LM2745, LM2748

SNOSAL2E – APRIL 2005 – REVISED APRIL 2013

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It is recommended to choose an AC coupling capacitance in the range of 50 pF to 100 pF. Exceeding the

recommended capacitance may inject excessive energy through the internal clamping diode structure present on

the FREQ/SYNC pin.

The typical trip level of the synchronization pin is 1.5V. To ensure proper synchronization and to avoid damaging

must be greater than 100 ns, and it's maximum width must be 100ns less than the period of the switching cycle.

The external clock synchronization process begins once the LM2745 is enabled and an external clock signal is

detected. During the external clock synchronization process the internal clock initially switches at approximately

1.5 MHz and decreases until it has matched the external clock’s frequency. The lock-in period is approximately

30 µs if the external clock is switching at 1 MHz, and about 100 µs if the external clock is at 200 kHz. When

there is no clock signal present, the LM2745 enters into fixed-frequency mode and begins switching at the

LM2745 will enter fixed-frequency mode within two clock cycles. If the external clock is removed within the 30 µs

lock-in period, the LM2745 will re-enter fixed-frequency mode within two internal clock cycles after the lock-in

period.

OUTPUT PRE-BIAS STARTUP

If there is a pre-biased load on the output of the LM2745/8 during startup, the IC will disable switching of the low-

side MOSFET and monitor the SW node voltage during the off-time of the high-side MOSFET. There is no load

current sensing while in pre-bias mode because the low-side MOSFET never turns on. The IC will remain in this

pre-bias mode until it sees the SW node stays below 0V during the entire high-side MOSFET's off-time. Once it

is determined that the SW node remained below 0V during the high-side off-time, the low-side MOSFET begins

switching during the next switching cycle. Figure 18 shows the SW node, HG, and LG signals during pre-bias

ensure that the High-Side MOSFET will be able to switch during startup.

Figure 18. Output Pre-Bias Mode Waveforms

TRACKING A VOLTAGE LEVEL

The LM2745/8 can track the output of a master power supply during soft-start by connecting a resistor divider to

the SS/TRACK pin. In this way, the output voltage slew rate of the LM2745/8 will be controlled by the master

supply for loads that require precise sequencing. When the tracking function is used no soft-start capacitor

soft-start pin and ground should be used.

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