lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 9

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

Block Diagram
Application Information
THEORY OF OPERATION
The LM2746 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 logically OR’ed 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
MOSFET.
START UP/SOFT-START
When V
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
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 LM2746 reference voltage
of 0.6V. At this point the reference voltage takes over at the
non-inverting error amplifier input. The capacitance of C
determines the length of the soft-start period, and can be
approximated by:
CC
exceeds 2.76V and the shutdown pin (SD) sees
DS
across the low side
SS
SS
is
9
Where C
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.
NORMAL OPERATION
While in normal operation mode, the LM2746 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:
The PWM frequency is adjustable between 50 kHz and 1
MHz and is set by an external resistor, R
FREQ pin and ground. The resistance needed for a desired
frequency is approximately:
SS
is in µF and t
SS
is in ms.
FADJ
, between the
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