NSC LQA28A

LM4805
3V, 1W Boosted Boomer
General Description
Key Specifications
The LM4805 is a boosted audio power amplifier designed for
driving 8ohm speakers in portable applications. It delivers at
least 1W continuous power to an 8Ω load from any input
voltage between 3V and 4.6V with less than 2% THD+N.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4805 does not require bootstrap capacitors, or snubber circuits. Therefore it is ideally
suited for portable applications requiring high power and
minimal size.
The LM4805 features a low-power consumption shutdown
mode along with an internal thermal shutdown protection
mechanism and short circuit protection.
The LM4805 contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during
turn-on and turn-off transitions. The LM4805 is unity-gain
stable and can be configured by external gain-setting resistors.
j Quiescent Power Supply Current
(VDD = 3V)
14mA (typ)
j Output Power
(VDD = 3.0V, RL = 8Ω, THD+N = 2%)
j Shutdown Current
1W (typ)
2µA (max)
Features
n Pop & click circuitry eliminates noise during turn-on and
turn-off transitions
n Low, 2µA (max) shutdown current
n Low, 14mA (typ) quiescent current
n Unity-gain stable
n External gain configuration capability
Applications
n Cellphone
n PTT (Push To Talk) mobile phones
Connection Diagrams
LM4805LQ (5x5)
LQ Marking
20126256
Top View
U = Wafer Fab
Z = Assembly Plant Code
XY = Date Code
TT = Die Run Code
20126284
Top View
Order Number LM4805LQ
See NS Package Number LQA28A
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2005 National Semiconductor Corporation
DS201262
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LM4805 3V, 1W Boosted Boomer
May 2005
LM4805
Typical Application
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* Cf2 is optional.
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
Junction Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Thermal Resistance
Supply Voltage (VDD)
6.5V
Supply Voltage (V1)
6.5V
Storage Temperature
θJA (LLP)
Operating Ratings
Temperature Range
−0.3V to VDD + 0.3V
Power Dissipation (Note 3)
TMIN ≤ TA ≤ TMAX
Internally limited
ESD Susceptibility (Note 4)
2000V
ESD Susceptibility (Note 5)
200V
59˚C/W
See AN-1187 ’Leadless Leadframe Packaging (LLP).’
−65˚C to +150˚C
Input Voltage
125˚C
−40˚C ≤ TA ≤ +85˚C
2.7V ≤ VDD ≤ 4.6V
Supply Voltage (VDD)
2.7V ≤ V1 ≤ 6.1V
Supply Voltage (V1)
Electrical Characteristics VDD = 4.2V (Notes 1, 2)
The following specifications apply for VDD = 4.2V, AV-BTL = 26dB, RL = 8Ω, CB = 1.0µF, R1 = 51.1kΩ, R2 = 15kΩ unless otherwise specified. Limits apply for TA = 25˚C. See Figure 1.
Symbol
Parameter
Conditions
LM4805
Typical
(Note 6)
Limit
(Notes 7, 8)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0, RLOAD = ∞
10
23
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND (Notes 9, 10)
0.1
2
µA (max)
VSDIH
Shutdown Voltage Input High
S/D1 and S/D2
1.5
V (min)
VSDIL
Shutdown Voltage Input Low
S/D1 and S/D2
0.4
V (max)
TWU
Wake-up Time
CB = 1.0µF
110
msec
(max)
VOS
Output Offset Voltage
TSD
Thermal Shutdown Temperature
80
5
POUT
Output Power
THD = 1% (max), f = 1kHz,
Mono BTL
THD+N
Total Harmomic Distortion + Noise
PO = 500mW, f = 1kHz
0.2
40
mV (max)
125
˚C (min)
1.2
0.9
W (min)
0.5
% (max)
eOS
Output Noise
A-Weighted Filter, VIN = 0V
105
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mVp-p, f = 100Hz,
inputs terminated
66
dB
VFB
Feedback Pin Reference Voltage
Note 11
1.23
V
Electrical Characteristics VDD = 3.0V (Notes 1, 2)
The following specifications apply for VDD = 3.0V, AV-BTL = 26dB, RL = 8Ω, CB = 1.0µF, R1 = 51.1kΩ, R2 = 15kΩ unless otherwise specified. Limits apply for TA = 25˚C.
Symbol
Parameter
Conditions
LM4805
Units
(Limits)
Typical
(Note 6)
Limit
(Notes 7, 8)
VDD = 3.2V, VIN = 0, RLOAD = ∞
14
27
mA (max)
0.1
2
µA (max)
IDD
Quiescent Power Supply Current
ISD
Shutdown Current
VSHUTDOWN = GND (Notes 9, 10)
VSDIH
Shutdown Voltage Input High
S/D1 and S/D2
1.5
V (min)
VSDIL
Shutdown Voltage Input Low
S/D1 and S/D2
0.4
V (max)
TWU
Wake-up Time
CB = 1.0µF
110
msec
(max)
VOS
Output Offset Voltage
TSD
Thermal Shutdown Temperature
80
5
40
mV (max)
125
˚C (min))
1
0.85
W (min)
0.55
% (max)
POUT
Output Power
THD = 2% (max), f = 1kHz,
Mono BTL
THD+N
Total Harmomic Distortion + Noise
PO = 500mW, fIN = 1kHz
0.25
eOS
Output Noise
A-Weighted Filter, VIN = 0V
105
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mVp-p, f = 100Hz
66
dB (min)
3
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LM4805
Absolute Maximum Ratings (Notes 1, 2)
LM4805
Electrical Characteristics VDD = 3.0V (Notes 1, 2)
(Continued)
The following specifications apply for VDD = 3.0V, AV-BTL = 26dB, RL = 8Ω, CB = 1.0µF, R1 = 51.1kΩ, R2 = 15kΩ unless otherwise specified. Limits apply for TA = 25˚C.
Symbol
VFB
Parameter
Feedback Pin Reference Voltage
Conditions
(Note 11)
LM4805
Typical
(Note 6)
Limit
(Notes 7, 8)
1.23
1.205
1.255
Units
(Limits)
V (max)
V (min)
Note 1: All voltages are measured with respect to the GND pin, unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The maximum
allowable power dissipation is PDMAX = (TJMAX − TA) / θJA or the given in Absolute Maximum Ratings, whichever is lower.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 5: Machine Model, 220pF–240pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: Shutdown current is measured at an ambient temperature of 25˚C. The Shutdown pin should be driven as close as possible to GND for minimum shutdown
current.
Note 10: Shutdown current is measured with components R1 and R2 removed.
Note 11: Feedback pin reference voltage is measured with the Audio Amplifier’s V1 (pin 26) floating and no addition load connected to the cathode of D1 (see Figure
1).
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LM4805
Typical Performance Characteristics
THD+N vs Frequency
VDD = 3V, AV = 6dB, RL = 16Ω
THD+N vs Frequency
VDD = 3V, AV = 6dB, RL = 8Ω
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THD+N vs Frequency
VDD = 3V, AV = 26dB, RL = 16Ω
THD+N vs Frequency
VDD = 3V, AV = 26dB, RL = 8Ω
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THD+N vs Frequency
VDD = 4.2V, AV = 6dB, RL = 16Ω
THD+N vs Frequency
VDD = 4.2V, AV = 6dB, RL = 8Ω
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20126216
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LM4805
Typical Performance Characteristics
(Continued)
THD+N vs Frequency
VDD = 4.2V, AV = 26dB, RL = 8Ω
THD+N vs Frequency
VDD = 4.2V, AV = 26dB, RL = 16Ω
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THD+N vs Output Power
VDD = 3V, AV = 6dB, RL = 16Ω
THD+N vs Output Power
VDD = 3V, AV = 6dB, RL = 8Ω
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THD+N vs Output Power
VDD = 3V, AV = 26dB, RL = 16Ω
THD+N vs Output Power
VDD = 3V, AV = 26dB, RL = 8Ω
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6
LM4805
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
VDD = 4.2V, AV = 6dB, RL = 8Ω
THD+N vs Output Power
VDD = 4.2V, AV = 6dB, RL = 16Ω
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THD+N vs Output Power
VDD = 4.2V, AV = 26dB, RL = 16Ω
THD+N vs Output Power
VDD = 4.2V, AV = 26dB, RL = 8Ω
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20126226
Supply Current vs Supply Voltage
Power Dissipation vs Output Power
VDD = 3V, RL = 8Ω, f = 1kHz
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LM4805
Typical Performance Characteristics
(Continued)
Power Dissipation vs Output Power
VDD = 3V, RL = 16Ω, f = 1kHz
Power Dissipation vs Output Power
VDD = 4.2V, RL = 8Ω, f = 1kHz
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Output Power vs Load Resistance
VDD = 3V
Power Dissipation vs Output Power
VDD = 4.2V, RL = 16Ω, f = 1kHz
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20126232
Output Power vs Supply Voltage
RL = 8Ω, f = 1kHz
Output Power vs Load Resistance
VDD = 4.2V
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LM4805
Typical Performance Characteristics
(Continued)
PSRR vs FREQUENCY
VDD = 3V, AV = 6dB
Vripple = 200mVP-P
Output Power vs Supply Voltage
RL = 16Ω, f = 1kHz
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PSRR vs FREQUENCY
VDD = 3V, AV = 26dB
Vripple = 200mVP-P
PSRR vs FREQUENCY
VDD = 4.2V, AV = 6dB
Vripple = 200mVP-P
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PSRR vs FREQUENCY
VDD = 4.2V, AV = 26dB
Vripple = 200mVP-P
Load Current vs VDD
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LM4805
Typical Performance Characteristics
(Continued)
Amplifier Frequency Response
vs Input Capacitor Size
Amplifier Open Loop
vs Frequency Respose
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Switch Current Limit
vs Duty Cycle - ”X”
Oscillator Frequency
vs Temperature - ”X”
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Feedback Voltage
vs Temperature
Feedback Bias Current
vs Temperature
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10
LM4805
Typical Performance Characteristics
(Continued)
Maximum Duty Cycle
vs Temperature - ”X”
RDS (ON)
vs Temperature
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20126245
RDS (ON)
vs VDD
20126248
AVD = 2 *(Rf/Ri)
to the load, thus doubling the output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same conditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power Amplifier
Design section.
The bridge configuration also creates a second advantage
over single-ended amplifiers. Since the differential outputs,
VO1 and VO2, are biased at half-supply, no net DC voltage
exists across the load. This eliminates the need for an output
coupling capacitor which is required in a single supply,
single-ended amplifier configuration. Without an output coupling capacitor, the half-supply bias across the load would
result in both increased internal IC power dissipation and
also possible loudspeaker damage.
By driving the load differentially through outputs VO1 and
VO2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classic single-ended amplifier configuration where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration. It provides differential drive
AMPLIFIER POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the amplifier portion of the
LM4805 has two operational amplifiers, the maximum inter-
Application Information
BRIDGE CONFIGURATION EXPLANATION
The Audio Amplifier portion of the LM4805 has two internal
amplifiers allowing different amplifier configurations. The first
amplifier’s gain is externally configurable, whereas the second amplifier is internally fixed in a unity-gain, inverting
configuration. The closed-loop gain of the first amplifier is set
by selecting the ratio of Rf to Ri while the second amplifier’s
gain is fixed by the two internal 20kΩ resistors. Figure 1
shows that the output of amplifier one serves as the input to
amplifier two. This results in both amplifiers producing signals identical in magnitude, but out of phase by 180˚. Consequently, the differential gain for the Audio Amplifier is
11
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LM4805
Application Information
may be connected to a large plane of continuous unbroken
copper. This plane forms a thermal mass, heat sink, and
radiation area. Further detailed and specific information concerning PCB layout, fabrication, and mounting an LD (LLP)
package is found in National Semiconductor’s Package Engineering Group under application note AN1187.
(Continued)
nal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation for a given BTL
application can be derived from Equation 1.
(1)
PDMAX(AMP) = 4(VDD)2 / (2π2RL)
SHUTDOWN FUNCTION
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch, and a pull-up resistor.
One terminal of the switch is connected to GND. The other
side is connected to the two shutdown pins and the terminal
of the pull-up resistor. The remaining resistance terminal is
connected to VDD. If the switch is open, then the external
pull-up resistor connected to VDD will enable the LM4805.
This scheme guarantees that the shutdown pins will not float
thus preventing unwanted state changes.
BOOST CONVERTER POWER DISSIPATION
At higher duty cycles, the increased ON-time of the switch
FET means the maximum output current will be determined
by power dissipation within the LM2731 FET switch. The
switch power dissipation from ON-time conduction is calculated by Equation 2.
(2)
PDMAX(SWITCH) = DC x IIND(AVE)2 x RDS(ON)
where DC is the duty cycle.
There will be some switching losses as well, so some derating needs to be applied when calculating IC power dissipation.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers, and switching boost converters, is critical for optimizing device and system performance.
Consideration to component values must be used to maximize overall system quality.
The best capacitors for use with the switching converter
portion of the LM4805 are multi-layer ceramic capacitors.
They have the lowest ESR (equivalent series resistance)
and highest resonance frequency, which makes them optimum for high frequency switching converters.
When selecting a ceramic capacitor, only X5R and X7R
dielectric types should be used. Other types such as Z5U
and Y5F have such severe loss of capacitance due to effects
of temperature variation and applied voltage, they may provide as little as 20% of rated capacitance in many typical
applications. Always consult capacitor manufacturer’s data
curves before selecting a capacitor. High-quality ceramic
capacitors can be obtained from Taiyo-Yuden, AVX, and
Murata.
TOTAL POWER DISSIPATION
The total power dissipation for the LM4805 can be calculated
by adding Equation 1 and Equation 2 together to establish
Equation 3:
PDMAX(TOTAL) = [4*(VDD)2/2π2RL] + [DC x IIND(AVE)2
x RDS(ON)]
(3)
The result from Equation 3 must not be greater than the
power dissipation that results from Equation 4:
PDMAX = (TJMAX - TA) / θJA
(4)
For the LQA28A, θJA = 59˚C/W. TJMAX = 125˚C for the
LM4805. Depending on the ambient temperature, TA, of the
system surroundings, Equation 4 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 3 is greater than that of
Equation 4, then either the supply voltage must be increased, the load impedance increased or TA reduced. For
the typical application of a 3V power supply, with V1 set to
5.5V and an 8Ω load, the maximum ambient temperature
possible without violating the maximum junction temperature
is approximately 111˚C provided that device operation is
around the maximum power dissipation point. Thus, for typical applications, power dissipation is not an issue. Power
dissipation is a function of output power and thus, if typical
operation is not around the maximum power dissipation
point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics
curves for power dissipation information for lower output
levels.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both V1 and VDD (Cs2 and Cs1) pins
should be as close to the device as possible.
SELECTING INPUT CAPACITOR FOR AUDIO
AMPLIFIER
One of the major considerations is the closedloop bandwidth
of the amplifier. To a large extent, the bandwidth is dictated
by the choice of external components shown in Figure 1. The
input coupling capacitor, Ci, forms a first order high pass filter
which limits low frequency response. This value should be
chosen based on needed frequency response for a few
distinct reasons.
High value input capacitors are both expensive and space
hungry in portable designs. Clearly, a certain value capacitor
is needed to couple in low frequencies without severe attenuation. However, speakers used in portable systems,
whether internal or external, have little ability to reproduce
signals below 100Hz to 150Hz. Thus, using a high value
input capacitor may not increase actual system performance.
In addition to system cost and size, click and pop performance is affected by the value of the input coupling capaci-
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4805’s exposed-DAP (die attach paddle) package
(LD) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. The low
thermal resistance allows rapid heat transfer from the die to
the surrounding PCB copper traces, ground plane, and surrounding air. The LD package should have its DAP soldered
to a copper pad on the PCB. The DAP’s PCB copper pad
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12
SETTING THE OUTPUT VOLTAGE (V1) OF BOOST
CONVERTER
(Continued)
tor, Ci. A high value input coupling capacitor requires more
charge to reach its quiescent DC voltage (nominally 1/2
VDD). This charge comes from the output via the feedback
and is apt to create pops upon device enable. Thus, by
minimizing the capacitor value based on desired low frequency response, turn-on pops can be minimized.
The output voltage is set using the external resistors R1 and
R2 (see Figure 1). A value of approximately 15k is recommended for R2 to establish a divider current of approximately 92µA. R1 is calculated using the formula:
R1 = R2 X (V1/1.23 − 1)
(5)
SELECTING BYPASS CAPACITOR FOR AUDIO
AMPLIFIER
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM4805’s internal Boost converter is internally
compensated, the external feed-forward capacitor Cf1 is
required for stability (see Figure 1). Adding this capacitor
puts a zero in the loop response of the converter. The
recommended frequency for the zero fz should be approximately 6kHz. Cf1 can be calculated using the formula:
Besides minimizing the input capacitor value, careful consideration should be paid to the bypass capacitor value. Bypass
capacitor, CB, is the most critical component to minimize
turn-on pops since it determines how fast the amplifier turns
on. The slower the amplifier’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on
pop. Choosing CB equal to 1.0µF along with a small value of
Ci (in the range of 0.039µF to 0.39µF), should produce a
virtually clickless and popless shutdown function. Although
the device will function properly, (no oscillations or motorboating), with CB equal to 0.1µF, the device will be much
more susceptible to turn-on clicks and pops. Thus, a value of
CB equal to 1.0µF is recommended in all but the most cost
sensitive designs.
Cf1 = 1 / (2 X R1 X fz)
(6)
SELECTING DIODES
The external diode used in Figure 1 should be a Schottky
diode. A 20V diode such as the MBR0520 is recommended.
The MBR05XX series of diodes are designed to handle a
maximum average current of 0.5A. For applications exceeding 0.5A average but less than 1A, a Microsemi UPS5817
can be used.
SELECTING FEEDBACK CAPACITOR FOR AUDIO
AMPLIFIER
The LM4805 is unity-gain stable which gives the designer
maximum system flexability. However, a typical application
requires a closed-loop differential gain of 10. In this case a
feedback capacitor (Cf2) can be used as shown in Figure 2
to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be
taken when calculating the -3dB frequency because an incorrect combination of Rf and Cf2 will cause rolloff before the
desired frequency
DUTY CYCLE
The maximum duty cycle of the boost converter determines
the maximum boost ratio of output-to-input voltage that the
converter can attain in continuous mode of operation. The
duty cycle for a given boost application is defined as:
Duty Cycle = VOUT + VDIODE - VIN / VOUT + VDIODE - VSW
SELECTING OUTPUT CAPACITOR (CO) FOR BOOST
CONVERTER
A single 4.7µF to 10µF ceramic capacitor will provide sufficient output capacitance for most applications. If larger
amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can be
used. Aluminum electrolytics with ultra low ESR such as
Sanyo Oscon can be used, but are usually prohibitively
expensive. Typical AI electrolytic capacitors are not suitable
for switching frequencies above 500 kHz because of significant ringing and temperature rise due to self-heating from
ripple current. An output capacitor with excessive ESR can
also reduce phase margin and cause instability.
In general, if electrolytics are used, we recommended that
they be paralleled with ceramic capacitors to reduce ringing,
switching losses, and output voltage ripple.
This applies for continuous mode operation.
INDUCTANCE VALUE
The first question we are usually asked is: “How small can I
make the inductor.” (because they are the largest sized
component and usually the most costly). The answer is not
simple and involves trade-offs in performance. Larger inductors mean less inductor ripple current, which typically means
less output voltage ripple (for a given size of output capacitor). Larger inductors also mean more load power can be
delivered because the energy stored during each switching
cycle is:
E = L/2 X (lp)2
SELECTING INPUT CAPACITOR (Cs1) FOR BOOST
CONVERTER
An input capacitor is required to serve as an energy reservoir
for the current which must flow into the coil each time the
switch turns ON. This capacitor must have extremely low
ESR, so ceramic is the best choice. We recommend a
nominal value of 4.7µF, but larger values can be used. Since
this capacitor reduces the amount of voltage ripple seen at
the input pin, it also reduces the amount of EMI passed back
along that line to other circuitry.
Where “lp” is the peak inductor current. An important point to
observe is that the LM4805 will limit its switch current based
on peak current. This means that since lp(max) is fixed,
increasing L will increase the maximum amount of power
available to the load. Conversely, using too little inductance
may limit the amount of load current which can be drawn
from the output.
Best performance is usually obtained when the converter is
operated in “continuous” mode at the load current range of
interest, typically giving better load regulation and less out13
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LM4805
Application Information
LM4805
Application Information
(Continued)
ILOAD = IIND(AVG) x (1 - DC)
put ripple. Continuous operation is defined as not allowing
the inductor current to drop to zero during the cycle. It should
be noted that all boost converters shift over to discontinuous
operation as the output load is reduced far enough, but a
larger inductor stays “continuous” over a wider load current
range.
(7)
Where "DC" is the duty cycle of the application. The switch
current can be found by:
ISW = IIND(AVG) + 1/2 (IRIPPLE)
To better understand these trade-offs, a typical application
circuit (5V to 12V boost with a 10µH inductor) will be analyzed. We will assume:
(8)
Inductor ripple current is dependent on inductance, duty
cycle, input voltage and frequency:
VIN = 5V, VOUT = 12V, VDIODE = 0.5V, VSW = 0.5V
IRIPPLE = DC x (VIN-VSW) / (f x L)
Since the frequency is 1.6MHz (nominal), the period is approximately 0.625µs. The duty cycle will be 62.5%, which
means the ON-time of the switch is 0.390µs. It should be
noted that when the switch is ON, the voltage across the
inductor is approximately 4.5V. Using the equation:
(9)
combining all terms, we can develop an expression which
allows the maximum available load current to be calculated:
ILOAD(max) = (1–DC)x(ISW(max)–DC(VIN-VSW))/fL (10)
The equation shown to calculate maximum load current
takes into account the losses in the inductor or turn-OFF
switching losses of the FET and diode.
V = L (di/dt)
We can then calculate the di/dt rate of the inductor which is
found to be 0.45 A/µs during the ON-time. Using these facts,
we can then show what the inductor current will look like
during operation:
DESIGN PARAMETERS VSW AND ISW
The value of the FET "ON" voltage (referred to as VSW in
equations 7 thru 10) is dependent on load current. A good
approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor current.
FET on resistance increases at VIN values below 5V, since
the internal N-FET has less gate voltage in this input voltage
range (see Typical Performance Characteristics curves).
Above VIN = 5V, the FET gate voltage is internally clamped
to 5V.
The maximum peak switch current the device can deliver is
dependent on duty cycle. For higher duty cycles, see Typical
Performance Characteristics curves.
20126255
INDUCTOR SUPPLIERS
Recommended suppliers of inductors for the LM4805 include, but are not limited to Taiyo-Yuden, Sumida, Coilcraft,
Panasonic, TDK and Murata. When selecting an inductor,
make certain that the continuous current rating is high
enough to avoid saturation at peak currents. A suitable core
type must be used to minimize core (switching) losses, and
wire power losses must be considered when selecting the
current rating.
FIGURE 2. 10µH Inductor Current
5V - 12V Boost (LM4805)
During the 0.390µs ON-time, the inductor current ramps up
0.176A and ramps down an equal amount during the OFFtime. This is defined as the inductor “ripple current”. It can
also be seen that if the load current drops to about 33mA,
the inductor current will begin touching the zero axis which
means it will be in discontinuous mode. A similar analysis
can be performed on any boost converter, to make sure the
ripple current is reasonable and continuous operation will be
maintained at the typical load current values.
PCB LAYOUT GUIDELINES
High frequency boost converters require very careful layout
of components in order to get stable operation and low
noise. All components must be as close as possible to the
LM4805 device. It is recommended that a 4-layer PCB be
used so that internal ground planes are available.
Some additional guidelines to be observed:
1. Keep the path between L1, D1, and Co extremely short.
Parasitic trace inductance in series with D1 and Co will
increase noise and ringing.
2. The feedback components R1, R2 and Cf 1 must be kept
close to the FB pin of U1 to prevent noise injection on the FB
pin trace.
3. If internal ground planes are available (recommended)
use vias to connect directly to ground at pin 2 of U1, as well
as the negative sides of capacitors Cs1 and Co.
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the application. This is illustrated in a graph in the typical performance characterization section which shows typical values
of switch current as a function of effective (actual) duty cycle.
CALCULATING OUTPUT CURRENT OF BOOST
CONVERTER (IAMP)
As shown in Figure 2 which depicts inductor current, the load
current is related to the average inductor current by the
relation:
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14
Single-Point Power / Ground Connection
(Continued)
The analog power traces should be connected to the digital
traces through a single point (link). A "Pi-filter" can be helpful
in minimizing high frequency noise coupling between the
analog and digital sections. It is further recommended to
place digital and analog power traces over the corresponding digital and analog ground traces to minimize noise coupling.
GENERAL MIXED-SIGNAL LAYOUT
RECOMMENDATION
This section provides practical guidelines for mixed signal
PCB layout that involves various digital/analog power and
ground traces. Designers should note that these are only
"rule-of-thumb" recommendations and the actual results will
depend heavily on the final layout.
Placement of Digital and Analog Components
Power and Ground Circuits
All digital components and high-speed digital signals traces
should be located as far away as possible from analog
components and circuit traces.
For 2 layer mixed signal design, it is important to isolate the
digital power and ground trace paths from the analog power
and ground trace paths. Star trace routing techniques (bringing individual traces back to a central point rather than daisy
chaining traces together in a serial manner) can have a
major impact on low level signal performance. Star trace
routing refers to using individual traces to feed power and
ground to each circuit or even device. This technique will
take require a greater amount of design time but will not
increase the final price of the board. The only extra parts
required may be some jumpers.
Avoiding Typical Design / Layout Problems
Avoid ground loops or running digital and analog traces
parallel to each other (side-by-side) on the same PCB layer.
When traces must cross over each other do it at 90 degrees.
Running digital and analog traces at 90 degrees to each
other from the top to the bottom side as much as possible will
minimize capacitive noise coupling and crosstalk.
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FIGURE 3. Demo Board Reference Schematic
15
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LM4805
Application Information
LM4805
Demonstration Board Layout
Composite Layer
20126251
Top Layer
20126253
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16
LM4805
Demonstration Board Layout
(Continued)
Silkscreen
20126252
Bottom Layer
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17
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LM4805 3V, 1W Boosted Boomer
Physical Dimensions
inches (millimeters)
unless otherwise noted
LQ Package
Order Number LM4805LQ
NS Package Number LQA28A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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