Datasheet

AGND

RT/CLK

External Clock

200 kHz to 1200 kHz

LMZ31704

SLVSBV8A –JUNE 2013–REVISED AUGUST 2013

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Synchronization (CLK)

An internal phase locked loop (PLL) has been implemented to allow synchronization between 200 kHz and

1200 kHz, and to easily switch from RT mode to CLK mode. To implement the synchronization feature, connect

a square wave clock signal to the RT/CLK pin with a duty cycle between 20% to 80%. The clock signal amplitude

must transition lower than 0.5 V and higher than 2.0 V. The start of the switching cycle is synchronized to the

falling edge of RT/CLK pin. In applications where both RT mode and CLK mode are needed, the device can be

configured as shown in Figure 35.

Before the external clock is present, the device works in RT mode and the switching frequency is set by RT

resistor. When the external clock is present, the CLK mode overrides the RT mode. The first time the CLK pin is

pulled above the RT/CLK high threshold (2.0 V), the device switches from RT mode to CLK mode and the

RT/CLK pin becomes high impedance as the PLL starts to lock onto the frequency of the external clock. It is not

recommended to switch from CLK mode back to RT mode because the internal switching frequency drops to

100 kHz first before returning to the switching frequency set by the RT resistor (R

Figure 35. RT/CLK Configuration

The synchronization frequency must be selected based on the output voltages of the devices being

synchronized. Table 9 shows the allowable frequencies for a given range of output voltages. For the most

efficient solution, always synchronize to the lowest allowable frequency. For example, an application requires

synchronizing three LMZ31704 devices with output voltages of 1.0 V, 1.2 V and 1.8 V, all powered from

PVIN = 12 V. Table 9 shows that all three output voltages should be synchronized to 300 kHz.

Table 9. Synchronization Frequency vs Output Voltage

PVIN = 12 V PVIN = 5 V

SYNCHRONIZATION

OUT

RANGE (V) V

OUT

RANGE (V)

FREQUENCY (kHz)

MIN MAX MIN MAX

200 0.6 1.3 0.6 1.5

300 0.8 2.0 0.6 4.3

400 1.1 2.5 0.6 4.3

500 1.4 3.4 0.6 4.3

600 1.6 5.0 0.7 4.3

700 1.9 5.5 0.8 4.3

800 2.1 5.5 0.9 4.3

900 2.4 5.5 1.0 4.3

1000 2.7 5.5 1.1 4.3

1100 2.9 5.5 1.3 4.3

1200 3.2 5.5 1.4 4.3

Product Folder Links: LMZ31704