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LM3478
SNVS085X – JULY 2000 – REVISED DECEMBER 2017
LM3478 High-Efficiency Low-Side N-Channel Controller for Switching Regulator
1 Features
3 Description
•
•
The LM3478 is a versatile Low-Side N-Channel
MOSFET controller for switching regulators. It is
suitable for use in topologies requiring a low side
MOSFET, such as boost, flyback, SEPIC, etc.
Moreover, the LM3478 can be operated at extremely
high switching frequency in order to reduce the
overall solution size. The switching frequency of the
LM3478 can be adjusted to any value between 100
kHz and 1 MHz by using a single external resistor.
Current mode control provides superior bandwidth
and transient response, besides cycle-by-cycle
current limiting. Output current can be programmed
with a single external resistor.
1
•
•
•
•
•
•
8-lead VSSOP-8 and SOIC-8 packages
Internal Push-Pull Driver With 1-A Peak Current
Capability
Current Limit and Thermal Shutdown
Frequency Compensation Optimized With a
Capacitor and a Resistor
Internal Soft Start
Current Mode Operation
Undervoltage Lockout With Hysteresis
Create a Custom Design Using the LM3478 with
the WEBENCH Power Designer
2 Applications
•
•
•
•
•
•
•
•
•
Distributed Power Systems
Battery Chargers
Offline Power Supplies
Telecom Power Supplies
Automotive Power Systems
Wide Supply Voltage Range of 2.97 V to 40 V
100-kHz to 1-MHz Adjustable Clock Frequency
±2.5% (Over Temperature) Internal Reference
10-µA Shutdown Current (Over Temperature)
The LM3478 has built-in features such as thermal
shutdown, short-circuit protection, over voltage
protection, etc. Power saving shutdown mode
reduces the total supply current to 5 µA and allows
power supply sequencing. Internal soft-start limits the
inrush current at start-up.
Device Information(1)
PART NUMBER
LM3478
PACKAGE
BODY SIZE (NOM)
SOIC (8)
4.90 mm x 3.91 mm
VSSOP (8)
3.00 mm x 3.00 mm
(1) For all available packages, see the orderable addendum at
the end of the datasheet.
Typical High Efficiency Step-Up (Boost) Converter
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM3478
SNVS085X – JULY 2000 – REVISED DECEMBER 2017
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
5
5
7
Absolute Maximum Ratings ......................................
ESD Ratings - LM3478 ............................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description ............................................ 11
7.1
7.2
7.3
7.4
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
11
12
12
15
8
Application and Implementation ........................ 16
8.1 Application Information............................................ 16
8.2 Typical Applications ................................................ 16
9 Power Supply Recommendations...................... 28
10 Layout................................................................... 28
10.1 Layout Guidelines ................................................. 28
10.2 Layout Example .................................................... 29
11 Device and Documentation Support ................. 30
11.1
11.2
11.3
11.4
11.5
11.6
11.7
Custom Design with WEBENCH Tools.................
Receiving Notification of Documentation Updates
Documentation Support .......................................
Related Links ........................................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
30
30
30
30
30
30
30
12 Mechanical, Packaging, and Orderable
Information ........................................................... 31
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision W (December 2014) to Revision X
Page
•
Deleted LM3478Q-Q1 device. See LM3478Q-Q1 data sheet ............................................................................................... 1
•
Changed package from "DCK" to "DGK" for LM3478 and LM3478-Q1 devices in the Thermal Information table;
changed pin complement from "3" to "8" for the D, and DGK packages ............................................................................... 5
•
Changed RθJA for the D package from "157.2" to "105.3" °C/W............................................................................................. 5
•
Changed RθJC(top) for the D package from "49.9" to "50.3" °C/W............................................................................................ 5
•
Changed RθJB for the D package from "77.1" to "55.8" °C/W................................................................................................. 5
•
Changed ψJT for the D package from "4.7" to "6.8" °C/W ...................................................................................................... 5
•
Changed ψJB for the D package from "75.8" to "54.7" °C/W .................................................................................................. 5
Changes from Revision V (February 2013) to Revision W
Page
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section ................................................................................................. 4
•
Deleted Thermal Resistance parameter from Electrical Characteristics ............................................................................... 6
Changes from Revision U (February 2013) to Revision V
2
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5 Pin Configuration and Functions
8-Lead VSSOP-8 Package
3 Pins
Top View
8-Lead SOIC-8
3 Pins
Top View
Pin Functions
PIN
NAME
NO.
I/O
DESCRIPTION
ISEN
1
I
Current sense input pin. Voltage generated across an external sense resistor is fed into this pin.
COMP
2
I
Compensation pin. A resistor, capacitor combination connected to this pin provides compensation for the
control loop.
FB
3
I
Feedback pin. The output voltage should be adjusted using a resistor divider to provide 1.26 V at this pin.
AGND
4
G
Analog ground pin.
PGND
5
G
Power ground pin.
DR
6
O
Drive pin. The gate of the external MOSFET should be connected to this pin.
FA/SD
7
I
Frequency adjust and Shutdown pin. A resistor connected to this pin sets the oscillator frequency. A high
level on this pin for longer than 30 µs will turn the device off. The device will then draw less than 10µA from
the supply.
VIN
8
P
Power Supply Input pin.
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature (unless otherwise noted)
(1)
MIN
MAX
UNIT
45
V
–0.4< V
V FB < 7
V
–0.4 < VFA/SD
VFA/SD< 7
V
1
A
+150
°C
Vapor Phase (60 s)
215
°C
Infrared (15 s)
260
°C
Input Voltage
FB Pin Voltage
FA/SD Pin Voltage
Peak Driver Output Current
(1.35 V) appears on the FA/SD pin, the
LM3478 stops switching and goes into a low current mode. The total supply current of the IC reduces to less
than 10 µA under these conditions. Figure 26 shows implementation of the shutdown function when operating in
frequency adjust mode. In this mode a high signal for more than 30 us shuts down the IC. However, the voltage
on the FA/SD pin should be always less than the absolute maximum of 7 V to avoid any damage to the device.
Figure 26. Shutdown Operation in Frequency Adjust Mode
7.3.4 Short-Circuit Protection
When the voltage across the sense resistor measured on the ISEN pin exceeds 343 mV, short circuit current limit
protection gets activated. A comparator inside the LM3478 reduces the switching frequency by a factor of 5 and
maintains this condition until the short is removed. In normal operation the sensed current will trigger the power
MOSFET to turn off. During the blanking interval the PWM comparator will not react to an over current so that
this additional 343 mV current limit threshold is implemented to protect the device in a short circuit or severe
overload condition.
7.4 Device Functional Modes
The device is set to run as soon as the input voltage crosses above the UVLO set point and at a frequency set
according to the FA/SD pin pulldown resistor. If the FA/SD pin is pulled high, the LM3481 enters shut-down
mode.
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LM3478 may be operated in either the continuous conduction mode (CCM) or the discontinuous current
conduction mode (DCM). The following applications are designed for the CCM operation. This mode of operation
has higher efficiency and usually lower EMI characteristics than the DCM.
8.2 Typical Applications
8.2.1 Typical High Efficiency Step-Up (Boost) Converter
Figure 27. Typical High Efficiency Step-Up (Boost) Converter Schematic
The boost converter converts a low input voltage into a higher output voltage. The basic configuration for a boost
converter is shown in Figure 28. In the CCM (when the inductor current never reaches zero at steady state), the
boost regulator operates in two states. In the first state of operation, MOSFET Q is turned on and energy is
stored in the inductor. During this state, diode D is reverse biased and load current is supplied by the output
capacitor, COUT.
In the second state, MOSFET Q is off and the diode is forward biased. The energy stored in the inductor is
transferred to the load and the output capacitor. The ratio of the switch on time to the total period is the duty
cycle D as shown in Equation 8.
D = 1 - (Vin / Vout)
(8)
Including the voltage drop across the MOSFET and the diode the definition for the duty cycle is shown in
Equation 9.
D = 1 - ((Vin - Vq)/(Vout + Vd))
(9)
Vd is the forward voltage drop of the diode and Vq is the voltage drop across the MOSFET when it is on.
16
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Typical Applications (continued)
A.
First Cycle Operation
B.
Second Cycle of Operation
Figure 28. Simplified Boost Converter
8.2.1.1 Design Requirements
To properly size the components for the application, the designer needs the following parameters: input voltage
range, output voltage, output current range, and required switching frequency. These four main parameters affect
the choices of component available to achieve a proper system behavior.
For the power supply, the input impedance of the supply rail should be low enough that the input current
transient does not drop below the UVLO value. The factors determining the choice of inductor used should be
the average inductor current, and the inductor current ripple. If the switching frequency is set high, the converter
can be operated with very small inductor values. The maximum current that can be delivered to the load is set by
the sense resistor, RSEN. Current limit occurs when the voltage generated across the sense resistor equals the
current sense threshold voltage, VSENSE. Also, a resistor RSL adds additional slope compensation, if required.
The following sections describe the design requirements for a typical LM3478 boost application.
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Custom Design with WEBENCH Tools
Click here to create a custom design using the LM3478 device with the WEBENCH® Power Designer.
1. Start by entering your VIN, VOUT and IOUT requirements.
2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and
compare this design with other possible solutions from Texas Instruments.
3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real
time pricing and component availability.
4. In most cases, you will also be able to:
– Run electrical simulations to see important waveforms and circuit performance,
– Run thermal simulations to understand the thermal performance of your board,
– Export your customized schematic and layout into popular CAD formats,
– Print PDF reports for the design, and share your design with colleagues.
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Typical Applications (continued)
5. Get more information about WEBENCH tools at www.ti.com/webench.
8.2.1.2.2 Power Inductor Selection
The inductor is one of the two energy storage elements in a boost converter. Figure 29 shows how the inductor
current varies during a switching cycle. The current through an inductor is quantified using Equation 10, which
shows the relationship of L, IL and VL.
(10)
The important quantities in determining a proper inductance value are IL (the average inductor current) and ΔIL
(the inductor current ripple). If ΔIL is larger than IL, the inductor current will drop to zero for a portion of the cycle
and the converter will operate in the DCM. All the analysis in this datasheet assumes operation in the CCM. To
operate in the CCM, the following condition must be met by using Equation 11.
(11)
Choose the minimum IOUT to determine the minimum inductance value. A common choice is to set ΔIL to 30% of
IL. Choosing an appropriate core size for the inductor involves calculating the average and peak currents
expected through the inductor. Use Equation 12, Equation 13, and Equation 14 to the peak inductor current in a
boost converter.
ILPEAK = Average IL(max) + ΔIL(max)
Average IL(max) = Iout / (1-D)
ΔIL(max) = D x Vin / (2 x fs x L)
(12)
(13)
(14)
An inductor size with ratings higher than these values has to be selected. If the inductor is not properly rated,
saturation will occur and may cause the circuit to malfunction.
The LM3478 can be set to switch at very high frequencies. When the switching frequency is high, the converter
can be operated with very small inductor values. The LM3478 senses the peak current through the switch which
is the same as the peak inductor current as calculated in the previous equation.
18
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Typical Applications (continued)
IL (A)
VIN
L
VIN
VOUT
L
'iL
IL_AVG
t (s)
D*Ts
Ts
(a)
ID (A)
VIN - V OUT
L
ID_AVG
=IOUT_AVG
t (s)
D*Ts
Ts
(b)
ISW (A)
VIN
L
ISW_AVG
t (s)
D*Ts
Ts
(C)
Figure 29. Inductor Current and Diode Current
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Typical Applications (continued)
8.2.1.2.3 Programming the Output Voltage
The output voltage can be programmed using a resistor divider between the output and the FB pin. The resistors
are selected such that the voltage at the FB pin is 1.26 V. Pick RF1 (the resistor between the output voltage and
the feedback pin) and RF2 (the resistor between the feedback pin and ground) can be selected using the
following equation,
RF2 = (1.26 V x RF1) / (Vout - 1.26 V)
(15)
A 100-pF capacitor may be connected between the feedback and ground pins to reduce noise.
8.2.1.2.4 Setting the Current Limit
The maximum amount of current that can be delivered to the load is set by the sense resistor, RSEN. Current limit
occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage,
VSENSE. When this threshold is reached, the switch will be turned off until the next cycle. Limits for VSENSE are
specified in the electrical characteristics section. VSENSE represents the maximum value of the internal control
signal VCS as shown in Figure 30. This control signal, however, is not a constant value and changes over the
course of a period as a result of the internal compensation ramp (VSL). Therefore the current limit threshold will
also change. The actual current limit threshold is a function of the sense voltage (VSENSE) and the internal
compensation ramp:
RSEN x ISWLIMIT = VCSMAX = VSENSE - (D x VSL)
(16)
Where ISWLIMIT is the peak switch current limit, defined by Equation 17.
120
VSL
DUTY CYCLE (%)
100
80
VSENSE
60
FS = 500 kHz
40
20
FS =
250 kHz
0
0.000 0.100
0.200
0.300
0.400
0.500
CURRENT SENSE VOLTAGE (V)
Figure 30. Current Sense Voltage vs Duty Cycle
Figure 30 shows how VCS (and current limit threshold voltage) change with duty cycle. The curve is equivalent to
the internal compensation ramp slope (Se) and is bounded at low duty cycle by VSENSE, shown as a dotted line.
As duty cycle increases, the control voltage is reduced as VSL ramps up. The graph also shows the short circuit
current limit threshold of 343 mV (typical) during the 325 ns (typical) blanking time. For higher frequencies this
fixed blanking time obviously occupies more duty cycle, percentage wise. Since current limit threshold varies with
duty cycle, the use Equation 17 to select RSEN and set the desired current limit threshold:
VSENSE - (D x VSL)
RSEN =
ISWLIMIT
(17)
The numerator of Equation 17 is VCS, and ISWLIMIT using Equation 18.
ISWLIMIT =
20
IOUT
+
(D x VIN)
(1-D) (2 x fS x L)
(18)
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Typical Applications (continued)
To avoid false triggering, the current limit value should have some margin above the maximum operating value,
typically 120%. Values for both VSENSE and VSL are specified in Electrical Characteristics. However, calculating
with the limits of these two specs could result in an unrealistically wide current limit or RSEN range. Therefore,
Equation 19 is recommended, using the VSL ratio value given in Electrical Characteristics.
VSENSE - (D x VSENSE x VSLratio)
RSEN =
ISWLIMIT
(19)
RSEN is part of the current mode control loop and has some influence on control loop stability. Therefore, once
the current limit threshold is set, loop stability must be verified. As described in the slope compensation section,
Equation 20 must hold true for a current mode converter to be stable.
Sf - Se < Sn + Se
(20)
To verify that this equation holds true, use Equation 21.
2 x VSL x fS x L
RSEN <
Vo - (2 x VIN)
(21)
If the selected RSEN is greater than this value, additional slope compensation must be added to ensure stability,
as described in the section below.
8.2.1.2.5 Current Limit with External Slope Compensation
RSL is used to add additional slope compensation when required. It is not necessary in most designs and RSL
should be no larger than necessary. Select RSL according to Equation 22.
RSEN x (Vo - 2VIN)
- VSL
2 x fS x L
RSL >
40 PA
(22)
Where RSEN is the selected value based on current limit. With RSL installed, the control signal includes additional
external slope to stabilize the loop, which will also have an effect on the current limit threshold. Therefore, the
current limit threshold must be re-verified, as illustrated in Equation 23, Equation 24, and Equation 25 below.
VCS = VSENSE – (D x (VSL + ΔVSL))
(23)
Where ΔVSL is the additional slope compensation generated as discussed in the slope compensation ramp
section and calculated using Equation 24.
ΔVSL = 40 µA x RSL
(24)
This changes the equation for current limit (or RSEN) as shown in Equation 25.
VSENSE - (D x(VSL + 'VSL))
ISWLIMIT =
RSEN
(25)
The RSEN and RSL values may have to be calculated iteratively in order to achieve both the desired current limit
and stable operation. In some designs RSL can also help to filter noise on the ISEN pin.
If the inductor is selected such that ripple current is the recommended 30% value, and the current limit threshold
is 120% of the maximum peak, a simpler method can be used to determine RSEN. Equation 26 below will provide
optimum stability without RSL, provided that the above 2 conditions are met.
VSENSE
RSEN =
Vo - Vi
xD
ISWLIMIT +
L x fS
(26)
8.2.1.2.6 Power Diode Selection
Observation of the boost converter circuit shows that the average current through the diode is the average load
current, and the peak current through the diode is the peak current through the inductor. The diode should be
rated to handle more than its peak current. The peak diode current can be calculated using Equation 27.
ID(Peak) = IOUT/ (1−D) + ΔIL
(27)
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Typical Applications (continued)
Thermally the diode must be able to handle the maximum average current delivered to the output. The peak
reverse voltage for boost converters is equal to the regulated output voltage. The diode must be capable of
handling this voltage. To improve efficiency, a low forward drop schottky diode is recommended.
8.2.1.2.7 Power MOSFET Selection
The drive pin of the LM3478 must be connected to the gate of an external MOSFET. The drive pin (DR) voltage
depends on the input voltage (see Typical Characteristics). In most applications, a logic level MOSFET can be
used. For very low input voltages, a sub logic level MOSFET should be used. The selected MOSFET has a great
influence on the system efficiency. The critical parameters for selecting a MOSFET are:
1. Minimum threshold voltage, VTH(MIN)
2. On-resistance, RDS(ON)
3. Total gate charge, Qg
4. Reverse transfer capacitance, CRSS
5. Maximum drain to source voltage, VDS(MAX)
The off-state voltage of the MOSFET is approximately equal to the output voltage. Vds(max) must be greater
than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and
switching losses. RDS(ON) is needed to estimate the conduction losses, Pcond:
Pcond = I2 x RDS(ON) x D
(28)
The temperature effect on the RDS(ON) usually is quite significant. Assume 30% increase at hot.
For the current I in Equation 28 the average inductor current may be used.
Especially at high switching frequencies the switching losses may be the largest portion of the total losses.
The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation.
Often the individual MOSFET's data sheet does not give enough information to yield a useful result. Equation 29
and Equation 30 give a rough idea how the switching losses are calculated:
PSW =
ILmax x Vout
2
x fSW x (tLH + tHL)
(29)
RdrOn
Qgs
x
tLH = Qgd +
Vdr - Vgsth
2
(30)
8.2.1.2.8 Input Capacitor Selection
Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous
and triangular as shown in Figure 29. The inductor ensures that the input capacitor sees fairly low ripple currents.
However, as the input capacitor gets smaller, the input ripple goes up. The RMS current in the input capacitor is
given using Equation 31.
(31)
The input capacitor should be capable of handling the RMS current. Although the input capacitor is not as critical
in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should
be chosen in the range of 10 µF to 20 µF. If a value lower than 10 µF is used, then problems with impedance
interactions or switching noise can affect the LM3478. To improve performance, especially with Vin below 8 volts,
it is recommended to use a 20 Ohm resistor at the input to provide an RC filter. The resistor is placed in series
with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 31). A 0.1-µF or 1-µF
ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the
other side of the resistor at the input power supply.
22
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Typical Applications (continued)
RIN
VIN
VIN
LM3478
CIN
CBYPASS
Figure 31. Reducing IC Input Noise
8.2.1.2.9 Output Capacitor Selection
The output capacitor in a boost converter provides all the output current when the inductor is charging. As a
result it sees very large ripple currents. The output capacitor should be capable of handling the maximum RMS
current. Equation 32 shows the RMS current in the output capacitor.
(32)
Where
(33)
The ESR and ESL of the capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the
output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic,
polymer tantalum, or multi-layer ceramic capacitors are recommended at the output.
For applications that require very low output voltage ripple, a second stage LC filter often is a good solution. Most
of the time it is lower cost to use a small second Inductor in the power path and an additional final output
capacitor than to reduce the output voltage ripple by purely increasing the output capacitor without an additional
LC filter.
8.2.1.2.10 Compensation
For detailed explanation on how to select the right compensation components to attach to the compensation pin
for a boost topology, please see AN-1286 Compensation For The LM3748 Boost Controller SNVA067.
8.2.1.3 Application Curves
Figure 32. Efficiency vs Load Current (9-V In and 12-V Out)
Figure 33. Efficiency vs Load Current (3.3-V In and 5-V
Out)
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Typical Applications (continued)
8.2.2 Typical SEPIC Converter
Figure 34. Typical SEPIC Converter
Since the LM3478 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary
Inductance Converter) applications. An example of a SEPIC using the LM3478 is shown in Figure 34. Note that
the output voltage can be higher or lower than the input voltage. The SEPIC uses two inductors to step-up or
step-down the input voltage. The inductors L1 and L2 can be two discrete inductors or two windings of a coupled
inductor since equal voltages are applied across the inductor throughout the switching cycle. Using two discrete
inductors allows use of catalog magnetics, as opposed to a custom inductor. The input ripple can be reduced
along with size by using the coupled windings for L1 and L2.
Due to the presence of the inductor L1 at the input, the SEPIC inherits all the benefits of a boost converter. One
main advantage of a SEPIC over a boost converter is the inherent input to output isolation. The capacitor CS
isolates the input from the output and provides protection against a shorted or malfunctioning load. Hence, the
SEPIC is useful for replacing boost circuits when true shutdown is required. This means that the output voltage
falls to 0V when the switch is turned off. In a boost converter, the output can only fall to the input voltage minus a
diode drop.
The duty cycle of a SEPIC is given using Equation 34.
(34)
In Equation 34, VQ is the on-state voltage of the MOSFET, Q, and VDIODE is the forward voltage drop of the
diode.
8.2.2.1 Design Requirements
To properly size the components for the application, the designer needs the following parameters: input voltage
range, output voltage, output current range, and required switching frequency. These four main parameters affect
the choices of component available to achieve a proper system behavior.
For the power supply, the input impedance of the supply rail should be low enough that the input current
transient does not drop below the UVLO value. The factors determining the choice of inductor used should be
the average inductor current, and the inductor current ripple. If the switching frequency is set high, the converter
can be operated with very small inductor values. The maximum current that can be delivered to the load is set by
the sense resistor, RSEN. Current limit occurs when the voltage generated across the sense resistor equals the
current sense threshold voltage, VSENSE. Also, a resistor RSL adds additional slope compensation, if required.
The following sections describe the design requirements for a typical LM3478 boost application.
24
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Typical Applications (continued)
8.2.2.2 Detailed Design Procedure
8.2.2.2.1 Power MOSFET Selection
As in a boost converter, parameters governing the selection of the MOSFET are the minimum threshold voltage,
VTH(MIN), the on-resistance, RDS(ON), the total gate charge, Qg, the reverse transfer capacitance, CRSS, and the
maximum drain to source voltage, VDS(MAX). The peak switch voltage in a SEPIC is given using Equation 35.
VSW(PEAK) = VIN + VOUT + VDIODE
(35)
The selected MOSFET should satisfy the condition:
VDS(MAX) > VSW(PEAK)
(36)
The peak switch current is given using Equation 37.
(37)
The RMS current through the switch is given using Equation 38.
(38)
8.2.2.2.2 Power Diode Selection
The Power diode must be selected to handle the peak current and the peak reverse voltage. In a SEPIC, the
diode peak current is the same as the switch peak current. The off-state voltage or peak reverse voltage of the
diode is VIN + VOUT. Similar to the boost converter, the average diode current is equal to the output current.
Schottky diodes are recommended.
8.2.2.2.3 Selection of Inductors L1 and L2
Proper selection of inductors L1 and L2 to maintain continuous current conduction mode requires calculations of
the following parameters.
Average current in the inductors can be calculated using Equation 39.
(39)
(40)
IL2AVE = IOUT
Peak to peak ripple current, to calculate core loss if necessary using Equation 41 and Equation 42.
(41)
(42)
Maintaining the condition IL > ΔiL/2 to ensure continuous current conduction yields Equation 43 and Equation 44.
(VIN - VQ)(1-D)
L1 >
2IOUTfS
(43)
(VIN - VQ)D
L2 >
2IOUTfS
(44)
Peak current in the inductor, use Equation 45 and Equation 46 to ensure the inductor does not saturate.
(45)
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Typical Applications (continued)
(46)
IL1PK must be lower than the maximum current rating set by the current sense resistor.
The value of L1 can be increased above the minimum recommended to reduce input ripple and output ripple.
However, once DIL1 is less than 20% of IL1AVE, the benefit to output ripple is minimal.
By increasing the value of L2 above the minimum recommended, ΔIL2 can be reduced, which in turn will reduce
the output ripple voltage:
'VOUT =
(
IOUT
1-D
+
'IL2
2
)
ESR
(47)
where ESR is the effective series resistance of the output capacitor.
If L1 and L2 are wound on the same core, then L1 = L2 = L. All of the previous equations will hold true if the
inductance is replaced by 2L.
8.2.2.2.4 Sense Resistor Selection
The peak current through the switch, ISW(PEAK) can be adjusted using the current sense resistor, RSEN, to provide
a certain output current. Resistor RSEN can be selected using Equation 48
VSENSE - D(VSL + 'VSL)
RSEN =
ISWPEAK
(48)
8.2.2.2.5 Sepic Capacitor Selection
The selection of the SEPIC capacitor, CS, depends on the RMS current. The RMS current of the SEPIC
capacitor is given by Equation 49.
(49)
The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes
the SEPIC much better suited to lower power applications where the RMS current through the capacitor is
relatively small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than
the maximum input voltage. There is an energy balance between CS and L1, which can be used to determine
the value of the capacitor. Equation 50 shows the basic energy balance.
(50)
where
(51)
is the ripple voltage across the SEPIC capacitor, and
(52)
is the ripple current through the inductor L1. The energy balance equation can be solved using Equation 53 to
provide a minimum value for CS.
(53)
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Typical Applications (continued)
8.2.2.2.6 Input Capacitor Selection
Similar to a boost converter, the SEPIC has an inductor at the input. Hence, the input current waveform is
continuous and triangular. The inductor ensures that the input capacitor sees fairly low ripple currents. However,
as the input capacitor gets smaller, the input ripple goes up. The RMS current in the input capacitor is given
using Equation 54.
(54)
The input capacitor should be capable of handling the RMS current. Although the input capacitor is not as critical
in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should
be chosen in the range of 10µF to 20µF. If a value lower than 10 µF is used, then problems with impedance
interactions or switching noise can affect the LM3478. To improve performance, especially with VIN below 8 volts,
TI recommends that the user uses a 20Ω resistor at the input to provide a RC filter. The resistor is placed in
series with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 31). A 0.1-µF or 1µF ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the
other side of the resistor with the input power supply.
8.2.2.2.7 Output Capacitor Selection
The output capacitor of the SEPIC sees very large ripple currents (similar to the output capacitor of a boost
converter). The RMS current through the output capacitor is given using Equation 55.
IRMS =
2
ISWPK2 - ISWPK ('IL1 + 'IL2)+ ('IL1 + 'IL2) (1-D) - IOUT2
3
(55)
The ESR and ESL of the output capacitor directly control the output ripple. Use low capacitors with low ESR and
ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer
electrolytic and polymer tantalum, Sanyo-OSCON, or multi-layer ceramic capacitors are recommended at the
output for low ripple.
8.2.2.3 Application Curves
Figure 35. Efficiency vs Load Current (3.3-V In and 12-V
Out)
Figure 36. Efficiency vs Load Current (5-V In and 12-V Out)
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9 Power Supply Recommendations
The LM3478 is designed to operate from various DC power supply including a car battery. If so, VIN input should
be protected from reversal voltage and voltage dump over 40 volts. The impedance of the input supply rail
should be low enough that the input current transient does not cause drop below VIN UVLO level. If the input
supply is connected by using long wires, additional bulk capacitance may be required in addition to normal input
capacitor.
10 Layout
10.1 Layout Guidelines
Good board layout is critical for switching controllers. First the ground plane area must be sufficient for thermal
dissipation purposes and second, appropriate guidelines must be followed to reduce the effects of switching
noise. Switching converters are very fast switching devices. In such devices, the rapid increase of input current
combined with the parasitic trace inductance generates unwanted Ldi/dt noise spikes. The magnitude of this
noise tends to increase as the output current increases. This parasitic spike noise may turn into electromagnetic
interference (EMI), and can also cause problems in device performance. Therefore, care must be taken in layout
to minimize the effect of this switching noise. The current sensing circuit in current mode devices can be easily
affected by switching noise. This noise can cause duty cycle jittering which leads to increased spectral noise.
Although the LM3478 has 325 ns blanking time at the beginning of every cycle to ignore this noise, some noise
may remain after the blanking time.
The most important layout rule is to keep the AC current loops as small as possible. Figure 37 shows the current
flow of a boost converter. The top schematic shows a dotted line which represents the current flow during onstate and the middle schematic shows the current flow during off-state. The bottom schematic shows the currents
we refer to as AC currents. They are the most critical ones since current is changing in very short time periods.
The dotted lined traces of the bottom schematic are the once to make as short as possible.
The PGND and AGND pins have to be connected to the same ground very close to the IC. To avoid ground loop
currents, attach all the grounds of the system only at one point.
A ceramic input capacitor should be connected as close as possible to the Vin pin and grounded close to the
GND pin.
For more information about layout in switch mode power supplies please refer to AN-1229 Simple Switcher PCB
Layout Guidelines, SNVA054.
Figure 37. Current Flow in a Boost Application
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Rs
GND
Rsn
Rc
COUT3
Cc
COUT2
COUT1
Cc2
Rfbb
Cff
Rfbt
10.2 Layout Example
Csn
OUTPUT+
LM3478
Cbyp
D1
Rbyp
Rfa
L1
Rdr
Q1
SW_TP
Q1
SD
INPUT+
GND
CIN1
CIN2
See evaluation modules for more detailed examples.
Figure 38. Typical Layout for a Boost Converter
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11 Device and Documentation Support
11.1 Custom Design with WEBENCH Tools
Click here to create a custom design using the LM3478 device with the WEBENCH® Power Designer.
1. Start by entering your VIN, VOUT and IOUT requirements.
2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and
compare this design with other possible solutions from Texas Instruments.
3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real
time pricing and component availability.
4. In most cases, you will also be able to:
– Run electrical simulations to see important waveforms and circuit performance,
– Run thermal simulations to understand the thermal performance of your board,
– Export your customized schematic and layout into popular CAD formats,
– Print PDF reports for the design, and share your design with colleagues.
5. Get more information about WEBENCH tools at www.ti.com/webench.
11.2 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
11.3 Documentation Support
Create a custom design using LM3478 with WEBENCH Power Designer.
11.3.1 Related Documentation
For related documentation see the following:
• AN-1286 Compensation for the LM3748 Boost Controller SNVA067
• AN-1229 Simple Switcher PCB Layout Guidelines SNVA054
11.4 Related Links
The table below lists quick access links. Categories include technical documents, support and community
resources, tools and software, and quick access to sample or buy.
Table 1. Related Links
PARTS
PRODUCT FOLDER
SAMPLE & BUY
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
LM3478
Click here
Click here
Click here
Click here
Click here
11.5 Trademarks
WEBENCH is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.6 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.7 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
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12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
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30-Sep-2021
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM3478MA/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
L3478
MA
LM3478MAX/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
L3478
MA
LM3478MM
NRND
VSSOP
DGK
8
1000
Non-RoHS
& Green
Call TI
Level-1-260C-UNLIM
-40 to 125
S14B
LM3478MM/NOPB
ACTIVE
VSSOP
DGK
8
1000
RoHS & Green
NIPDAUAG | SN
Level-1-260C-UNLIM
-40 to 125
S14B
LM3478MMX/NOPB
ACTIVE
VSSOP
DGK
8
3500
RoHS & Green
NIPDAUAG | SN
Level-1-260C-UNLIM
-40 to 125
S14B
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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