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LM2745MTCX датащи(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
номер детали LM2745MTCX
подробное описание детали  Synchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization
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производитель  NSC [National Semiconductor (TI)]
домашняя страница  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2745MTCX датащи(HTML) 8 Page - National Semiconductor (TI)

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Block Diagram
20137403
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 convert-
ers. 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 goes out of regulation. Current limit is
achieved by sensing the voltage V
DS across the low side
MOSFET. The 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
When V
CC exceeds 2.79V and the shutdown pin (SD) sees
a logic high, the soft-start period begins. Then an internal,
fixed 10 µA source begins charging the soft-start capacitor.
During soft-start the voltage on the soft-start capacitor C
SS is
connected internally to the non-inverting input of the error
amplifier. The soft-start period lasts until the voltage on the
soft-start capacitor exceeds the LM2745/8 reference voltage
of 0.6V. At this point the reference voltage takes over at the
non-inverting error amplifier input. The capacitance of C
SS
determines the length of the soft-start period, and can be
approximated by:
Where C
SS is in µF and tSS is in ms.
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 fre-
quency is adjustable between 50 kHz and 1 MHz and is set
by an external resistor, R
FADJ, between the FREQ/SYNC pin
and ground. The resistance needed for a desired frequency
www.national.com
8


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