NSC LM2742

March 2004
LM2742
N-Channel FET Synchronous Buck Regulator Controller
for Low Output Voltages
General Description
Features
The LM2742 is a high-speed, synchronous, switching regulator 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 convert from a 1V to 16V power rail. The part utilizes a fixedfrequency, 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 onresistance 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 designer 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 further 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.
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% (0˚C - 125˚C)
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
n
n
n
n
n
POL power supply modules
Cable Modems
Set-Top Boxes/ Home Gateways
DDR Core Power
High-Efficiency Distributed Power
Local Regulation of Core Power
Typical Application
20087510
© 2004 National Semiconductor Corporation
DS200875
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LM2742 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages
PRELIMINARY
LM2742
Connection Diagram
20087511
14-Lead Plastic TSSOP
θJA = 155˚C/W
NS Package Number MTC14
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 compensating the control loop.
FREQ (Pin 11) - The switching frequency is set by connecting 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 20µA
pull-up connects this pin to VCC. 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 MOSFET. This signal is interlocked with LG to avoid shootthrough problems.
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.
VCC (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 VCC
or other voltage source through a pull-up resistor.
ISEN (Pin 7) - Current limit threshold setting. This sources a
fixed 50µA 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.
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If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Lead Temperature
(soldering, 10sec)
260˚C
Infrared or Convection (20sec)
235˚C
ESD Rating
2 kV
7V
VCC
BOOTV
21V
LG and HG to GND (Note 3)
Operating Ratings
-2V to 21V
Junction Temperature
150˚C
Storage Temperature
−65˚C to 150˚C
Supply Voltage (VCC)
4.5V to 5.5V
Junction Temperature Range
−40˚C to +125˚C
Thermal Resistance (θJA)
Soldering Information
155˚C/W
Electrical Characteristics
VCC = 5V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA=TJ=+25˚C. 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
VFB_ADJ
VON
IQ-V5
Parameter
FB Pin Voltage
UVLO Thresholds
Operating VCC Current
Min
Typ
Max
VCC = 4.5V, 0˚C to +125˚C
Conditions
0.591
0.6
0.609
VCC = 5V, 0˚C to +125˚C
0.591
0.6
0.609
VCC = 5.5V, 0˚C to +125˚C
0.591
0.6
0.609
VCC = 4.5V, −40˚C to +125˚C
0.589
0.6
0.609
VCC = 5V, −40˚C to +125˚C
0.589
0.6
0.609
VCC = 5.5V, −40˚C to +125˚C
0.589
0.6
0.609
Rising
Falling
4.2
3.6
Units
V
V
SD = 5V, FB = 0.55V
Fsw = 600kHz
1
1.5
2
SD = 5V, FB = 0.65V
Fsw = 600kHz
0.8
1.7
2.2
0.15
0.4
0.7
mA
Shutdown VCC Current
SD = 0V
tPWGD1
PWGD Pin Response Time
FB Voltage Going Up
6
µs
tPWGD2
PWGD Pin Response Time
FB Voltage Going Down
6
µs
20
µA
ISD
ISS-ON
ISS-OC
ISEN-TH
SD Pin Internal Pull-up Current
SS Pin Source Current
SS Voltage = 2.5V
0˚C to +125˚C
-40˚C to +125˚C
8
5
SS Pin Sink Current During Over SS Voltage = 2.5V
Current
ISEN Pin Source Current Trip
Point
0˚C to +125˚C
-40˚C to +125˚C
11
11
15
15
95
35
28
50
50
mA
µA
µA
65
65
µA
ERROR AMPLIFIER
GBW
G
Error Amplifier Unity Gain
Bandwidth
5
MHz
Error Amplifier DC Gain
60
dB
SR
Error Amplifier Slew Rate
IFB
FB Pin Bias Current
FB = 0.55V
FB = 0.65V
6
IEAO
EAO Pin Current Sourcing and
Sinking
VEAO = 2.5, FB = 0.55V
VEAO = 2.5, FB = 0.65V
2.8
0.8
mA
VEA
Error Amplifier Maximum Swing
Minimum
Maximum
1.2
3.2
V
0
0
3
15
30
V/µA
100
155
nA
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LM2742
Absolute Maximum Ratings (Note 1)
LM2742
Electrical Characteristics
(Continued)
VCC = 5V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA=TJ=+25˚C. 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
BOOT = 12V, EN = 0
0˚C to +125˚C
-40˚C to +125˚C
95
95
160
215
µA
GATE DRIVE
IQ-BOOT
BOOT Pin Quiescent Current
RDS1
Top FET Driver Pull-Up ON
resistance
BOOT-SW = [email protected]
3
Ω
RDS2
Top FET Driver Pull-Down ON
resistance
BOOT-SW = [email protected]
2
Ω
RDS3
Bottom FET Driver Pull-Up ON
resistance
BOOT-SW = [email protected]
3
Ω
RDS4
Bottom FET Driver Pull-Down
ON resistance
BOOT-SW = [email protected]
2
Ω
OSCILLATOR
fOSC
D
ton-min
PWM Frequency
Max Duty Cycle
RFADJ = 590kΩ
50
RFADJ = 88.7kΩ
300
RFADJ = 42.2kΩ, 0˚C to +125˚C
500
600
700
RFADJ = 42.2kΩ, -40˚C to +125˚C
490
600
700
kHz
RFADJ = 17.4kΩ
1400
RFADJ = 11.3kΩ
2000
fPWM = 300kHz
fPWM = 600kHz
90
88
%
40
ns
Minimum on-time
LOGIC INPUTS AND OUTPUTS
VSD-IH
SD Pin Logic High Trip Point
VSD-IL
SD Pin Logic Low Trip Point
0˚C to +125˚C
-40˚C to +125˚C
1.3
1.25
1.6
1.6
PWGD Pin Trip Points
FB Voltage Going Down
0˚C to +125˚C
-40˚C to +125˚C
0.413
0.410
0.430
0.430
0.446
0.446
V
FB Voltage Going Up
0˚C to +125˚C
-40˚C to +125˚C
0.691
0.688
0.710
0.710
0.734
0.734
V
VPWGD-TH-LO
VPWGD-TH-HI
VPWGD-HYS
PWGD Pin Trip Points
PWGD Hysteresis
2.6
FB Voltage Going Down FB Voltage
Going Up
35
110
3.5
V
V
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.
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LM2742
Typical Performance Characteristics
Efficiency (VO = 3.3V)
FSW = 300kHz, TA = 25˚C
Efficiency (VO = 1.5V)
FSW = 300kHz, TA = 25˚C
20087512
20087513
Bootpin Current vs Temperature for BOOTV = 12V
FSW = 600kHz, Si4826DY FET, No-Load
VCC Operating Current vs Temperature
FSW = 600kHz, No-Load
20087515
20087514
PWM Frequency vs Temperature
for RFADJ = 43.2kΩ
Bootpin Current vs Temperature with 5V Bootstrap
FSW = 600kHz, Si4826DY FET, No-Load
20087516
20087517
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LM2742
Typical Performance Characteristics
(Continued)
RFADJ vs PWM Frequency
(in 100 to 800kHz range), TA = 25˚C
RFADJ vs PWM Frequency
(in 900 to 2000kHz range), TA = 25˚C
20087518
20087519
Switch Waveforms (HG Falling)
VIN = 5V, VO = 1.8V
IO = 3A, CSS = 10nF
FSW = 600kHz
VCC Operating Current Plus Boot Current vs
PWM Frequency (Si4826DY FET, TA = 25˚C)
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20087520
Start-Up (No-Load)
VIN = 10V, VO = 1.2V
CSS = 10nF, FSW = 300kHz
Switch Waveforms (HG Rising)
VIN = 5V, VO = 1.8V
IO = 3A, FSW = 600kHz
20087524
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20087521
6
LM2742
Typical Performance Characteristics
(Continued)
Start-Up (Full-Load)
VIN = 10V, VO = 1.2V
IO = 10A, CSS = 10nF
FSW = 300kHz
Start Up (No-Load, 10x CSS)
VIN = 10V, VO = 1.2V
CSS = 100nF, FSW = 300kHz
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20087526
Start Up (Into 1.2V Pre-Bias)
VIN = 12V, VO = 2.5V
No Load, No Soft Start Capacitor
FSW = 300kHz
Start Up (Full Load, 10x CSS)
VIN = 10V, VO = 1.2V
IO = 10A, CSS = 100nF
FSW = 300kHz
20087548
20087525
Shutdown
VIN = 12V, VO = 1.2V
IO = 10A, CSS = 10nF
FSW = 300kHz
Start Up (Into 1.2V Pre-Bias)
VIN = 12V, VO = 2.5V
No Load, CSS = 10nF
FSW = 300kHz
20087527
20087549
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LM2742
Typical Performance Characteristics
(Continued)
Shutdown (No Load)
VIN = 12V, VO = 1.2V
IO = 10A, CSS = 10nF
FSW = 300kHz
Load Transient Response (IO = 0 to 4A)
VIN = 12V, VO = 1.2V
FSW = 300kHz
20087533
20087528
Line Transient Response (VIN =5V to 12V)
VO = 1.2V, IO = 5A
FSW = 300kHz
Load Transient Response (IO = 4 to 0A)
VIN = 12V, VO = 1.2V
FSW = 300kHz
20087529
20087530
Line Transient Response
VO = 1.2V, IO = 5A
FSW = 300kHz
Line Transient Response (VIN =12V to 5V)
VO = 1.2V, IO = 5A
FSW = 300kHz
20087531
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20087532
8
LM2742
Block Diagram
20087501
case CSS would be 400nF. (390 10%) During soft start the
PWRGD flag is forced low and is released when the voltage
reaches a set value. At this point this chip enters normal
operation mode and the Power Good flag is released.
Since the output is floating when the LM2742 is turned off, it
is possible that the output capacitor may be precharged to
some positive value. During start-up, the LM2742 operates
fully synchronous and will discharge the output capacitor to
some extent depending on the output voltage, soft start
capacitance, and the size of the output capacitor.
Application Information
THEORY OF OPERATION
The LM2742 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 converters. It has power good (PWRGD), and output shutdown
(SD). Current limit is achieved by sensing the voltage VDS
across the low side FET. During current limit the high side
gate is turned off and the low side gate turned on. The soft
start capacitor is discharged by a 95µA source (reducing the
maximum duty cycle) until the current is under control.
NORMAL OPERATION
While in normal operation mode, the LM2742 regulates the
output voltage by controlling the duty cycle of the high side
and low side FETs. The equation governing output voltage
is:
VO = 0.6 x (RFB1 + RFB2) / RFB1
The PWM frequency is adjustable between 50kHz and
2MHz and is set by an external resistor, RFADJ, between the
FREQ pin and ground. The resistance needed for a desired
frequency is approximately:
START UP
When VCC exceeds 4.2V and the shutdown pin SD sees a
logic high the soft start capacitor begins charging through an
internal fixed 10µA source. During this time the output of the
error amplifier is allowed to rise with the voltage of the soft
start capacitor. This capacitor, CSS, determines soft start
time, and can be determined approximately by:
An application for a microprocessor might need a delay of
3ms, in which case CSS would be 12nF. For a different
device, a 100ms delay might be more appropriate, in which
9
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LM2742
Application Information
until VCC rises above 4.2V. As with shutdown, the soft start
capacitor is discharged through a FET, ensuring that the next
start-up will be smooth.
(Continued)
MOSFET GATE DRIVERS
The LM2742 has two gate drivers designed for driving
N-channel MOSFETs in a synchronous mode. Power for the
drivers is supplied through the BOOT pin. For the high side
gate (HG) to fully turn on the top FET, the BOOT voltage
must be at least one VGS(th) greater than Vin. (BOOT ≥
2*Vin) This voltage can be supplied by a separate, higher
voltage source, or supplied from a local charge pump structure. In a system such as a desktop computer, both 5V and
12V are usually available. Hence if Vin was 5V, the 12V
supply could be used for BOOT. 12V is more than 2*Vin, so
the HG would operate correctly. For a BOOT of 12V, the
initial gate charging current is 2A, and the initial gate discharging current is typically 6A.
CURRENT LIMIT
Current limit is realized by sensing the voltage across the
low side FET while it is on. The RDSON of the FET is a known
value, hence the current through the FET can be determined
as:
VDS = I * RDSON
The current limit is determined by an external resistor, RCS,
connected between the switch node and the ISEN pin. A
constant current of 50µA is forced through Rcs, causing a
fixed voltage drop. This fixed voltage is compared against
VDS and if the latter is higher, the current limit of the chip has
been reached. RCS can be found by using the following:
RCS = RDSON(LOW) * ILIM/50µA
For example, a conservative 15A current limit in a 10A
design with a minimum RDSON of 10mΩ would require a
3.3kΩ resistor. Because current sensing is done across the
low side FET, no minimum high side on-time is necessary. In
the current limit mode the LM2742 will turn the high side off
and the keep low side on for as long as necessary. The chip
also discharges the soft start capacitor through a fixed 95µA
source. In this way, smooth ramping up of the output voltage
as with a normal soft start is ensured. The output of the
LM2742 internal error amplifier is limited by the voltage on
the soft start capacitor. Hence, discharging the soft start
capacitor reduces the maximum duty cycle D of the controller. During severe current limit, this reduction in duty cycle
will reduce the output voltage, if the current limit conditions
lasts for an extended time.
During the first few nanoseconds after the low side gate
turns on, the low side FET body diode conducts. This causes
an additional 0.7V drop in VDS. The range of VDS is normally
much lower. For example, if RDSON were 10mΩ and the
current through the FET was 10A, VDS would be 0.1V. The
current limit would see 0.7V as a 70A current and enter
current limit immediately. Hence current limit is masked during the time it takes for the high side switch to turn off and the
low side switch to turn on.
20087502
FIGURE 1. BOOT Supplied by Charge Pump
In a system without a separate, higher voltage, a charge
pump (bootstrap) can be built using a diode and small capacitor, Figure 1. The capacitor serves to maintain enough
voltage between the top FET gate and source to control the
device even when the top FET is on and its source has risen
up to the input voltage level.
The LM2742 gate drives use a BiCMOS design. Unlike some
other bipolar control ICs, the gate drivers have rail-to-rail
swing, ensuring no spurious turn-on due to capacitive coupling.
SHUT DOWN
If the shutdown pin SD is pulled low, the LM2742 discharges
the soft start capacitor through a MOSFET switch. The high
side and low side switches are turned off. The LM2742
remains in this state until SD is released.
DESIGN CONSIDERATIONS
The following is a design procedure for all the components
needed to create the circuit shown in Figure 3 in the Example Circuits section, a 5V in to 1.2V out converter, capable
of delivering 10A with an efficiency of 85%. The switching
frequency is 300kHz. The same procedures can be followed
to create many other designs with varying input voltages,
output voltages, and output currents.
POWER GOOD SIGNAL
The power good signal is the or-gated flag representing
over-voltage and under-voltage protection. If the output voltage is 18% over it’s nominal value, VFB = 0.7V, or falls 30%
below that value, VFB = 0.41V, the power good flag goes low.
It will return to a logic high whenever the feedback pin
voltage is between 70% and 118% of 0.6V. The power good
pin is an open drain output that can be pulled up to logic
voltages of 5V or less with a 10kΩ resistor.
INPUT CAPACITOR
The input capacitors in a Buck switching converter are subjected to high stress due to the input current waveform,
which is a square wave. Hence input caps are selected for
their ripple current capability and their ability to withstand the
heat generated as that ripple current runs through their ESR.
Input rms ripple current is approximately:
UVLO
The 4.2V turn-on threshold on VCC has a built in hysteresis
of 0.6V. Therefore, if VCC drops below 3.6V, the chip enters
UVLO mode. UVLO consists of turning off the top FET,
turning off the bottom FET, and remaining in that condition
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OUTPUT INDUCTOR
(Continued)
The output inductor forms the first half of the power stage in
a Buck converter. It is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple. (∆Io) The inductance is chosen by
selecting between tradeoffs in output ripple, efficiency, and
response time. The smaller the output inductor, the more
quickly the converter can respond to transients in the load
current. If the inductor value is increased, the ripple through
the output capacitor is reduced and thus the output ripple is
reduced. As shown in the efficiency calculations, a smaller
inductor requires a higher switching frequency to maintain
the same level of output current ripple. An increase in frequency can mean increasing loss in the FETs due to the
charging and discharging of the gates. Generally the switching frequency is chosen so that conduction loss outweighs
switching loss. The equation for output inductor selection is:
The power dissipated by each input capacitor is:
Here, n is the number of capacitors, and indicates that power
loss in each cap decreases rapidly as the number of input
caps increase. The worst-case ripple for a Buck converter
occurs during full load, when the duty cycle D = 50%.
In the 5V to 1.2V case, D = 1.2/5 = 0.24. With a 10A
maximum load the ripple current is 4.3A. The Sanyo
10MV5600AX aluminum electrolytic capacitor has a ripple
current rating of 2.35A, up to 105˚C. Two such capacitors
make a conservative design that allows for unequal current
sharing between individual caps. Each capacitor has a maximum ESR of 18mΩ at 100 kHz. Power loss in each device is
then 0.05W, and total loss is 0.1W. Other possibilities for
input and output capacitors include MLCC, tantalum,
OSCON, SP, and POSCAPS.
A good range for ∆Io is 25 to 50% of the output current. In the
past, 30% was considered a maximum value for output
currents higher than about 2Amps, but as output capacitor
technology improves the ripple current can be allowed to
increase. Plugging in the values for output current ripple,
input voltage, output voltage, switching frequency, and assuming a 40% peak-to-peak output current ripple yields an
inductance of 1.5µH. The output inductor must be rated to
handle the peak current (also equal to the peak switch
current), which is (Io + 0.5*∆Io). This is 12A for a 10A design.
The Coilcraft D05022-152HC is 1.5µH, is rated to 15Arms,
and has a DCR of 4mΩ.
INPUT INDUCTOR
The input inductor serves two basic purposes. First, in high
power applications, the input inductor helps insulate the
input power supply from switching noise. This is especially
important if other switching converters draw current from the
same supply. Noise at high frequency, such as that developed by the LM2742 at 1MHz operation, could pass through
the input stage of a slower converter, contaminating and
possibly interfering with its operation.
An input inductor also helps shield the LM2742 from high
frequency noise generated by other switching converters.
The second purpose of the input inductor is to limit the input
current slew rate. During a change from no-load to full-load,
the input inductor sees the highest voltage change across it,
equal to the full load current times the input capacitor ESR.
This value divided by the maximum allowable input current
slew rate gives the minimum input inductance:
OUTPUT CAPACITOR
The output capacitor forms the second half of the power
stage of a Buck switching converter. It is used to control the
output voltage ripple (∆Vo) and to supply load current during
fast load transients.
In this example the output current is 10A and the expected
type of capacitor is an aluminum electrolytic, as with the
input capacitors. (Other possibilities include ceramic, tantalum, and solid electrolyte capacitors, however the ceramic
type often do not have the large capacitance needed to
supply current for load transients, and tantalums tend to be
more expensive than aluminum electrolytic.) Aluminum capacitors tend to have very high capacitance and fairly low
ESR, meaning that the ESR zero, which affects system
stability, will be much lower than the switching frequency.
The large capacitance means that at switching frequency,
the ESR is dominant, hence the type and number of output
capacitors is selected on the basis of ESR. One simple
formula to find the maximum ESR based on the desired
output voltage ripple, ∆Vo and the designed output current
ripple, ∆Io, is:
In the case of a desktop computer system, the input current
slew rate is the system power supply or "silver box" output
current slew rate, which is typically about 0.1A/µs. Total input
capacitor ESR is 9mΩ, hence ∆V is 10*0.009 = 90 mV, and
the minimum inductance required is 0.9µH. The input inductor should be rated to handle the DC input current, which is
approximated by:
In this example, in order to maintain a 2% peak-to-peak
output voltage ripple and a 40% peak-to-peak inductor current ripple, the required maximum ESR is 6mΩ. Three Sanyo
10MV5600AX capacitors in parallel will give an equivalent
ESR of 6mΩ. The total bulk capacitance of 16.8mF is
In this case IIN-DC is about 2.8A. One possible choice is the
TDK SLF12575T-1R2N8R2, a 1.2µH device that can handle
8.2Arms, and has a DCR of 7mΩ.
11
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LM2742
Application Information
LM2742
Application Information
RIN and CIN are standard filter components designed to
ensure smooth DC voltage for the chip supply. Depending on
noise, RIN should be 10 to 100Ω, and CIN should be between
0.1 and 2.2 µF. CBOOT is the bootstrap capacitor, and should
be 0.1µF. (In the case of a separate, higher supply to the
BOOT pin, this 0.1µF cap can be used to bypass the supply.)
Using a Schottky device for the bootstrap diode allows the
minimum drop for both high and low side drivers. The On
Semiconductor BAT54 or MBR0520 work well.
(Continued)
enough to supply even severe load transients. Using the
same capacitors for both input and output also keeps the bill
of materials simple.
MOSFETS
MOSFETS are a critical part of any switching controller and
have a direct impact on the system efficiency. In this case
the target efficiency is 85% and this is the variable that will
determine which devices are acceptable. Loss from the capacitors, inductors, and the LM2742 itself are detailed in the
Efficiency section, and come to about 0.54W. To meet the
target efficiency, this leaves 1.45W for the FET conduction
loss, gate charging loss, and switching loss. Switching loss
is particularly difficult to estimate because it depends on
many factors. When the load current is more than about 1 or
2 amps, conduction losses outweigh the switching and gate
charging losses. This allows FET selection based on the
RDSON of the FET. Adding the FET switching and gatecharging losses to the equation leaves 1.2W for conduction
losses. The equation for conduction loss is:
PCnd = D(I2o * RDSON *k) + (1-D)(I2o * RDSON *k)
The factor k is a constant which is added to account for the
increasing RDSON of a FET due to heating. Here, k = 1.3. The
Si4442DY has a typical RDSON of 4.1mΩ. When plugged into
the equation for PCND the result is a loss of 0.533W. If this
design were for a 5V to 2.5V circuit, an equal number of
FETs on the high and low sides would be the best solution.
With the duty cycle D = 0.24, it becomes apparent that the
low side FET carries the load current 76% of the time.
Adding a second FET in parallel to the bottom FET could
improve the efficiency by lowering the effective RDSON. The
lower the duty cycle, the more effective a second or even
third FET can be. For a minimal increase in gate charging
loss (0.054W) the decrease in conduction loss is 0.15W.
What was an 85% design improves to 86% for the added
cost of one SO-8 MOSFET.
Rp is a standard pull-up resistor for the open-drain power
good signal, and should be 10kΩ. If this feature is not
necessary, it can be omitted.
RCS is the resistor used to set the current limit. Since the
design calls for a peak current magnitude (Io + 0.5 * ∆Io) of
12A, a safe setting would be 15A. (This is well below the
saturation current of the output inductor, which is 25A.)
Following the equation from the Current Limit section, use a
3.3kΩ resistor.
RFADJ is used to set the switching frequency of the chip.
Following the equation in the Theory of Operation section,
the closest 1% tolerance resistor to obtain fSW = 300kHz is
88.7kΩ.
CSS depends on the users requirements. Based on the
equation for CSS in the Theory of Operation section, for a
3ms delay, a 12nF capacitor will suffice.
EFFICIENCY CALCULATIONS
A reasonable estimation of the efficiency of a switching
controller can be obtained by adding together the loss is
each current carrying element and using the equation:
The following shows an efficiency calculation to complement
the Circuit of Figure 3. Output power for this circuit is 1.2V x
10A = 12W.
Chip Operating Loss
PIQ = IQ-VCC *VCC
2mA x 5V = 0.01W
FET Gate Charging Loss
PGC = n * VCC * QGS * fOSC
The value n is the total number of FETs used. The Si4442DY
has a typical total gate charge, QGS, of 36nC and an rds-on of
4.1mΩ. For a single FET on top and bottom:
2*5*36E-9*300,000 = 0.108W
FET Switching Loss
PSW = 0.5 * Vin * IO * (tr + tf)* fOSC
CONTROL LOOP COMPONENTS
The circuit is this design example and the others shown in
the Example Circuits section have been compensated to
improve their DC gain and bandwidth. The result of this
compensation is better line and load transient responses.
For the LM2742, the top feedback divider resistor, Rfb2, is
also a part of the compensation. For the 10A, 5V to 1.2V
design, the values are:
Cc1 = 4.7pF 10%, Cc2 = 1nF 10%, Rc = 229kΩ 1%. These
values give a phase margin of 63˚ and a bandwidth of
29.3kHz.
SUPPORT CAPACITORS AND RESISTORS
The Cinx capacitors are high frequency bypass devices,
designed to filter harmonics of the switching frequency and
input noise. Two 1µF ceramic capacitors with a sufficient
voltage rating (10V for the Circuit of Figure 3) will work well
in almost any case.
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The Si4442DY has a typical rise time tr and fall time tf of 11
and 47ns, respectively. 0.5*5*10*58E-9*300,000 = 0.435W
12
LM2742
Application Information
Input Inductor Loss
(Continued)
PLin = I2in * DCRinput-L
FET Conduction Loss
PCn = 0.533W
Input Capacitor Loss
2.822*0.007 = 0.055W
Output Inductor Loss
PLout = I2o * DCRoutput-L
2
10 *0.004 = 0.4W
System Efficiency
2
4.28 *0.018/2 = 0.164W
Example Circuits
20087503
FIGURE 2. 5V-16V to 3.3V, 10A, 300kHz
This circuit and the one featured on the front page have been
designed to deliver high current and high efficiency in a small
package, both in area and in height The tallest component in
this circuit is the inductor L1, which is 6mm tall. The compensation has been designed to tolerate input voltages from
5 to 16V.
13
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LM2742
Example Circuits
(Continued)
20087504
FIGURE 3. 5V to 1.2V, 10A, 300kHz
This circuit design, detailed in the Design Considerations
section, uses inexpensive aluminum capacitors and off-theshelf inductors. It can deliver 10A at better than 85% efficiency. Large bulk capacitance on input and output ensure
stable operation.
20087505
FIGURE 4. 5V to 1.8V, 3A, 600kHz
The example circuit of Figure 4 has been designed for
minimum component count and overall solution size. A
switching frequency of 600kHz allows the use of small input/
output capacitors and a small inductor. The availability of
separate 5V and 12V supplies (such as those available from
desk-top computer supplies) and the low current further
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reduce component count. Using the 12V supply to power the
MOSFET drivers eliminates the bootstrap diode, D1. At low
currents, smaller FETs or dual FETs are often the most
efficient solutions. Here, the Si4826DY, an asymmetric dual
FET in an SO-8 package, yields 92% efficiency at a load of
2A.
14
LM2742
Example Circuits
(Continued)
20087506
FIGURE 5. 3.3V to 0.8V, 5A, 500kHz
The circuit of Figure 5 demonstrates the LM2742 delivering a
low output voltage at high efficiency (87%). A separate 5V
supply is required to run the chip, however the input voltage
can be as low as 2.2
15
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LM2742
Example Circuits
(Continued)
20087507
FIGURE 6. 1.8V and 3.3V, 1A, 1.4MHz, Simultaneous
The circuits in Figure 6 are intended for ADSL applications,
where the high switching frequency keeps noise out of the
data transmission range. In this design, the 1.8 and 3.3V
outputs come up simultaneously by using the same softstart
capacitor. Because two current sources now charge the
same capacitor, the capacitance must be doubled to achieve
www.national.com
the same softstart time. (Here, 40nF is used to achieve a
5ms softstart time.) A common softstart capacitor means
that, should one circuit enter current limit, the other circuit
will also enter current limit. The additional compensation
components Rc2 and Cc3 are needed for the low ESR, all
ceramic output capacitors, and the wide (3x) range of Vin.
16
LM2742
Example Circuits
(Continued)
20087508
FIGURE 7. 12V Unregulated to 3.3V, 3A, 750kHz
This circuit shows the LM2742 paired with a cost effective
solution to provide the 5V chip power supply, using no extra
components other than the LM78L05 regulator itself. The
input voltage comes from a ’brick’ power supply which does
not regulate the 12V line tightly. Additional, inexpensive 10uF
ceramic capacitors (Cinx and Cox) help isolate devices with
sensitive databands, such as DSL and cable modems, from
switching noise and harmonics.
20087509
FIGURE 8. 12V to 5V, 1.8A, 100kHz
In situations where low cost is very important, the LM2742
can also be used as an asynchronous controller, as shown in
the above circuit. Although a a schottky diode in place of the
bottom FET will not be as efficient, it will cost much less than
the FET. The 5V at low current needed to run the LM2742
could come from a zener diode or inexpensive regulator,
such as the one shown in Figure 7. Because the LM2742
senses current in the low side MOSFET, the current limit
feature will not function in an asynchronous design. The
ISEN pin should be left open in this case.
17
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LM2742
TABLE 1. Bill of Materials for Typical Application Circuit
ID
U1
Part Number
LM2742
Q1, Q2
Si4884DY
L1
RLF7030T-1R5N6R1
Type
Synchronous
Controller
Size
Parameters
Qty.
Vendor
TSSOP-14
TSSOP-14
1
NSC
N-MOSFET
Inductor
SO-8
30V, 13mΩ, 15nC
1
Vishay
7.1x7.1x3.2mm
1.5µH, 6.1A 9.6mΩ
1
TDK
TDK
Cin1, Cin2
C2012X5R1J106M
MLCC
0805
10µF 6.3V
2
Cinx
C3216X7R1E105K
Capacitor
1206
1µF, 25V
1
TDK
Co1, Co2
6MV2200WG
10mm D 20mm H
2200µF 6.3V125mΩ
2
Sanyo
Vishay
AL-E
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
Cc1
VJ1206A2R2KXX
Capacitor
1206
2.2pF 10%
1
Vishay
Cc2
VJ1206A181KXX
Capacitor
1206
180pF 10%
1
Vishay
Rin
CRCW1206100J
Resistor
1206
10Ω 5%
1
Vishay
Rfadj
CRCW12066342F
Resistor
1206
63.4kΩ 1%
1
Vishay
Rc1
CRCW12063923F
Resistor
1206
392kΩ 1%
1
Vishay
Rfb1
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Rfb2
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Rcs
CRCW1206222J
Resistor
1206
2.2kΩ 5%
1
Vishay
TABLE 2. Bill of Materials for Circuit of Figure 2
(Identical to BOM for 1.5V except as noted below)
ID
Part Number
Size
Parameters
Qty.
Vendor
L1
RLF12560T-2R7N110
Inductor
Type
12.5x12.8x6mm
2.7µH, 14.4A 4.5mΩ
1
TDK
Co1, Co2,
Co3, Co4
10TPB100M
POSCAP
7.3x4.3x2.8mm
100µF 10V 1.9Arms
4
Sanyo
Cc1
VJ1206A6R8KXX
Capacitor
1206
6.8pF 10%
1
Vishay
Cc2
VJ1206A271KXX
Capacitor
1206
270pF 10%
1
Vishay
Cc3
VJ1206A471KXX
Capacitor
1206
470pF 10%
1
Vishay
Rc2
CRCW12068451F
Resistor
1206
8.45kΩ 1%
1
Vishay
Rfb1
CRCW12061102F
Resistor
1206
11kΩ 1%
1
Vishay
Qty.
Vendor
1
NSC
TABLE 3. Bill of Materials for Circuit of Figure 3
ID
Part Number
Type
Synchronous
Controller
Size
Parameters
U1
LM2742
Q1
Si4442DY
N-MOSFET
SO-8
30V, 4.1mΩ, @ 4.5V,
36nC
1
Vishay
Q2
Si4442DY
N-MOSFET
SO-8
30V, 4.1mΩ, @ 4.5V,
36nC
1
Vishay
Schottky Diode
TSSOP-14
D1
BAT-54
SOT-23
30V
1
Vishay
Lin
SLF12575T-1R2N8R2
Inductor
12.5x12.5x7.5mm
12µH, 8.2A, 6.9mΩ
1
Coilcraft
L1
D05022-152HC
Inductor
22.35x16.26x8mm
1.5µH, 15A,4mΩ
1
Coilcraft
16mm D 25mm H
5600µF10V 2.35Arms
2
Sanyo
Cin1, Cin2
10MV5600AX
Aluminum
Electrolytic
Cinx
C3216X7R1E105K
Capacitor
1206
1µF, 25V
1
TDK
Co1, Co2,
Co3
10MV5600AX
Aluminum
Electrolytic
16mm D 25mm H
5600µF10V 2.35Arms
2
Sanyo
Vishay
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
www.national.com
18
ID
Part Number
Size
Parameters
Qty.
Vendor
Cc1
VJ1206A4R7KXX
Capacitor
Type
1206
4.7pF 10%
1
Vishay
Cc2
VJ1206A102KXX
Capacitor
1206
1nF 10%
1
Vishay
Vishay
Rin
CRCW1206100J
Resistor
1206
10Ω 5%
1
Rfadj
CRCW12068872F
Resistor
1206
88.7kΩ 1%
1
Vishay
Rc1
CRCW12062293F
Resistor
1206
229kΩ 1%
1
Vishay
Rfb1
CRCW12064991F
Resistor
1206
4.99kΩ 1%
1
Vishay
Rfb2
CRCW12064991F
Resistor
1206
4.99kΩ 1%
1
Vishay
Rcs
CRCW1206152J
Resistor
1206
1.5kΩ 5%
1
Vishay
ID
Part Number
Qty.
Vendor
U1
LM2742
1
NSC
Q1/Q2
Si4826DY
L1
DO3316P-222
Cin1
Co1
TABLE 4. Bill of Materials for Circuit of Figure 4
Type
Synchronous
Controller
Size
Parameters
TSSOP-14
Asymetric Dual
N-MOSFET
SO-8
30V, 24mΩ/ 8nC
Top 16.5mΩ/ 15nC
1
Vishay
Inductor
12.95x9.4x
5.21mm
2.2µH, 6.1A, 12mΩ
1
Coilcraft
10TPB100ML
POSCAP
7.3x4.3x3.1mm
100µF 10V 1.9Arms
1
Sanyo
4TPB220ML
POSCAP
7.3x4.3x3.1mm
220µF 4V 1.9Arms
1
Sanyo
Cc
C3216X7R1E105K
Capacitor
1206
1µF, 25V
1
TDK
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
Cc1
VJ1206A100KXX
Capacitor
1206
10pF 10%
1
Vishay
Cc2
VJ1206A561KXX
Capacitor
1206
560pF 10%
1
Vishay
Rin
CRCW1206100J
Resistor
1206
10Ω 5%
1
Vishay
Rfadj
CRCW12064222F
Resistor
1206
42.2kΩ 1%
1
Vishay
Rc1
CRCW12065112F
Resistor
1206
51.1kΩ 1%
1
Vishay
Rfb1
CRCW12062491F
Resistor
1206
2.49kΩ 1%
1
Vishay
Rfb2
CRCW12064991F
Resistor
1206
4.99kΩ 1%
1
Vishay
Rcs
CRCW1206272J
Resistor
1206
2.7kΩ 5%
1
Vishay
Qty.
Vendor
1
NSC
TABLE 5. Bill of Materials for Circuit of Figure 5
ID
Part Number
U1
LM2742
Type
Q1
Si4884DY
N-MOSFET
SO-8
30V, 13.5mΩ, @ 4.5V
15.3nC
1
Vishay
Q2
Si4884DY
N-MOSFET
SO-8
30V, 13.5mΩ, @ 4.5V
15.3nC
1
Vishay
Synchronous
Controller
Schottky Diode
Size
Parameters
TSSOP-14
D1
BAT-54
SOT-23
30V
1
Vishay
Lin
P1166.102T
Inductor
7.29x7.29 3.51mm
1µH, 11A 3.7mΩ
1
Pulse
L1
P1168.102T
Inductor
12x12x4.5 mm
1µH, 11A, 3.7mΩ
1
Pulse
Cin1
10MV5600AX
Aluminum
Electrolytic
16mm D 25mm H
5600µF 10V 2.35Arms
1
Sanyo
Cinx
C3216X7R1E105K
Capacitor
1206
1µF, 25V
1
TDK
Co1, Co2,
Co3
16MV4700WX
Aluminum
Electrolytic
12.5mm D 30mm
H
4700µF 16V 2.8Arms
2
Sanyo
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
Vishay
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
19
www.national.com
LM2742
TABLE 3. Bill of Materials for Circuit of Figure 3 (Continued)
LM2742
TABLE 5. Bill of Materials for Circuit of Figure 5 (Continued)
ID
Part Number
Size
Parameters
Qty.
Cc1
VJ1206A4R7KXX
Capacitor
Type
1206
4.7pF 10%
1
Vendor
Vishay
Cc2
VJ1206A681KXX
Capacitor
1206
680pF 10%
1
Vishay
Vishay
Rin
CRCW1206100J
Resistor
1206
10Ω 5%
1
Rfadj
CRCW12064992F
Resistor
1206
49.9kΩ 1%
1
Vishay
Rc1
CRCW12061473F
Resistor
1206
147kΩ 1%
1
Vishay
Rfb1
CRCW12061492F
Resistor
1206
14.9kΩ 1%
1
Vishay
Rfb2
CRCW12064991F
Resistor
1206
4.99kΩ 1%
1
Vishay
Rcs
CRCW1206332J
Resistor
1206
3.3kΩ 5%
1
Vishay
ID
Part Number
Qty.
Vendor
U1
LM2742
1
NSC
Q1/Q2
Si4826DY
Assymetric Dual
N-MOSFET
SO-8
30V, 24mΩ/ 8nC
Top 16.5mΩ/ 15nC
1
Vishay
D1
BAT-54
Schottky Diode
SOT-23
30V
1
Vishay
TDK
TABLE 6. Bill of Materials for Circuit of Figure 6
Type
Synchronous
Controller
Size
Parameters
TSSOP-14
Lin
RLF7030T-1R0N64
Inductor
6.8x7.1x3.2mm
1µH, 6.4A, 7.3mΩ
1
L1
RLF7030T-3R3M4R1
Inductor
6.8x7.1x3.2mm
3.3µH, 4.1A, 17.4mΩ
1
TDK
Cin1
C4532X5R1E156M
MLCC
1812
15µF 25V 3.3Arms
1
Sanyo
Co1
C4532X5R1E156M
MLCC
1812
15µF 25V 3.3Arms
1
Sanyo
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
TDK
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X393KXX
Capacitor
1206
39nF, 25V
1
Vishay
Cc1
VJ1206A220KXX
Capacitor
1206
22pF 10%
1
Vishay
Cc2
VJ1206A681KXX
Capacitor
1206
680pF 10%
1
Vishay
Cc3
VJ1206A681KXX
Capacitor
1206
680pF 10%
1
Vishay
Rin
CRCW1206100J
Resistor
1206
10Ω 5%
1
Vishay
Rfadj
CRCW12061742F
Resistor
1206
17.4kΩ 1%
1
Vishay
Rc1
CRCW12061072F
Resistor
1206
10.7kΩ 1%
1
Vishay
Rc2
CRCW120666R5F
Resistor
1206
66.5Ω 1%
1
Vishay
Rfb1
CRCW12064991F
Resistor
1206
4.99kΩ 1%
1
Vishay
Rfb2
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Rcs
CRCW1206152J
Resistor
1206
1.5kΩ 5%
1
Vishay
Qty.
Vendor
TABLE 7. Bill of Materials for 3.3V Circuit of Figure 6
(Identical to BOM for 1.8V except as noted below)
ID
Part Number
Type
Inductor
Size
Parameters
L1
RLF7030T-4R7M3R4
6.8x7.1x 3.2mm
4.7µH, 3.4A, 26mΩ
1
TDK
Cc1
VJ1206A270KXX
Capacitor
1206
27pF 10%
1
Vishay
Cc2
VJ1206X102KXX
Capacitor
1206
1nF 10%
1
Vishay
Cc3
VJ1206A821KXX
Capacitor
1206
820pF 10%
1
Vishay
Rc1
CRCW12061212F
Resistor
1206
12.1kΩ 1%
1
Vishay
Rc2
CRCW12054R9F
Resistor
1206
54.9Ω 1%
1
Vishay
Rfb1
CRCW12062211F
Resistor
1206
2.21kΩ 1%
1
Vishay
Rfb2
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Qty.
Vendor
1
NSC
TABLE 8. Bill of Materials for Circuit of Figure 7
ID
Part Number
U1
LM2742
www.national.com
Type
Synchronous
Controller
Size
TSSOP-14
20
Parameters
ID
Part Number
U2
LM78L05
Voltage
Regulator
Type
SO-8
Q1/Q2
Si4826DY
Assymetric Dual
N-MOSFET
SO-8
Schottky Diode
Size
Parameters
Qty.
Vendor
1
NSC
30V, 24mΩ/ 8nC
Top 16.5mΩ/ 15nC
1
Vishay
D1
BAT-54
SOT-23
30V
1
Vishay
Lin
RLF7030T-1R0N64
Inductor
6.8x7.1x3.2mm
1µH, 6.4A, 7.3mΩ
1
TDK
L1
SLF12565T-4R2N5R5
Inductor
12.5x12.5x6.5mm
4.2µH, 5.5A, 15mΩ
1
TDK
Cin1
16MV680WG
D: 10mm L:
12.5mm
680µF 16V 3.4Arms
1
Sanyo
Al-E
Cinx
C3216X5R1C106M
MLCC
1210
10µF 16V 3.4Arms
1
TDK
Co1 Co2
16MV680WG
MLCC
1812
15µF 25V 3.3Arms
1
Sanyo
Cox
C3216X5R10J06M
MLCC
1206
10µF 6.3V 2.7A
TDK
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
Vishay
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
Cc1
VJ1206A8R2KXX
Capacitor
1206
8.2pF 10%
1
Vishay
Cc2
VJ1206X102KXX
Capacitor
1206
1nF 10%
1
Vishay
Cc3
VJ1206X472KXX
Capacitor
1206
4.7nF 10%
1
Vishay
Rfadj
CRCW12063252F
Resistor
1206
32.5kΩ 1%
1
Vishay
Rc1
CRCW12065232F
Resistor
1206
52.3kΩ 1%
1
Vishay
Rc2
CRCW120662371F
Resistor
1206
2.37Ω 1%
1
Vishay
Rfb1
CRCW12062211F
Resistor
1206
2.21kΩ 1%
1
Vishay
Rfb2
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Rcs
CRCW1206202J
Resistor
1206
2kΩ 5%
1
Vishay
Qty.
Vendor
1
NSC
Vishay
TABLE 9. Bill of Materials for Circuit of Figure 8
ID
Part Number
U1
LM2742
Q1
Si4894DY
D2
MBRS330T3
L1
SLF12565T-470M2R4
D1
MBR0520
Cin1
16MV680WG
Cinx
C3216X5R1C106M
Co1, Co2
16MV680WG
Type
Synchronous
Controller
Size
Parameters
TSSOP-14
N-MOSFET
SO-8
30V, 15mΩ, 11.5nC
1
Schottky Diode
SO-8
30V, 3A
1
ON
12.5x12.8x 4.7mm
47µH, 2.7A 53mΩ
1
TDK
Inductor
Schottky Diode
1812
20V 0.5A
1
ON
Al-E
1206
680µF, 16V, 1.54Arms
1
Sanyo
MLCC
1206
10µF, 16V, 3.4Arms
1
TDK
D: 10mm L:
12.5mm
680µF 16V 26mΩ
2
Sanyo
Al-E
Cox
C3216X5R10J06M
MLCC
1206
10µF, 6.3V 2.7A
1
TDK
Cboot
VJ1206X104XXA
Capacitor
1206
0.1µF, 25V
1
Vishay
Cin
C3216X7R1E225K
Capacitor
1206
2.2µF, 25V
1
TDK
Css
VJ1206X123KXX
Capacitor
1206
12nF, 25V
1
Vishay
Cc1
VJ1206A561KXX
Capacitor
1206
56pF 10%
1
Vishay
Cc2
VJ1206X392KXX
Capacitor
1206
3.9nF 10%
1
Vishay
Cc3
VJ1206X223KXX
Capacitor
1206
22nF 10%
1
Vishay
Rfadj
CRCW12062673F
Resistor
1206
267kΩ 1%
1
Vishay
Rc1
CRCW12066192F
Resistor
1206
61.9kΩ 1%
1
Vishay
Vishay
Rc2
CRCW12067503F
Resistor
1206
750kΩ 1%
1
Rfb1
CRCW12061371F
Resistor
1206
1.37kΩ 1%
1
Vishay
Rfb2
CRCW12061002F
Resistor
1206
10kΩ 1%
1
Vishay
Rcs
CRCW1206122F
Resistor
1206
1.2kΩ 5%
1
Vishay
21
www.national.com
LM2742
TABLE 8. Bill of Materials for Circuit of Figure 7 (Continued)
LM2742 N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages
Physical Dimensions
inches (millimeters) unless otherwise noted
TSSOP-14 Pin Package
NS Package Number MTC14
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