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LM2745 Datasheet Preview

LM2745 Datasheet

Synchronous Buck Controller

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LM2745, LM2748
www.ti.com
SNOSAL2E – APRIL 2005 – REVISED APRIL 2013
LM2745 / LM2748 Synchronous Buck Controller with Pre-Bias Startup, and Optional Clock
Synchronization
Check for Samples: LM2745, LM2748
FEATURES
1
2 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
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 include the ability to startup with a pre-
biased load on the output, soft-start, input
undervoltage lockout (UVLO) and Power Good
(based on both undervoltage and overvoltage
detection). In addition, the shutdown pin of the IC can
be used for providing startup delay, and the soft-start
pin can be used for implementing precise tracking, for
the purpose of sequencing with respect to an external
rail.
Typical Application
VCC = 3.3V
D1
CBOOT
RCC
RPULL-UP
CCC
Clk
CClk
RFADJ
CSS
VCC
HG
LM2745
SD BOOT
PWGD
ISEN
FREQ/SYNC
LG
SS/TRACK
PGND
SGND
PGND
EAO
FB
CC1
CC2
RC1
Q1
RCS
Q2
L1
RC2
CC3
VIN = 3.3V
+ CIN1,2
VOUT = 1.2V@16A
RFB2
+
CO1,2
RFB1
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
2
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2005–2013, Texas Instruments Incorporated




etcTI

LM2745 Datasheet Preview

LM2745 Datasheet

Synchronous Buck Controller

No Preview Available !

LM2745, LM2748
SNOSAL2E – APRIL 2005 – REVISED APRIL 2013
www.ti.com
1 14
2 BOOT LM2745
HG
13
LG PGND
3
PGND
12
SD
4
SGND
11
FREQ/SYNC
5
VCC
10
FB
6
PWGD
9
SS/TRACK
7 ISEN
EAO 8
1 14
BOOT
HG
2 13
LG
3
LM2748 PGND 12
PGND
SD
4
SGND
11
FREQ
5
VCC
10
FB
6
PWGD
9
SS/TRACK
7 ISEN
EAO 8
Figure 1. 14-Lead Plastic TSSOP, θJA = 155°C/W
Top View
Package Number PW0014A
Figure 2. 14-Lead Plastic TSSOP, θJA = 155°C/W
Top View
Package Number PW0014A
Pin Descriptions
BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the high-side gate driver. When the high-side MOSFET turns on, the voltage on this
pin should be at least one gate threshold above the regulator input voltage VIN to properly turn on the MOSFET. See MOSFET Gate Drivers
in the Application Information section for more details on how to select MOSFETs.
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 the pins must be connected together on
the PCB and form a ground plane, which is usually also 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.
VCC (Pin 5) Supply rail for the control sections of the IC.
PWGD (Pin 6) - Power Good pin. This is an open drain output, which is typically meant to be connected to VCC or 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. A recommended
value for the pull-up resistor is 100 kfor most applications. The voltage on this pin is thus pulled low under output undervoltage or
overvoltage fault conditions and also under input UVLO.
ISEN (Pin 7) - Current limit threshold setting pin. This sources 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 k.
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 reference voltage. For the basic soft-start function, a
capacitor of minimum value 1 nF is connected from this pin to ground. To track the rising ramp of another power supply’s output, connect 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/SYNC (Pin 11) - Frequency adjust pin. The switching frequency is set by connecting a resistor of suitable value between this pin and
ground. Some typical values (rounded up to the nearest standard values) are 150 kfor 200 kHz, 100 kfor 300 kHz, 51.1 kfor 500
kHz, 18.7 kfor 1 MHz. This pin is also used in the LM2745 to synchronize to an external clock.
SD (Pin 12) - IC shutdown pin. Pull this pin to VCC to ensure 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.
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
2 Submit Documentation Feedback
Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM2745 LM2748


Part Number LM2745
Description Synchronous Buck Controller
Maker etcTI
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