LM5145 datasheet(20/62 Pages) TI

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frequency, use a resistor from the RT pin to AGND, or synchronize the LM5145-Q1 to an external clock signal

through the SYNCIN pin.

8.3.6.1 Frequency Adjust

Adjust the free-running switching frequency by using a resistor from the RT pin to AGND. The switching

frequency range is from 100 kHz to 1 MHz. The frequency set resistance, RRT, is governed by Equation 3. E96

standard-value resistors for common switching frequencies are given in Table 8-1.

kHz

(3)

Table 8-1. Frequency Set Resistors

SWITCHING FREQUENCY

(kHz)

FREQUENCY SET RESISTANCE

(kΩ)

100

200

49.9

250

40.2

300

33.2

400

24.9

500

750

13.3

1000

8.3.6.2 Clock Synchronization

Apply an external clock synchronization signal to the LM5145-Q1 to synchronize switching in both frequency and

phase. Requirements for the external clock SYNC signal are:

• Clock frequency range: 100 kHz to 1 MHz

• Clock frequency: –20% to +50% of the free-running frequency set by RRT

• Clock maximum voltage amplitude: 13 V

• Clock minimum pulse width: 50 ns

1 Ps/DIV

SYNCIN

2 V/DIV

SW 10 V/DIV

Figure 8-5. Typical 400-kHz SYNCIN and SW Voltage Waveforms

Figure 8-5 shows a clock signal at 400 kHz and the corresponding SW node waveform (VIN = 48 V, VOUT = 5 V,

free-running frequency = 280 kHz). The SW voltage waveform is synchronized with respect to the rising edge of

SYNCIN. The rising edge of the SW voltage is phase delayed relative to SYNCIN by approximately 100 ns.

LM5145-Q1

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