constant-frequency pulse-skipping

mode, or forced continuous conduction mode. To select

the forced continuous operation, tie the MODE/PLLIN pin

to a DC voltage below 0.8V. To select pulse-skipping mode

of operation, tie the MODE/PLLIN pin to INTVCC. To select

Burst Mode operation, ﬂoat the MODE/PLLIN pin.

Frequency Synchronization

A phase-lock loop is available on the LTM4619 to synchro-

nize the internal clock to an external clock source connected

on the MODE/PLLIN pin. The clock high level needs to be

higher than 1.6V and the clock low level needs to be lower

than 1V. The frequency programming voltage and or the

programming voltage divider must be removed from the

FREQ/PLLFLTR pin when synchronizing to an external

clock. The FREQ/PLLFLTR pin has the required onboard

PLL ﬁlter components for clock synchronization. The LTM

will default to forced continuous mode while being clock

synchronized. Channel 1 is synchronized to the rising

edge on the external clock, and channel 2 is 180 degrees

out-of-phase with the external clock.

APPLICATIONS INFORMATION

Frequency Selection

The switching frequency of the LTM4619’s controllers can

be selected using the FREQ/PLLFLTR pin. If the MODE/

PLLIN pin is not being driven by an external clock source,

the FREQ/PLLFLTR pin can be set from 0V to 2.4V to pro-

gram the controller’s operating frequency from 250kHz to

780kHz using a voltage divider to INTVCC (see Figure 19).

The typical frequency is 780kHz. If the output is too low

or the minimum on-time is reached, the frequency needs

to decrease to enlarge the turn-on time. Otherwise, a

signiﬁcant amount of cycle skipping can occur with cor-

respondingly larger current and voltage ripple.

Refer to the ﬁgure of Output Voltage vs Minimum On-Time

to choose a proper frequency.

Figure 2. Switching Frequency vs FREQ/PLLFLTR Pin Voltage

FREQ PIN VOLTAGE (V)

0.5

1.5

4619 F02

2.5

100

300

400

500

900

800

700

200

600

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