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LM2742
N-Channel FET Synchronous Buck Regulator Controller
for Low Output Voltages
General Description
The LM2742 is a high-speed, synchronous, switching regu-
lator controller. It is intended to control currents of 0.7A to
20A with up to 95% conversion efficiencies. Power up and
down sequencing is achieved with the power-good flag,
adjustable soft-start and output enable features. The
LM2742 operates from a low-current 5V bias and can con-
vert from a 1V to 16V power rail. The part utilizes a fixed-
frequency, voltage-mode, PWM control architecture and the
switching frequency is adjustable from 50kHz to 2MHz by
setting the value of an external resistor. Current limit is
achieved by monitoring the voltage drop across the on-
resistance of the low-side MOSFET, which enables on-times
on the order of 40ns, one of the best in the industry. The wide
range of operating frequencies gives the power supply de-
signer the flexibility to fine-tune component size, cost, noise
and efficiency. The adaptive, non-overlapping MOSFET
gate-drivers and high-side bootstrap structure helps to fur-
ther maximize efficiency. The high-side power FET drain
voltage can be from 1V to 16V and the output voltage is
adjustable down to 0.6V.
Features
n
Input power from 1V to 16V
n
Output voltage adjustable down to 0.6V
n
Power Good flag, adjustable soft-start and output enable
for easy power sequencing
n
Reference Accuracy: 1.5% (0C - 125C)
n
Current limit without sense resistor
n
Soft start
n
Switching frequency from 50 kHz to 2 MHz
n
40ns typical minimum on-time
n
TSSOP-14 package
Applications
n
POL power supply modules
n
Cable Modems
n
Set-Top Boxes/ Home Gateways
n
DDR Core Power
n
High-Efficiency Distributed Power
n
Local Regulation of Core Power
Typical Application
20087510
PRELIMINARY
March 2004
LM2742
N-Channel
FET
Synchronous
Buck
Regulator
Controller
for
Low
Output
V
oltages
2004 National Semiconductor Corporation
DS200875
www.national.com
Connection Diagram
20087511
14-Lead Plastic TSSOP
JA
= 155C/W
NS Package Number MTC14
Pin Description
BOOT (Pin 1) - Supply rail for the N-channel MOSFET gate
drive. The voltage should be at least one gate threshold
above the regulator input voltage to properly turn on the
high-side N-FET.
LG (Pin 2) - Gate drive for the low-side N-channel MOSFET.
This signal is interlocked with HG to avoid shoot-through
problems.
PGND (Pins 3, 13) - Ground for FET drive circuitry. It should
be connected to system ground.
SGND (Pin 4) - Ground for signal level circuitry. It should be
connected to system ground.
V
CC
(Pin 5) - Supply rail for the controller.
PWGD (Pin 6) - Power Good. This is an open drain output.
The pin is pulled low when the chip is in UVP, OVP, or UVLO
mode. During normal operation, this pin is connected to V
CC
or other voltage source through a pull-up resistor.
ISEN (Pin 7) - Current limit threshold setting. This sources a
fixed 50A current. A resistor of appropriate value should be
connected between this pin and the drain of the low-side
FET.
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 (Pin 9) - Soft start pin. A capacitor connected between
this pin and ground sets the speed at which the output
voltage ramps up. Larger capacitor value results in slower
output voltage ramp but also lower inrush current.
FB (Pin 10) - This is the inverting input of the error amplifier,
which is used for sensing the output voltage and compen-
sating the control loop.
FREQ (Pin 11) - The switching frequency is set by connect-
ing a resistor between this pin and ground.
SD (Pin 12) - IC Logic Shutdown. When this pin is pulled low
the chip turns off both the high side and low side switches.
While this pin is low, the IC will not start up. An internal 20A
pull-up connects this pin to V
CC
. For a device which turns on
the low side switch during shutdown, see the pin compatible
LM2737.
HG (Pin 14) - Gate drive for the high-side N-channel MOS-
FET. This signal is interlocked with LG to avoid shoot-
through problems.
LM2742
www.national.com
2
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
CC
7V
BOOTV
21V
LG and HG to GND (Note 3)
-2V to 21V
Junction Temperature
150C
Storage Temperature
-65C to 150C
Soldering Information
Lead Temperature
(soldering, 10sec)
260C
Infrared or Convection (20sec)
235C
ESD Rating
2 kV
Operating Ratings
Supply Voltage (V
CC
)
4.5V to 5.5V
Junction Temperature Range
-40C to +125C
Thermal Resistance (
JA
)
155C/W
Electrical Characteristics
V
CC
= 5V unless otherwise indicated. Typicals and limits appearing in plain type apply for T
A
=T
J
=+25C. Limits appearing in
boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design,
test, or statistical analysis.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
FB_ADJ
FB Pin Voltage
V
CC
= 4.5V, 0C to +125C
0.591
0.6
0.609
V
V
CC
= 5V, 0C to +125C
0.591
0.6
0.609
V
CC
= 5.5V, 0C to +125C
0.591
0.6
0.609
V
CC
= 4.5V, -40C to +125C
0.589
0.6
0.609
V
CC
= 5V, -40C to +125C
0.589
0.6
0.609
V
CC
= 5.5V, -40C to +125C
0.589
0.6
0.609
V
ON
UVLO Thresholds
Rising
Falling
4.2
3.6
V
I
Q-V5
Operating V
CC
Current
SD = 5V, FB = 0.55V
Fsw = 600kHz
1
1.5
2
mA
SD = 5V, FB = 0.65V
Fsw = 600kHz
0.8
1.7
2.2
Shutdown V
CC
Current
SD = 0V
0.15
0.4
0.7
mA
t
PWGD1
PWGD Pin Response Time
FB Voltage Going Up
6
s
t
PWGD2
PWGD Pin Response Time
FB Voltage Going Down
6
s
I
SD
SD Pin Internal Pull-up Current
20
A
I
SS-ON
SS Pin Source Current
SS Voltage = 2.5V
0C to +125C
-40C to +125C
8
5
11
11
15
15
A
I
SS-OC
SS Pin Sink Current During Over
Current
SS Voltage = 2.5V
95
A
I
SEN-TH
I
SEN
Pin Source Current Trip
Point
0C to +125C
-40C to +125C
35
28
50
50
65
65
A
ERROR AMPLIFIER
GBW
Error Amplifier Unity Gain
Bandwidth
5
MHz
G
Error Amplifier DC Gain
60
dB
SR
Error Amplifier Slew Rate
6
V/A
I
FB
FB Pin Bias Current
FB = 0.55V
FB = 0.65V
0
0
15
30
100
155
nA
I
EAO
EAO Pin Current Sourcing and
Sinking
V
EAO
= 2.5, FB = 0.55V
V
EAO
= 2.5, FB = 0.65V
2.8
0.8
mA
V
EA
Error Amplifier Maximum Swing
Minimum
Maximum
1.2
3.2
V
LM2742
www.national.com
3
Electrical Characteristics
(Continued)
V
CC
= 5V unless otherwise indicated. Typicals and limits appearing in plain type apply for T
A
=T
J
=+25C. Limits appearing in
boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design,
test, or statistical analysis.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
GATE DRIVE
I
Q-BOOT
BOOT Pin Quiescent Current
BOOT = 12V, EN = 0
0C to +125C
-40C to +125C
95
95
160
215
A
R
DS1
Top FET Driver Pull-Up ON
resistance
BOOT-SW = 5V
@
350mA
3
R
DS2
Top FET Driver Pull-Down ON
resistance
BOOT-SW = 5V
@
350mA
2
R
DS3
Bottom FET Driver Pull-Up ON
resistance
BOOT-SW = 5V
@
350mA
3
R
DS4
Bottom FET Driver Pull-Down
ON resistance
BOOT-SW = 5V
@
350mA
2
OSCILLATOR
f
OSC
PWM Frequency
R
FADJ
= 590k
50
kHz
R
FADJ
= 88.7k
300
R
FADJ
= 42.2k
, 0C to +125C
500
600
700
R
FADJ
= 42.2k
, -40C to +125C
490
600
700
R
FADJ
= 17.4k
1400
R
FADJ
= 11.3k
2000
D
Max Duty Cycle
f
PWM
= 300kHz
f
PWM
= 600kHz
90
88
%
t
on-min
Minimum on-time
40
ns
LOGIC INPUTS AND OUTPUTS
V
SD-IH
SD Pin Logic High Trip Point
2.6
3.5
V
V
SD-IL
SD Pin Logic Low Trip Point
0C to +125C
-40C to +125C
1.3
1.25
1.6
1.6
V
V
PWGD-TH-LO
PWGD Pin Trip Points
FB Voltage Going Down
0C to +125C
-40C to +125C
0.413
0.410
0.430
0.430
0.446
0.446
V
V
PWGD-TH-HI
PWGD Pin Trip Points
FB Voltage Going Up
0C to +125C
-40C to +125C
0.691
0.688
0.710
0.710
0.734
0.734
V
V
PWGD-HYS
PWGD Hysteresis
FB Voltage Going Down FB Voltage
Going Up
35
110
mV
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device
operates correctly. Opearting Ratings do not imply guaranteed performance limits.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin.
Note 3: The LG and HG pin can have -2V to -0.5V applied for a maximum duty cycle of 10% with a maximum period of 1 second. There is no duty cycle or maximum
period limitation for a LG and HG pin voltage range of -0.5V to 21V.
LM2742
www.national.com
4
Typical Performance Characteristics
Efficiency (V
O
= 1.5V)
F
SW
= 300kHz, T
A
= 25C
Efficiency (V
O
= 3.3V)
F
SW
= 300kHz, T
A
= 25C
20087512
20087513
V
CC
Operating Current vs Temperature
F
SW
= 600kHz, No-Load
Bootpin Current vs Temperature for BOOTV = 12V
F
SW
= 600kHz, Si4826DY FET, No-Load
20087514
20087515
Bootpin Current vs Temperature with 5V Bootstrap
F
SW
= 600kHz, Si4826DY FET, No-Load
PWM Frequency vs Temperature
for R
FADJ
= 43.2k
20087516
20087517
LM2742
www.national.com
5
Document Outline
- LM2742
- General Description
- Features
- Applications
- Typical Application
- Connection Diagram
- Pin Description
- Absolute Maximum Ratings
- Operating Ratings
- Electrical Characteristics
- Typical Performance Characteristics
- Block Diagram
- Application Information
- Example Circuits
- FIGURE 2. 5V-16V to 3.3V, 10A, 300kHz
- FIGURE 3. 5V to 1.2V, 10A, 300kHz
- FIGURE 4. 5V to 1.8V, 3A, 600kHz
- FIGURE 5. 3.3V to 0.8V, 5A, 500kHz
- FIGURE 6. 1.8V and 3.3V, 1A, 1.4MHz, Simultaneous
- FIGURE 7. 12V Unregulated to 3.3V, 3A, 750kHz
- FIGURE 8. 12V to 5V, 1.8A, 100kHz
- TABLE 1. Bill of Materials for Typical Application Circuit
- TABLE 2. Bill of Materials for Circuit of (Identical to BOM for 1.5V except as noted below)
- TABLE 3. Bill of Materials for Circuit of
- TABLE 4. Bill of Materials for Circuit of
- TABLE 5. Bill of Materials for Circuit of
- TABLE 6. Bill of Materials for Circuit of
- TABLE 7. Bill of Materials for 3.3V Circuit of (Identical to BOM for 1.8V except as noted below)
- TABLE 8. Bill of Materials for Circuit of
- TABLE 9. Bill of Materials for Circuit of
- Physical Dimensions
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