External Clock

Signal

To FREQ/SYNC Pin

+

60

LM2745, LM2748

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SNOSAL2E – APRIL 2005 – REVISED APRIL 2013

APPLICATION INFORMATION

The LM2745/8 is a voltage-mode, high-speed synchronous buck regulator with a PWM control scheme. It is

designed for use in set-top boxes, thin clients, DSL/Cable modems, and other applications that require high

efficiency buck converters. It has output shutdown (SD), input undervoltage lock-out (UVLO) mode and power

good (PWGD) flag (based on overvoltage and undervoltage detection). The overvoltage and undervoltage

signals are OR-gated to drive the power good signal and provide a logic signal to the system if the output voltage

LM2745/8 is also able to start-up with the output pre-biased with a load. The LM2745 also allows for the

switching frequency to be synchronized with an external clock source.

START UP/SOFT-START

internal, fixed 10 µA source begins charging the soft-start capacitor. During soft-start the voltage on the soft-start

until the voltage on the soft-start capacitor exceeds the LM2745/8 reference voltage of 0.6V. At this point the

length of the soft-start period, and can be approximated by:

During soft start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of

0.6V. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage

monitoring starts.

SETTING THE OUTPUT VOLTAGE

The LM2745/8 regulates the output voltage by controlling the duty cycle of the high side and low side MOSFETs

(see Typical Application Circuit).The equation governing output voltage is:

SETTING THE SWITCHING FREQUENCY

During fixed-frequency mode of operation the PWM frequency is adjustable between 50 kHz and 1 MHz and is

desired frequency is approximated by the curve FREQUENCY vs. FREQUENCY ADJUST RESISTOR in the

Typical Performance Characteristics section.

When it is desired to synchronize the switching frequency with an external clock source, the LM2745 has the

unique ability to synchronize from this external source within the range of 250 kHz to 1 MHz. The external clock

that the fixed frequency is approximately within ±30% of the external synchronizing clock frequency. An internal

protection diode clamps the low level of the synchronizing signal to approximately -0.5V. The internal clock

sychronizes to the rising edge of the external clock.

Figure 17. AC Coupled Clock

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Product Folder Links: LM2745 LM2748