NSC LM27342MYX 2 mhz 1.5a/2a wide input range step-down dc-dc regulator with frequency synchronization Datasheet

February 9, 2010
2 MHz 1.5A/2A Wide Input Range Step-Down DC-DC
Regulator with Frequency Synchronization
General Description
Features
The LM27341 and LM27342 regulators are monolithic, high
frequency, PWM step-down DC-DC converters in 10-pin LLP
and 10-pin eMSOP packages. They contain all the active
functions to provide local DC-DC conversion with fast transient response and accurate regulation in the smallest possible PCB area.
With a minimum of external components the LM27341 and
LM27342 are easy to use. The ability to drive 1.5A or 2A loads
respectively, with an internal 150 mΩ NMOS switch results in
the best power density available. The world-class control circuitry allows for on-times as low as 65 ns, thus supporting
exceptionally high frequency conversion. Switching frequency is internally set to 2 MHz and synchronizable from 1 to 2.35
MHz, which allows the use of extremely small surface mount
inductors and chip capacitors. Even though the operating frequency is very high, efficiencies up to 90% are easy to
achieve. External shutdown is included featuring an ultra-low
shutdown current of 70 nA. The LM27341and LM27342 utilize
peak current-mode control and internal compensation to provide high-performance regulation over a wide range of operating conditions. Additional features include internal soft-start
circuitry to reduce inrush current, pulse-by-pulse current limit,
thermal shutdown, and output over-voltage protection.
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Space saving 3 x 3 mm LLP-10 & eMSOP-10 package
Wide input voltage range
3V to 20V
Wide output voltage range
1V to 18V
LM27341 delivers 1.5A maximum output current
LM27342 delivers 2A maximum output current
High switching frequency
2 MHz
Frequency synchronization
1.00 MHz < fSW <
2.35 MHz
150 mΩ NMOS switch with internal bootstrap supply
70 nA shutdown current
Internal voltage reference accuracy of 1%
Peak Current-Mode, PWM operation
Thermal shutdown
LM27341Q and LM27342Q are AEC-Q100 Grade 1
qualified and are manufactured on an Automotive Grade
Flow
Applications
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Local 12V to Vcore Step-Down Converters
Radio Power Supply
Core Power in HDDs
Set-Top Boxes
Automotive
USB Powered Devices
DSL Modems
Typical Application Circuit
30005674
30005676
Efficiency vs Load Current
VOUT = 5V, fsw = 2 MHz
© 2010 National Semiconductor Corporation
300056
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LM27341/LM27342/LM27341Q/LM27342Q 2 MHz 1.5/2A Wide Input Range Step-Down DC-DC
Regulator with Frequency Synchronization
LM27341/LM27342/
LM27341Q/LM27342Q
LM27341/LM27342
Connection Diagrams
Top View
Top View
30005604
30005605
10 - Lead LLP
10 - Lead eMSOP
Ordering Information
Order Number
Package
Type
NSC Package
Drawing
Package
Marking
Supplied As
LM27341MY
eMSOP -10
MUC10A
SSCB
1000 Units on Tape and Reel
LM27341MYX
eMSOP -10
MUC10A
SSCB
3500 Units on Tape and Reel
LM27342MY
eMSOP -10
MUC10A
SSCA
1000 Units on Tape and Reel
LM27342MYX
eMSOP -10
MUC10A
SSCA
3500 Units on Tape and Reel
LM27341SD
LLP - 10
SDA10A
L231B
1000 Units on Tape and Reel
LM27341SDX
LLP - 10
SDA10A
L231B
4500 Units on Tape and Reel
1000 Units on Tape and Reel
LM27342SD
LLP - 10
SDA10A
L231A
LM27342SDX
LLP - 10
SDA10A
L231A
4500 Units on Tape and Reel
LM27341QMY
eMSOP -10
MUC10A
SSJB
1000 Units Per Anti-Static Tube
LM27341QMYX
eMSOP -10
MUC10A
SSJB
3500 Units on Tape and Reel
LM27342QMY
eMSOP -10
MUC10A
SSJA
1000 Units Per Anti-Static Tube
LM27342QMYX
eMSOP -10
MUC10A
SSJA
3500 Units on Tape and Reel
Feature
AEC-Q100 Grade 1
qualified. Automotive
Grade Production Flow*
*Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies.
Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified
with the letter Q. For more information go to http://www.national.com/automotive.
Pin Descriptions
Pin
Name
Function
1,2
SW
3
BOOST
Output switch. Connects to the inductor, catch diode, and bootstrap capacitor.
Boost voltage that drives the internal NMOS control switch. A bootstrap capacitor is connected between
the BOOST and SW pins.
4
EN
Enable control input. Logic high enables operation. Do not allow this pin to float or be greater than VIN
+ 0.3V.
5
SYNC
Frequency synchronization input. Drive this pin with an external clock or pulse train. Ground it to use
the internal clock.
6
FB
7
GND
Feedback pin. Connect FB to the external resistor divider to set output voltage.
Signal and Power Ground pin. Place the bottom resistor of the feedback network as close as possible
to this pin for accurate regulation.
8
AVIN
Supply voltage for the control circuitry.
9,10
PVIN
Supply voltage for output power stage. Connect a bypass capacitor to this pin.
DAP
GND
Signal / Power Ground and thermal connection. Tie this directly to GND (pin 7). See Application
Information regarding optimum thermal layout.
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2
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
AVIN, PVIN
SW Voltage
Boost Voltage
Boost to SW Voltage
FB Voltage
SYNC Voltage
EN Voltage
Storage Temperature Range
Junction Temperature
Operating Ratings
-0.5V to 24V
-0.5V to 24V
-0.5V to 28V
-0.5V to 6.0V
-0.5V to 3.0V
-0.5V to 6.0V
-0.5V to (VIN + 0.3V)
−65°C to +150°C
150°C
2kV
260°C
(Note 1)
AVIN, PVIN
SW Voltage
Boost Voltage
Boost to SW Voltage
Junction Temperature Range
3V to 20V
-0.5V to 20V
-0.5V to 24V
3.0V to 5.5V
−40°C to +125°C
Thermal Resistance (θJA) LLP10 (Note 3)
33°C/W
Thermal Resistance (θJA) eMSOP10 (Note 3)
45°C/W
Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C, and those in boldface type apply over the full Operating Temperature
Range (TJ = -40°C to 125°C). VIN = 12V, and VBOOST - VSW = 4.3V unless otherwise specified. Datasheet min/max specification
limits are guaranteed by design, test, or statistical analysis.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
TJ = 0°C to 85°C
0.990
1.0
1.010
TJ = -40°C to 125°C
0.984
1.0
1.014
SYSTEM PARAMETERS
VFB
ΔVFB/ΔVIN
IFB
OVP
UVLO
SS
IQ
IBOOST
Feedback Voltage
Feedback Voltage Line Regulation
VIN = 3V to 20V
0.003
Feedback Input Bias Current
20
Over Voltage Protection, VFB at
which PWM Halts.
Undervoltage Lockout
UVLO Hysteresis
%/V
nA
100
1.13
V
VIN Rising until VSW is Switching
2.60
2.75
2.90
VIN Falling from UVLO
0.30
0.47
0.6
0.5
1
1.5
Soft Start Time
V
V
ms
Quiescent Current, IQ = IQ_AVIN +
IQ_PVIN
VFB = 1.1 (not switching)
2.4
mA
Quiescent Current, IQ = IQ_AVIN +
IQ_PVIN
VEN = 0V (shutdown)
70
nA
fSW= 2 MHz
8.2
10
fSW= 1 MHz
4.4
6
2
2.3
Boost Pin Current
mA
OSCILLATOR
fSW
Switching Frequency
VFB_FOLD
FB Pin Voltage where SYNC input
is overridden.
fFOLD_MIN
Frequency Foldback Minimum
SYNC = GND
1.75
MHz
0.53
VFB = 0V
220
V
250
kHz
2.35
MHz
LOGIC INPUTS (EN, SYNC)
fSYNC
SYNC Frequency Range
1
VIL
EN, SYNC Logic low threshold
Logic Falling Edge
VIH
EN, SYNC Logic high threshold
Logic Rising Edge
0.4
1.8
V
tSYNC_HIGH
SYNC, Time Required above VIH to
Ensure a Logical High.
100
ns
tSYNC_LOW
SYNC, Time Required below VIL to
Ensure a Logical Low.
100
ns
ISYNC
SYNC Pin Current
IEN
Enable Pin Current
VSYNC < 5V
20
nA
VEN = 3V
6
15
VIN = VEN = 20V
50
100
3
µA
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LM27341/LM27342
ESD Susceptibility (Note 2)
Soldering Information
Infrared Reflow (5sec)
Absolute Maximum Ratings (Note 1)
LM27341/LM27342
Symbol
Parameter
Conditions
Min
Typ
Max
Units
150
320
mΩ
INTERNAL MOSFET
RDS(ON)
ICL
DMAX
Switch ON Resistance
Switch Current Limit
Maximum Duty Cycle
LM27342
2.5
4.0
LM27341
2.0
3.7
SYNC = GND
85
93
A
%
tMIN
Minimum on time
65
ns
ISW
Switch Leakage Current
40
nA
Boost LDO Output Voltage
3.9
V
BOOST LDO
VLDO
THERMAL
TSHDN
Thermal Shutdown Temperature
Junction temperature rising
165
°C
Thermal Shutdown Hysteresis
Junction temperature falling
15
°C
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the recommended Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and should not be
operated beyond such conditions.
Note 2: Human body model, 1.5 kΩ in series with 100 pF.
Note 3: Thermal shutdown will occur if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX) , θJA and TA . The
maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/θJA . All numbers apply for packages soldered directly onto a 3” x 3” PC
board with 2oz. copper on 4 layers in still air.
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All curves taken at VIN = 12V, VBOOST - VSW = 4.3V and
Efficiency vs Load Current
VOUT = 5V, fSW = 2 MHz
Refer to Figure 10
Load Transient
VOUT = 5V, IOUT = 100 mA - 2A @ slewrate = 2A / µs
Refer to Figure 10
300056100
30005676
Efficiency vs Load Current
VOUT = 3.3V, fSW = 2 MHz
Refer to Figure 12
Load Transient
VOUT = 3.3V, IOUT = 100 mA - 2A @ slewrate = 2A / µs
Refer to Figure 12
300056102
30005680
Efficiency vs Load Current
VOUT = 1.8V, fSW = 2 MHz
Refer to Figure 15
Load Transient
VOUT = 1.8V, IOUT = 100 mA - 2A @ slewrate = 2A / µs
Refer to Figure 15
300056105
30005684
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LM27341/LM27342
Typical Performance Characteristics
TA = 25°C, unless specified otherwise.
LM27341/LM27342
Line Transient
VIN = 10 to 15V, VOUT = 3.3V, no CFF
Refer to Figure 13
Line Transient
VIN = 10 to 15V, VOUT = 3.3V
Refer to Figure 12
300056108
300056109
Short Circuit
Short Circuit Release
300056110
300056111
Soft Start
Soft Start with EN Tied to VIN
30005653
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30005654
6
VIN = 12V, VOUT = 3.3V, L = 1.5 µH COUT = 44 µF Iout =1A
Refer to Figure 12
300056112
300056113
VIN = 5V, VOUT = 1.8V, L = 1.0 µH COUT = 44 µF Iout =1A
Refer to Figure 15
VIN = 5V, VOUT = 1.2V, L = 0.56 µH COUT = 68 µF Iout =1A
Refer to Figure 17
300056115
300056114
Sync Functionality
Loss of Synchronization
30005656
30005655
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LM27341/LM27342
VIN = 12V, VOUT = 5 V, L = 2.2 µH, COUT = 44 µF Iout =1A
Refer to Figure 10
LM27341/LM27342
Oscillator Frequency vs Temperature
VSYNC = GND, fSW = 2 MHz
Oscillator Frequency vs VFB
30005629
30005628
VFB vs Temperature
VFB vs VIN
30005630
30005631
Current Limit vs Temperature
VIN = 12V
RDSON vs Temperature
30005633
30005632
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LM27341/LM27342
IQ (Shutdown) vs Temperature
IQ = IAVIN + IPVIN
IEN vs VEN
30005635
30005634
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LM27341/LM27342
Block Diagram
30005698
FIGURE 1.
voltage (VFB) and VREF. When the output of the PWM comparator goes high, the switch turns off until the next switching
cycle begins. During the switch off-time (tOFF), inductor current discharges through the catch diode D1, which forces the
SW pin (VSW) to swing below ground by the forward voltage
(VD1) of the catch diode. The regulator loop adjusts the duty
cycle (D) to maintain a constant output voltage.
Application Information
THEORY OF OPERATION
The LM27341/LM27342 is a constant-frequency, peak current-mode PWM buck regulator IC that delivers a 1.5 or 2A
load current. The regulator has a preset switching frequency
of 2 MHz. This high frequency allows the LM27341/LM27342
to operate with small surface mount capacitors and inductors,
resulting in a DC-DC converter that requires a minimum
amount of board space. The LM27341/LM27342 is internally
compensated, which reduces design time, and requires few
external components.
The following operating description of the LM27341/LM27342
will refer to the Block Diagram (Figure 1) and to the waveforms
in Figure 2. The LM27341/LM27342 supplies a regulated output voltage by switching the internal NMOS switch at a constant frequency and varying the duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the
internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS switch. During this ontime, the SW pin voltage (VSW) swings up to approximately
VIN, and the inductor current (iL) increases with a linear slope.
The current-sense amplifier measures iL, which generates an
output proportional to the switch current typically called the
sense signal. The sense signal is summed with the regulator’s
corrective ramp and compared to the error amplifier’s output,
which is proportional to the difference between the feedback
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30005607
FIGURE 2. LM27341/LM27342 Waveforms of SW Pin
Voltage and Inductor Current
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LM27341/LM27342
BOOST FUNCTION
Capacitor C2 in Figure 1, commonly referred to as CBOOST, is
used to store a voltage VBOOST. When the LM27341/LM27342
starts up, an internal LDO charges CBOOST ,via an internal
diode, to a voltage sufficient to turn the internal NMOS switch
on. The gate drive voltage supplied to the internal NMOS
switch is VBOOST - VSW.
During a normal switching cycle, when the internal NMOS
control switch is off (tOFF) (refer to Figure 2), VBOOST equals
VLDO minus the forward voltage of the internal diode (VD2). At
the same time the inductor current (iL) forward biases the
catch diode D1 forcing the SW pin to swing below ground by
the forward voltage drop of the catch diode (VD1). Therefore,
the voltage stored across CBOOST is
VBOOST - VSW = VLDO - VD2 + VD1
30005636
Thus,
VBOOST = VSW + VLDO - VD2 + VD1
FIGURE 4. Minimum Load Current for L = 1.5 µH
When the NMOS switch turns on (tON), the switch pin rises to
VSW = VIN – (RDSON x IL),
ENABLE PIN / SHUTDOWN MODE
Connect the EN pin to a voltage source greater than 1.8V to
enable operation of the LM27341/LM27342. Apply a voltage
less than 0.4V to put the part into shutdown mode. In shutdown mode the quiescent current drops to typically 70 nA.
Switch leakage adds another 40 nA from the input supply. For
proper operation, the LM27341/LM27342 EN pin should never be left floating, and the voltage should never exceed VIN +
0.3V.
The simplest way to enable the operation of the LM27341/
LM27342 is to connect the EN pin to AVIN which allows self
start-up of the LM27341/LM27342 when the input voltage is
applied.
When the rise time of VIN is longer than the soft-start time of
the LM27341/LM27342 this method may result in an overshoot in output voltage. In such applications, the EN pin
voltage can be controlled by a separate logic signal, or tied to
a resistor divider, which reaches 1.8V after VIN is fully established (see Figure 5). This will minimize the potential for
output voltage overshoot during a slow VIN ramp condition.
Use the lowest value of VIN , seen in your application when
calculating the resistor network, to ensure that the 1.8V minimum EN threshold is reached.
reverse biasing D1, and forcing VBOOST to rise. The voltage
at VBOOST is then
VBOOST = VIN – (RDSON x IL) + VLDO – VD2 + VD1
which is approximately
VIN + VLDO- 0.4V
VBOOST has pulled itself up by its "bootstraps", or boosted to
a higher voltage.
LOW INPUT VOLTAGE CONSIDERATIONS
When the input voltage is below 5V and the duty cycle is
greater than 75 percent, the gate drive voltage developed
across CBOOST might not be sufficient for proper operation of
the NMOS switch. In this case, CBOOST should be charged via
an external Schottky diode attached to a 5V voltage rail, see
Figure 3. This ensures that the gate drive voltage is high
enough for proper operation of the NMOS switch in the triode
region. Maintain VBOOST - VSW less than the 6V absolute maximum rating.
30005626
FIGURE 3. External Diode Charges CBOOST
30005608
FIGURE 5. Resistor Divider on EN
HIGH OUTPUT VOLTAGE CONSIDERATIONS
When the output voltage is greater than 3.3V, a minimum load
current is needed to charge CBOOST, see Figure 4. The minimum load current forward biases the catch diode D1 forcing
the SW pin to swing below ground. This allows CBOOST to
charge, ensuring that the gate drive voltage is high enough
for proper operation. The minimum load current depends on
many factors including the inductor value.
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LM27341/LM27342
The UVLO threshold has approximately 470 mV of hysteresis,
so the part will operate until VIN drops below 2.28V(typ). Hysteresis prevents the part from turning off during power up if
VIN has finite impedance.
FREQUENCY SYNCHRONIZATION
The LM27341/LM27342 switching frequency can be synchronized to an external clock, between 1.00 and 2.35 MHz,
applied at the SYNC pin. At the first rising edge applied to the
SYNC pin, the internal oscillator is overridden and subsequent positive edges will initiate switching cycles. If the external SYNC signal is lost during operation, the LM27341/
LM27342 will revert to its internal 2 MHz oscillator within 1.5
µs. To disable Frequency Synchronization and utilize the internal 2 MHz oscillator, connect the SYNC pin to GND.
The SYNC pin gives the designer the flexibility to optimize
their design. A lower switching frequency can be chosen for
higher efficiency. A higher switching frequency can be chosen
to keep EMI out of sensitive ranges such as the AM radio
band. Synchronization can also be used to eliminate beat frequencies generated by the interaction of multiple switching
power converters. Synchronizing multiple switching power
converters will result in cleaner power rails.
The selected switching frequency (fSYNC) and the minimum
on-time (tMIN) limit the minimum duty cycle (DMIN) of the device.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the internal NMOS switch when the IC junction temperature
exceeds 165°C (typ). After thermal shutdown occurs, hysteresis prevents the internal NMOS switch from turning on
until the junction temperature drops to approximately 150°C.
Design Guide
INDUCTOR SELECTION
Inductor selection is critical to the performance of the
LM27341/LM27342. The selection of the inductor affects stability, transient response and efficiency. A key factor in inductor selection is determining the ripple current (ΔiL) (see Figure
2).
The ripple current (ΔiL) is important in many ways.
First, by allowing more ripple current, lower inductance values
can be used with a corresponding decrease in physical dimensions and improved transient response. On the other
hand, allowing less ripple current will increase the maximum
achievable load current and reduce the output voltage ripple
(see Output Capacitor section for more details on calculating
output voltage ripple). Increasing the maximum load current
is achieved by ensuring that the peak inductor current (ILPK)
never exceeds the minimum current limit of 2.0A min
(LM27341) or 2.5A min (LM27342) .
DMIN= tMIN x fSYNC
Operation below DMIN is not reccomended. The LM27341/
LM27342 will skip pulses to keep the output voltage in regulation, and the current limit is not guaranteed. The switching
is in phase but no longer at the same switching frequency as
the SYNC signal.
CURRENT LIMIT
The LM27341 and LM27342 use cycle-by-cycle current limiting to protect the output switch. During each switching cycle,
a current limit comparator detects if the output switch current
exceeds 2.0A min (LM27341) or 2.5A min (LM27342) , and
turns off the switch until the next switching cycle begins.
ILPK = IOUT + ΔiL / 2
Secondly, the slope of the ripple current affects the current
control loop. The LM27341/LM27342 has a fixed slope corrective ramp. When the slope of the current ripple becomes
significantly less than the converter’s corrective ramp (see
Figure 1), the inductor pole will move from high frequencies
to lower frequencies. This negates one advantage that peak
current-mode control has over voltage-mode control, which
is, a single low frequency pole in the power stage of the converter. This can reduce the phase margin, crossover frequency and potentially cause instability in the converter. Contrarily,
when the slope of the ripple current becomes significantly
greater than the converter’s corrective ramp, resonant peaking can occur in the control loop. This can also cause instability (Sub-Harmonic Oscillation) in the converter. For the
power supply designer this means that for lower switching
frequencies the current ripple must be increased to keep the
inductor pole well above crossover. It also means that for
higher switching frequencies the current ripple must be decreased to avoid resonant peaking.
With all these factors, how is the desired ripple current selected? The ripple ratio (r) is defined as the ratio of inductor
ripple current (ΔiL) to output current (IOUT), evaluated at maximum load:
FREQUENCY FOLDBACK
The LM27341/LM27342 employs frequency foldback to protect the device from current run-away during output shortcircuit. Once the FB pin voltage falls below regulation, the
switch frequency will smoothly reduce with the falling FB voltage until the switch frequency reaches 220 kHz (typ). If the
device is synchronized to an external clock, synchronization
is disabled until the FB pin voltage exceeds 0.53V
SOFT-START
The LM27341/LM27342 has a fixed internal soft-start of 1 ms
(typ). During soft-start, the error amplifier’s reference voltage
ramps from 0.0 V to its nominal value of 1.0 V in approximately
1 ms. This forces the regulator output to ramp in a controlled
fashion, which helps reduce inrush current. Upon soft-start
the part will initially be in frequency foldback and the frequency will rise as FB rises. The regulator will gradually rise to 2
MHz. The LM27341/LM27342 will allow synchronization to an
external clock at FB > 0.53V.
OUTPUT OVERVOLTAGE PROTECTION
The overvoltage comparator turns off the internal power
NFET when the FB pin voltage exceeds the internal reference
voltage by 13% (VFB > 1.13 * VREF). With the power NFET
turned off the output voltage will decrease toward the regulation level.
A good compromise between physical size, transient response and efficiency is achieved when we set the ripple ratio
between 0.2 and 0.4. The recommended ripple ratio vs. duty
cycle shown below (see Figure 6) is based upon this compromise and control loop optimizations. Note that this is just
UNDERVOLTAGE LOCKOUT
Undervoltage lockout (UVLO) prevents the LM27341/
LM27342 from operating until the input voltage exceeds
2.75V(typ).
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DMAX
= (VOUT + VD1) / (VIN + VD1 - VDS)
= (3.3V + 0.5V) / (7V + 0.5V - 0.30V)
= 0.528
Using Figure 6 gives us a recommended ripple ratio = 0.4.
Now the minimum duty cycle is calculated.
DMIN
= (VOUT + VD1) / (VIN + VD1 - VDS)
= (3.3V + 0.5V) / (16V + 0.5V - 0.30V)
= 0.235
The inductance can now be calculated.
L
= (1 - DMIN) x (VOUT + VD1) / (IOUT x r x fsw)
= (1 - 0.235) x (3.3V + .5V) / (2A x 0.4 x 2 MHz)
= 1.817 µH
This is close to the standard inductance value of 1.8 µH. This
leads to a 1% deviation from the recommended ripple ratio,
which is now 0.4038.
Finally, we check that the peak current does not reach the
minimum current limit of 2.5A.
30005627
FIGURE 6. Recommended Ripple Ratio Vs. Duty Cycle
ILPK = IOUT x (1 + r / 2)
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (VOUT) to input voltage (VIN):
= 2A x (1 + .4038 / 2 )
= 2.404A
The peak current is less than 2.5A, so the DC load specification can be met with this ripple ratio. To design for the
LM27341 simply replace IOUT = 1.5A in the equations for
ILPK and see that ILPK does not exceed the LM27341's current
limit of 2.0A (min).
The application's lowest input voltage should be used to calculate the ripple ratio. The catch diode forward voltage drop
(VD1) and the voltage drop across the internal NFET (VDS)
must be included to calculate a more accurate duty cycle.
Calculate D by using the following formula:
INDUCTOR MATERIAL SELECTION
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. To prevent
the inductor from saturating over the entire -40 °C to 125 °C
range, pick an inductor with a saturation current higher than
the upper limit of ICL listed in the Electrical Characteristics table.
Ferrite core inductors are recommended to reduce AC loss
and fringing magnetic flux. The drawback of ferrite core inductors is their quick saturation characteristic. The current
limit circuit has a propagation delay and so is oftentimes not
fast enough to stop a saturated inductor from going above the
current limit. This has the potential to damage the internal
switch. To prevent a ferrite core inductor from getting into saturation, the inductor saturation current rating should be higher
than the switch current limit ICL. The LM27341/LM27342 is
quite robust in handling short pulses of current that are a few
amps above the current limit. Saturation protection is provided by a second current limit which is 30% higher than the
cycle by cycle current limit. When the saturation protection is
triggered the part will turn off the output switch and attempt to
soft-start. (When a compromise has to be made, pick an inductor with a saturation current just above the lower limit of
the ICL.) Be sure to validate the short-circuit protection over
the intended temperature range.
An inductor's saturation current is usually lower when hot. So
consult the inductor vendor if the saturation current rating is
only specified at room temperature.
Soft saturation inductors such as the iron powder types can
also be used. Such inductors do not saturate suddenly and
VDS can be approximated by:
VDS = IOUT x RDS(ON)
The diode forward drop (VD1) can range from 0.3V to 0.5V
depending on the quality of the diode. The lower VD1 is, the
higher the operating efficiency of the converter.
Now that the ripple current or ripple ratio is determined, the
required inductance is calculated by:
where DMIN is the duty cycle calculated with the maximum
input voltage, fsw is the switching frequency, and IOUT is the
maximum output current of 2A. Using IOUT = 2A will minimize
the inductor's physical size.
INDUCTOR CALCULATION EXAMPLE
Operating conditions for the LM27342 are:
VIN = 7 - 16V
VOUT = 3.3V
fSW = 2 MHz
VD1 = 0.5V
IOUT = 2A
First the maximum duty cycle is calculated.
13
www.national.com
LM27341/LM27342
a guideline. Please see Application note AN-1197 for further
considerations.
LM27341/LM27342
the initial current of a load transient. Capacitance can be increased significantly with little detriment to the regulator stability. However, increasing the capacitance provides dimininshing improvement over 100 uF in most applications,
because the bandwidth of the control loop decreases as output capacitance increases. If improved transient performance
is required, add a feed forward capacitor. This becomes especially important for higher output voltages where the bandwidth of the LM27341/LM27342 is lower. See Feed Forward
Capacitor and Frequency Synchronization sections.
Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet the following condition:
therefore are safer when there is a severe overload or even
shorted output. Their physical sizes are usually smaller than
the Ferrite core inductors. The downside is their fringing flux
and higher power dissipation due to relatively high AC loss,
especially at high frequencies.
INPUT CAPACITOR
An input capacitor is necessary to ensure that VIN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and Equivalent Series Inductance (ESL).
The recommended input capacitance is 10 µF, although 4.7
µF works well for input voltages below 6V. The input voltage
rating is specifically stated by the capacitor manufacturer.
Make sure to check any recommended deratings and also
verify if there is any significant change in capacitance at the
operating input voltage and the operating temperature. The
input capacitor maximum RMS input current rating (IRMS-IN)
must be greater than:
where IOUT is the output current, and r is the ripple ratio.
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is recommended for its fast switching times
and low forward voltage drop. The catch diode should be
chosen so that its current rating is greater than:
where r is the ripple ratio defined earlier, IOUT is the output
current, and D is the duty cycle. It can be shown from the
above equation that maximum RMS capacitor current occurs
when D = 0.5. Always calculate the RMS at the point where
the duty cycle, D, is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional
area of the current path. A large leaded capacitor will have
high ESL and a 0805 ceramic chip capacitor will have very
low ESL. At the operating frequencies of the LM27341/
LM27342, certain capacitors may have an ESL so large that
the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount
capacitors are strongly recommended. Sanyo POSCAP, Tantalum or Niobium, Panasonic SP or Cornell Dubilier Low ESR
are all good choices for input capacitors and have acceptable
ESL. Multilayer ceramic capacitors (MLCC) have very low
ESL. For MLCCs it is recommended to use X7R or X5R dielectrics. Consult the capacitor manufacturer's datasheet to
see how rated capacitance varies over operating conditions.
ID1 = IOUT x (1-D)
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To improve efficiency choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE (OPTIONAL)
For circuits with input voltages VIN < 5V and duty cycles (D)
>0.75V. a small-signal Schottky diode is recommended. A
good choice is the BAT54 small signal diode. The cathode of
the diode is connected to the BOOST pin and the anode to a
5V voltage rail.
BOOST CAPACITOR
A ceramic 0.1 µF capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between VOUT and the FB pin. A good starting value for R2 is 1 kΩ.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired output ripple and transient response. The LM27341/2's loop
compensation is designed for ceramic capacitors. A minimum
of 22 µF is required at 2 MHz (33 uF at 1 MHz) while 47 - 100
µF is recommended for improved transient response and
higher phase margin. The output voltage ripple of the converter is:
FEED FORWARD CAPACITOR (OPTIONAL)
A feed forward capacitor CFF can improve the transient response of the converter. Place CFF in parallel with R1. The
value of CFF should place a zero in the loop response at, or
above, the pole of the output capacitor and RLOAD. The CFF
capacitor will increase the crossover frequency of the design,
thus a larger minimum output capacitance is required for designs using CFF. CFF should only be used with an output
capacitance greater than or equal to 44 uF.
When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain
amount of switching edge noise will couple through parasitic
capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not.
The transient response is determined by the speed of the
control loop and the ability of the output capacitor to provide
www.national.com
14
The complete LM27341/LM27342 DC-DC converter efficiency can be calculated in the following manner.
PSWF = 1/2(VIN x IOUT x fSW x tFALL)
PSWR = 1/2(VIN x IOUT x fSW x tRISE)
PSW = PSWF + PSWR
Or
Typical Rise and Fall Times vs Input Voltage
Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are
not discussed.
Power loss (PLOSS) is the sum of two basic types of losses in
the converter, switching and conduction. Conduction losses
usually dominate at higher output loads, where as switching
losses remain relatively fixed and dominate at lower output
loads. The first step in determining the losses is to calculate
the duty cycle (D).
VIN
tRISE
tFALL
5V
8ns
8ns
10V
9ns
9ns
15V
10ns
10ns
IC QUIESCENT LOSSES
Another loss is the power required for operation of the internal
circuitry:
PQ = IQ x VIN
IQ is the quiescent operating current, and is typically around
2.4 mA.
MOSFET DRIVER LOSSES
The other operating power that needs to be calculated is that
required to drive the internal NFET:
PBOOST = IBOOST x VBOOST
VDS is the voltage drop across the internal NFET when it is
on, and is equal to:
VBOOST is normally between 3VDC and 5VDC. The IBOOST rms
current is dependant on switching frequency fSW. IBOOST is
approximately 8.2 mA at 2 MHz and 4.4 mA at 1 MHz.
VDS = IOUT x RDSON
TOTAL POWER LOSSES
Total power losses are:
VD is the forward voltage drop across the Schottky diode. It
can be obtained from the Electrical Characteristics section of
the schottky diode datasheet. If the voltage drop across the
inductor (VDCR) is accounted for, the equation becomes:
PLOSS = PCOND + PSWR + PSWF + PQ + PBOOST + PDIODE + PIND
Losses internal to the LM27341/LM27342 are:
PINTERNAL = PCOND + PSWR + PSWF + PQ + PBOOST
EFFICIENCY CALCULATION EXAMPLE
Operating conditions are:
VDCR usually gives only a minor duty cycle change, and has
been omitted in the examples for simplicity.
SCHOTTKY DIODE CONDUCTION LOSSES
The conduction losses in the free-wheeling Schottky diode
are calculated as follows:
VIN = 12V
VOUT = 3.3V
IOUT = 2A
fSW = 2 MHz
VD1 = 0.5V
RDCR = 20 mΩ
Internal Power Losses are:
PCOND
PDIODE = VD1 x IOUT (1-D)
= IOUT2 x RDSON x D
= 22 x 0.15Ω x 0.314
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode that
has a low forward voltage drop.
PSW
= (VIN x IOUT x fSW x tFALL)
= (12V x 2A x 2 MHz x 10ns)
PQ
INDUCTOR CONDUCTION LOSSES
Another significant external power loss is the conduction loss
in the output inductor. The equation can be simplified to:
PIND = IOUT2 x RDCR
= 29 mW
= IBOOST x VBOOST
= 8.2 mA x 4.5V
MOSFET CONDUCTION LOSSES
The LM27341/LM27342 conduction loss is mainly associated
with the internal NFET:
= 480 mW
= IQ x VIN
= 2.4 mA x 12V
PBOOST
= 188 mW
= 37 mW
________
PINTERNAL = PCOND + PSW + PQ + PBOOST
= 733 mW
Total Power Losses are:
PCOND = IOUT2 x RDSON x D
15
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LM27341/LM27342
MOSFET SWITCHING LOSSES
Switching losses are also associated with the internal NFET.
They occur during the switch on and off transition periods,
where voltages and currents overlap resulting in power loss.
The simplest means to determine this loss is to empirically
measuring the rise and fall times (10% to 90%) of the switch
at the switch node:
Calculating Efficiency, and Junction
Temperature
LM27341/LM27342
PDIODE
= 0.5V x 2 x (1 - 0.314)
PIND
With this information we can estimate the junction temperature of the LM27341/LM27342.
= VD1 x IOUT (1 - D)
= 686 mW
CALCULATING THE LM27341/LM27342 JUNCTION
TEMPERATURE
= IOUT2 x RDCR
= 22 x 20 mΩ
= 80 mW
Thermal Definitions:
TJ = IC junction temperature
TA = Ambient temperature
RθJC = Thermal resistance from IC junction to device case
RθJA = Thermal resistance from IC junction to ambient air
________
PLOSS
= PINTERNAL + PDIODE + PIND
= 1.499 W
The efficiency can now be estimated as:
30005666
FIGURE 7. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.
Heat in the LM27341/LM27342 due to internal power dissipation is removed through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional
areas of material. Depending on the material, the transfer of
heat can be considered to have poor to good thermal conductivity properties (insulator vs conductor).
Heat Transfer goes as:
layers within the PCB. The type and number of thermal vias
can also make a large difference in the thermal impedance.
Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to the ground plane.
Six to nine thermal vias should be placed under the exposed
pad to the ground plane. Placing more than nine thermal vias
results in only a small reduction to RθJA for the same copper
area. These vias should have 8 mil holes to avoid wicking
solder away from the DAP. See AN-1187 and AN-1520 for
more information on package thermal performance. If a compromise for cost needs to be made, the thermal vias for the
eMSOP package can range from 8-14 mils, this will increase
the possibility of solder wicking.
To predict the silicon junction temperature for a given application, three methods can be used. The first is useful before
prototyping and the other two can more accurately predict the
junction temperature within the application.
Method 1:
The first method predicts the junction temperature by extrapolating a best guess RθJA from the table or graph. The tables
and graph are for natural convection. The internal dissipation
can be calculated using the efficiency calculations. This allows the user to make a rough prediction of the junction
temperature in their application. Methods two and three can
later be used to determine the junction temperature more accurately.
The two tables below have values of RθJA for the LLP and the
eMSOP package.
Silicon→Lead Frame→PCB
Convection: Heat transfer is by means of airflow. This could
be from a fan or natural convection. Natural convection occurs
when air currents rise from the hot device to cooler air.
Thermal impedance is defined as:
Thermal impedance from the silicon junction to the ambient
air is defined as:
This impedance can vary depending on the thermal properties of the PCB. This includes PCB size, weight of copper
used to route traces , the ground plane, and the number of
www.national.com
16
Number
Size of
Size of Top Number
of Board Bottom Layer Layer Copper of 10 mil
Layers
Copper
Connected to Thermal
Connected to
Dap
Vias
DAP
RθJA
2
0.25 in2
0.05 in2
8
80.6 °
C/W
2
0.5625 in2
0.05 in2
8
70.9 °
C/W
2
1 in2
0.05 in2
8
62.1 °
C/W
2
1.3225 in2
0.05 in2
8
54.6 °
C/W
4 (Eval
Board)
3.25 in2
2.25 in2
14
Therefore:
TJ = (RθJC x PLOSS) + TC
35.3 °
C/W
METHOD 2 EXAMPLE
The operating conditions are the same as the previous Efficiency Calculation:
RθJA values for the LLP @ 1Watt dissipation:
Number
Size of
Size of Top Number
of Board Bottom Layer Layer Copper of 8 mil
Layers
Copper
Connected to Thermal
Connected to
Dap
Vias
DAP
2
0.25 in2
0.05 in2
8
RθJA
VIN = 12V
VOUT = 3.3V
IOUT = 2A
fSW = 2 MHz
VD1 = 0.5V
RDCR = 20 mΩ
Internal Power Losses are:
PCOND
78 °C/
W
= IOUT2 x RDSON x D
= 188 mW
= 22 x 0.15Ω x 0.314
2
0.5625 in2
0.05 in2
8
65.6 °
C/W
PSW
2
1 in2
0.05 in2
8
58.6 °
C/W
PQ
2
1.3225 in2
0.05 in2
8
50 °C/
W
PBOOST
4 (Eval
Board)
3.25 in2
2.25 in2
15
30.7 °
C/W
= (VIN x IOUT x fSW x tFALL)
= (12V x 2A x 2 MHz x 10ns)
= 480 mW
= IQ x VIN
= 1.5 mA x 12V
= 29 mW
= IBOOST x VBOOST
= 7 mA x 4.5V
= 37 mW
________
PINTERNAL = PCOND + PSW + PQ + PBOOST
= 733 mW
The junction temperature can now be estimated as:
TJ = (RθJC x PINTERNAL) + TC
A National Semiconductor eMSOP evaluation board was
used to determine the TJ of the LM27341/LM27342. The four
layer PCB is constructed using FR4 with 2oz copper traces.
There is a ground plane on the internal layer directly beneath
the device, and a ground plane on the bottom layer. The
ground plane is accessed by fourteen 10 mil vias. The board
measures 2in x 2in (50.8mm x 50.8mm). It was placed in a
container with no airflow. The case temperature measured on
this LM27342MY Demo Board was 48.7°C. Therefore,
TJ = (9.5 °C/W x 733 mW) + 48.7 °C
TJ = 55.66 °C
To keep the Junction temperature below 125 °C for this layout, the ambient temperature must stay below 94.33 °C.
30005690
TA_MAX = TJ_MAX - TJ +TA
FIGURE 8. Estimate of Thermal Resistance vs. Ground
Copper Area
Eight Thermal Vias and Natural Convection
TA_MAX = 125 °C - 55.66 °C + 25 °C
TA_MAX = 94.33 °C
Method 3:
The third method can also give a very accurate estimate of
silicon junction temperature. The first step is to determine
Method 2:
17
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LM27341/LM27342
The second method requires the user to know the thermal
impedance of the silicon junction to case. (RθJC) is approximately 9.5°C/W for the eMSOP package or 9.1°C/W for the
LLP. The case temperature should be measured on the bottom of the PCB at a thermal via directly under the DAP of the
LM27341/LM27342. The solder resist should be removed
from this area for temperature testing. The reading will be
more accurate if it is taken midway between pins 2 and 9,
where the NMOS switch is located. Knowing the internal dissipation from the efficiency calculation given previously, and
the case temperature (TC) we have:
RθJA values for the eMSOP @ 1Watt dissipation:
LM27341/LM27342
structed using FR4 with 2oz copper traces. There is a ground
plane on the internal layer directly beneath the device, and a
ground plane on the bottom layer. The ground plane is accessed by fourteen 10 mil vias. The board measures 2in x 2in
(50.8mm x 50.8mm). It was placed in an oven with no forced
airflow.
The ambient temperature was raised to 132 °C, and at that
temperature, the device went into thermal shutdown. RθJA can
now be calculated.
RθJA of the application. The LM27341/LM27342 has overtemperature protection circuitry. When the silicon temperature reaches 165 °C, the device stops switching. The
protection circuitry has a hysteresis of 15 °C. Once the silicon
temperature has decreased to approximately 150 °C, the device will start to switch again. Knowing this, the RθJA for any
PCB can be characterized during the early stages of the design by raising the ambient temperature in the given application until the circuit enters thermal shutdown. If the SW-pin is
monitored, it will be obvious when the internal NFET stops
switching indicating a junction temperature of 165 °C. We can
calculate the internal power dissipation from the above methods. All that is needed for calculation is the estimate of
RDSON at 165 °C. This can be extracted from the graph of
RDSON vs. Temperature. The value is approximately 0.267
ohms. With this, the junction temperature, and the ambient
temperature RθJA can be determined.
To keep the Junction temperature below 125 °C for this layout, the ambient temperature must stay below 92 °C.
TA_MAX = TJ_MAX - (RθJA x PINTERNAL)
TA_MAX = 125 °C - (37.46 °C/W x 0.881 W)
TA_MAX = 92 °C
This calculation of the maximum ambient temperature is only
2.3 °C different from the calculation using method 2. The
methods described above to find the junction temperature in
the eMSOP package can also be used to calculate the junction temperature in the LLP package. The 10 pin LLP package
has a RθJC = 9.1°C/W, while RθJA can vary depending on the
layout. RθJA can be calculated in the same manner as described in method 3.
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
METHOD 3 EXAMPLE
The operating conditions are the same as the previous Efficiency Calculation:
VIN = 12V
VOUT = 3.3V
IOUT = 2A
fSW = 2 MHz
VD1 = 0.5V
RDCR = 20 mΩ
PCB Layout Considerations
COMPACT LAYOUT
The performance of any switching converter depends as
much upon the layout of the PCB as the component selection.
The following guidelines will help the user design a circuit with
maximum rejection of outside EMI and minimum generation
of unwanted EMI.
Parasitic inductance can be reduced by keeping the power
path components close together and keeping the area of the
loops small, on which high currents travel. Short, thick traces
or copper pours (shapes) are best. In particular, the switch
node (where L1, D1, and the SW pin connect) should be just
large enough to connect all three components without excessive heating from the current it carries. The LM27341/
LM27342 operates in two distinct cycles (see Figure 2) whose
high current paths are shown below in Figure 9:
Internal Power Losses are:
PCOND
= IOUT2 x RDSON x D
= 22 x 0.267Ω x .314
PSW
= (VIN x IOUT x fSW x tFALL)
= (12V x 2A x 2 MHz x 10nS)
PQ
= 480 mW
= IQ x VIN
= 1.5 mA x 12V
PBOOST
= 335 mW
= 29 mW
= IBOOST x VBOOST
= 7 mA x 4.5V
= 37 mW
________
PINTERNAL = PCOND + PSW + PQ + PBOOST
= 881 mW
Using a National Semiconductor eMSOP evaluation board to
determine the RθJA of the board. The four layer PCB is con-
30005660
FIGURE 9. Buck Converter Current Loops
www.national.com
18
The most important consideration when completing the
layout is the close coupling of the GND connections of
the CIN capacitor and the catch diode D1. These ground
connections should be immediately adjacent, with
multiple vias in parallel at the pad of the input capacitor
connected to GND. Place CIN and D1 as close to the IC
as possible.
3. Next in importance is the location of the GND connection
of the COUT capacitor, which should be near the GND
connections of CIN and D1.
4. There should be a continuous ground plane on the
copper layer directly beneath the converter. This will
reduce parasitic inductance and EMI.
5. The FB pin is a high impedance node and care should
be taken to make the FB trace short to avoid noise pickup
and inaccurate regulation. The feedback resistors should
be placed as close as possible to the IC, with the GND
of R2 placed as close as possible to the GND of the IC.
The VOUT trace to R1 should be routed away from the
inductor and any other traces that are switching.
6. High AC currents flow through the VIN, SW and VOUT
traces, so they should be as short and wide as possible.
However, making the traces wide increases radiated
noise, so the layout designer must make this trade-off.
Radiated noise can be decreased by choosing a shielded
inductor.
The remaining components should also be placed as close
as possible to the IC. Please see Application Note AN-1229
for further considerations and the LM27342 demo board as
an example of a four-layer layout.
GROUND PLANE AND SHAPE ROUTING
The diagram of Figure 9 is also useful for analyzing the flow
of continuous current vs. the flow of pulsating currents. The
circuit paths with current flow during both the on-time and offtime are considered to be continuous current, while those that
carry current during the on-time or off-time only are pulsating
currents. Preference in routing should be given to the pulsating current paths, as these are the portions of the circuit most
likely to emit EMI. The ground plane of a PCB is a conductor
and return path, and it is susceptible to noise injection just like
any other circuit path. The path between the input source and
the input capacitor and the path between the catch diode and
the load are examples of continuous current paths. In contrast, the path between the catch diode and the input capacitor
carries a large pulsating current. This path should be routed
with a short, thick shape, preferably on the component side
of the PCB. Multiple vias in parallel should be used right at
the pad of the input capacitor to connect the component side
shapes to the ground plane. A second pulsating current loop
that is often ignored is the gate drive loop formed by the SW
and BOOST pins and boost capacitor CBOOST. To minimize
this loop and the EMI it generates, keep CBOOST close to the
SW and BOOST pins.
FB LOOP
The FB pin is a high-impedance input, and the loop created
by R2, the FB pin and ground should be made as small as
possible to maximize noise rejection. R2 should therefore be
placed as close as possible to the FB and GND pins of the IC.
PCB SUMMARY
1. Minimize the parasitic inductance by keeping the power
path components close together and keeping the area of
the high-current loops small.
19
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LM27341/LM27342
2.
The dark grey, inner loop represents the high current path
during the MOSFET on-time. The light grey, outer loop represents the high current path during the off-time.
LM27341/LM27342
LM27341/LM27342 Circuit Examples
30005615
FIGURE 10. VIN = 7 - 16V, VOUT = 5V, fSW = 2 MHz, IOUT = Full Load
300056100
30005677
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 10
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
22 µF
C3225X7R1C226K
TDK
COUT
C4
22 µF
C3225X7R1C226K
TDK
CFF
C5
0.18 µF
0603ZC184KAT2A
AVX
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
2.2 µH
CDRHD5D28RHPNP
Sumida
Feedback Resistor
R1
560Ω
CRCW0603560RFKEA
Vishay
Feedback Resistor
R2
140Ω
CRCW0603140RFKEA
Vishay
www.national.com
20
LM27341/LM27342
30005673
FIGURE 11. VIN = 7 - 16V, VOUT = 5V, fSW = 1 MHz, IOUT = Full Load
300056101
30005679
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 11
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
47 µF
GRM32ER61A476KE20L
Murata
COUT
C4
22 µF
C3225X7R1C226K
TDK
CFF
C5
0. 27 µF
C0603C274K4RACTU
Kemet
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
3.3 µH
CDRH6D26HPNP
Sumida
Feedback Resistor
R1
560Ω
CRCW0603560RFKEA
Vishay
Feedback Resistor
R2
140Ω
CRCW0603140RFKEA
Vishay
21
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LM27341/LM27342
30005615
FIGURE 12. VIN = 5 - 16V, VOUT = 3.3V, fSW = 2 MHz, IOUT = Full Load
300056102
30005681
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 12
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
22 µF
C3225X7R1C226K
TDK
COUT
C4
22 µF
C3225X7R1C226K
TDK
CFF
C5
0.18 µF
0603ZC184KAT2A
AVX
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
1.5 µH
CDRH5D18BHPNP
Sumida
Feedback Resistor
R1
430 Ω
CRCW0603430RFKEA
Vishay
Feedback Resistor
R2
187 Ω
CRCW0603187RFKEA
Vishay
www.national.com
22
LM27341/LM27342
30005614
FIGURE 13. VIN = 5 - 16V, VOUT = 3.3V, fSW = 2 MHz, IOUT = Full Load
300056103
30005681
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 13
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
22 µF
C3225X7R1C226K
TDK
COUT
C4
22 µF
C3225X7R1C226K
TDK
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
1.5 µH
CDRH5D18BHPNP
Sumida
Feedback Resistor
R1
430 Ω
CRCW0603430RFKEA
Vishay
Feedback Resistor
R2
187 Ω
CRCW0603187RFKEA
Vishay
23
www.national.com
LM27341/LM27342
30005673
FIGURE 14. VIN = 5 - 16V, VOUT = 3.3V, fSW = 1 MHz, IOUT = Full Load
300056104
30005683
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 14
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
47 µF
GRM32ER61A476KE20L
Murata
COUT
C4
22 µF
C3225X7R1C226K
TDK
CFF
C5
0.27 µF
C0603C274K4RACTU
Kemet
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
2.7 µH
CDRH5D18BHPNP
Sumida
Feedback Resistor
R1
430 Ω
CRCW0603430RFKEA
Vishay
Feedback Resistor
R2
187 Ω
CRCW0603187RFKEA
Vishay
www.national.com
24
LM27341/LM27342
30005614
FIGURE 15. VIN = 3.3 - 16V, VOUT = 1.8V, fSW = 2 MHz, IOUT = Full Load
300056105
30005685
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 15
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
22 µF
C3225X7R1C226K
TDK
COUT
C4
22 µF
C3225X7R1C226K
TDK
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
1.0 µH
CDRH5D18BHPNP
Sumida
Feedback Resistor
R1
12 kΩ
CRCW060312K0FKEA
Vishay
Feedback Resistor
R2
15 kΩ
CRCW060315K0FKEA
Vishay
25
www.national.com
LM27341/LM27342
30005673
FIGURE 16. VIN = 3.3 - 16V, VOUT = 1.8V, fSW = 1 MHz, IOUT = Full Load
300056106
30005687
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 16
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
22 uF
C3225X7R1C226K
TDK
COUT
C4
22 uF
C3225X7R1C226K
TDK
CFF
C5
3.9 nF
GRM188R71H392KA01D
Murata
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
1.8 µH
CDRH5D18BHPNP
Sumida
Feedback Resistor
R1
12 kΩ
CRCW060312K0FKEA
Vishay
Feedback Resistor
R2
15 kΩ
CRCW060315K0FKEA
Vishay
www.national.com
26
LM27341/LM27342
30005615
FIGURE 17. VIN = 3.3 - 9V, VOUT = 1.2V, fSW = 2 MHz, IOUT = Full Load
300056107
30005689
Transient Response
IOUT = 100 mA - 2A @ slewrate = 2A / µs
LM27342 Efficiency vs. Load Current
Bill of Materials for Figure 17
Part Name
Part ID Part Value
Part Number
Manufacturer
Buck Regulator
U1
1.5 or 2A Buck Regulator
LM27341 / LM27342
National Semiconductor
CPVIN
C1
10 µF
GRM32DR71E106KA12L
Murata
CBOOST
C2
0.1 µF
GRM188R71C104KA01D
Murata
COUT
C3
47 µF
GRM32ER61A476KE20L
Murata
COUT
C4
22 µF
C3225X7R1C226K
TDK
CFF
C5
NOT MOUNTED
Catch Diode
D1
Schottky Diode Vf = 0.32V
CMS06
Toshiba
Inductor
L1
0.56 µH
CDRH2D18/HPNP
Sumida
Feedback Resistor
R1
1.02 kΩ
CRCW06031K02FKEA
Vishay
Feedback Resistor
R2
5.10 kΩ
CRCW06035K10FKEA
Vishay
27
www.national.com
LM27341/LM27342
Physical Dimensions inches (millimeters) unless otherwise noted
10-Lead LLP Package
NS Package Number SDA10A
10-Lead eMSOP Package
NS Package Number MUC10A
www.national.com
28
LM27341/LM27342
Notes
29
www.national.com
LM27341/LM27342/LM27341Q/LM27342Q 2 MHz 1.5/2A Wide Input Range Step-Down DC-DC
Regulator with Frequency Synchronization
Notes
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