lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 2

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

www.national.com
Connection Diagram
Ordering Information
Pin Description
BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the
gate drivers. When the high-side MOSFET turns on, the
voltage on this pin should be at least one gate threshold
above the regulator input voltage V
MOSFET. See MOSFET Gate Drivers in the Application
Information section for more details on how to select MOS-
FETs.
LG (Pin 2) - Low-gate drive pin. This is the gate drive for the
low-side N-channel MOSFET. This signal is interlocked with
the high-side gate drive HG (Pin 14), so as to avoid shoot-
through.
PGND (Pins 3, 13) - Power ground. This is also the ground
for the low-side MOSFET driver. Both pins must be con-
nected together on the PCB to form a ground plane, which is
usually the system ground.
SGND (Pin 4) - Signal ground. It should be connected
appropriately to the ground plane with due regard to good
layout practices in switching power regulator circuits.
V
PWGD (Pin 6) - Power Good pin. This is an open drain
output, which is typically meant to be connected to V
any other low voltage source through a pull-up resistor.
Choose the pull-up resistor so that the current going into this
pin is kept below 1 mA. For most applications a recom-
mended value for the pull-up resistor is 100 kΩ. The voltage
on this pin is thus pulled low under output undervoltage or
overvoltage fault conditions and also under input UVLO.
I
a fixed 40 µA current. A resistor of appropriate value should
be connected between this pin and the drain of the low-side
MOSFET (switch node). The minimum value for this resistor
is 1 kΩ.
SEN
CC
(Pin 5) Supply rail for the control sections of the IC.
(Pin 7) - Current limit threshold setting pin. This sources
Order Number
LM2746MXAX
LM2746MXA
IN
to properly turn on the
Package Type
eTSSOP-14
eTSSOP-14
14-Lead Plastic eTSSOP
CC
or
(MXA14A)
2
NSC Package Drawing
EAO (Pin 8) - Output of the error amplifier. The voltage level
on this pin is compared with an internally generated ramp
signal to determine the duty cycle. This pin is necessary for
compensating the control loop.
SS/TRACK (Pin 9) - Soft-start and tracking pin. This pin is
internally connected to the non-inverting input of the error
amplifier during soft-start, and in fact any time the SS/
TRACK pin voltage happens to be below the internal refer-
ence voltage. For the basic soft-start function, a capacitor of
minimum value 1nF is connected from this pin to ground. To
track the rising ramp of another power supply’s output, con-
nect a resistor divider from the output of that supply to this
pin as described in Application Information.
FB (Pin 10) - Feedback pin. This is the inverting input of the
error amplifier, which is used for sensing the output voltage
and compensating the control loop.
FREQ (Pin 11) - Frequency adjust pin. The switching fre-
quency is set by connecting a resistor of suitable value
between this pin and ground. The equation for calculating
the exact value is provided in Application Information, but
some typical values (rounded up to the nearest standard
values) are 324 kΩ for 100 kHz, 97.6 kΩ for 300 kHz, 56.2
kΩ for 500 kHz, 24.9 kΩ for 1 MHz.
SD (Pin 12) - IC shutdown pin. Pull this pin to V
the IC is enabled. Connect to ground to disable the IC. Under
shutdown, both high-side and low-side drives are off. This
pin also features a precision threshold for power supply
sequencing purposes, as well as a low threshold to ensure
minimal quiescent current.
HG (Pin 14) - High-gate drive pin. This is the gate drive for
the high-side N-channel MOSFET. This signal is interlocked
with LG (Pin 2) to avoid shoot-through.
EXPOSED PAD - Connect to ground.
MXA14A
MXA14A
20147702
2500 Units on Tape and Reel
94 Units, Rail
Supplied As
CC
to ensure