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LM2748MTC View Datasheet(PDF) - National ->Texas Instruments

Part NameLM2748MTC National-Semiconductor
National ->Texas Instruments National-Semiconductor
DescriptionSynchronous Buck Controller with Pre-bias Startup, and Optional Clock Synchronization
LM2748MTC Datasheet PDF : 23 Pages
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Block Diagram
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 VDS 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 VCC 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 CSS 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 CSS
determines the length of the soft-start period, and can be
approximated by:
20137403
Where CSS 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, RFADJ, between the FREQ/SYNC pin
and ground. The resistance needed for a desired frequency
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DESCRIPTION
The LM2745/8 are high-speed synchronous buck regulator controllers with an accurate feedback voltage accuracy of ±1.5%. It can provide simple down conversion to output voltages as low as 0.6V. Though the control sections of the IC are rated for 3 to 6V, the driver sections are designed to accept input supply rails as high as 14V. The use of adaptive non-overlapping MOSFET gate drivers helps avoid potential shoot-through problems while maintaining high efficiency. The IC is designed for the more cost effective option of driving only N-channel MOSFETs in both the high-side and low-side positions. It senses the low-side switch voltage drop for providing a simple, adjustable current limit.
The LM2745/8 features a fixed-frequency voltage-mode PWM control architecture which is adjustable from 50 kHz to 1 MHz with one external resistor. In addition, the LM2745 also allows the switching frequency to be synchronized to an external clock signal over the range of 250 kHz to 1 MHz. This wide range of switching frequency gives the power supply designer the flexibility to make better tradeoffs between component size, cost and efficiency.

FEATURES
• Switching Frequency from 50 kHz to 1 MHz
• Switching Frequency Synchronize Range 250 kHz to 1 MHz (LM2745 Only)
• Startup with a Pre-biased Output Load
• Power Stage Input Voltage from 1V to 14V
• Control Stage Input Voltage from 3V to 6V
• Output Voltage Adjustable Down to 0.6V
• Power Good Flag and Shutdown
• Output Overvoltage and Undervoltage Detection
• ±1.5% Feedback Voltage Accuracy Over Temperature
• Low-side Adjustable Current Sensing
• Adjustable Soft-start
• Tracking and Sequencing with Shutdown and Soft Start Pins
• TSSOP-14 Package

APPLICATIONS
• Down Conversion from 3.3V
• Cable Modem, DSL and ADSL
• Laser Jet and Ink Jet Printers
• Low Voltage Power Modules
• DSP, ASIC, Core and I/O

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