SYNC

LM25574

UP TO 5 TOTAL

DEVICES

LM25574

SYNC

SYNC

AGND

LM25574

SW

CLK

SYNC

SW

500 ns

- 580 x 10

-9

135 x 10

-12

F

1

LM25574

SNVS483F – JANUARY 2007 – REVISED FEBRUARY 2009

www.ti.com

The SD pin can also be used to implement various remote enable / disable functions. Pulling the SD pin below

the 0.7V threshold totally disables the controller. If the SD pin voltage is above 1.225V the regulator will be

operational.

Oscillator and Sync Capability

The LM25574 oscillator frequency is set by a single external resistor connected between the RT pin and the

calculated from the following equation:

(1)

The SYNC pin can be used to synchronize the internal oscillator to an external clock. The external clock must be

output is the recommended interface from the external clock to the SYNC pin. The clock pulse duration should

be greater than 15ns.

Figure 3. Sync from External Clock

Figure 4. Sync from Multiple Devices

Multiple LM25574 devices can be synchronized together simply by connecting the SYNC pins together. In this

configuration all of the devices will be synchronized to the highest frequency device. The diagram in Figure 5

illustrates the SYNC input/output features of the LM25574. The internal oscillator circuit drives the SYNC pin with

a strong pull-down / weak pull-up inverter. When the SYNC pin is pulled low either by the internal oscillator or an

external clock, the ramp cycle of the oscillator is terminated and a new oscillator cycle begins. Thus, if the SYNC

pins of several LM25574 IC’s are connected together, the IC with the highest internal clock frequency will pull the

connected SYNC pins low first and terminate the oscillator ramp cycles of the other IC’s. The LM25574 with the

highest programmed clock frequency will serve as the master and control the switching frequency of the all the

devices with lower oscillator frequency.

10

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