NSC LM4801MH

LM4801
1W Stereo Audio Amplifier Plus Adjustable LDO
General Description
Key Specifications
The LM4801 combines a bridge-connected (BTL) stereo
audio power amplifier with a low dropout voltage regulator
(LDO). The audio amplifier delivers 1.0W to a 8Ω load with a
less than 1.0% THD+N while operating on a 5V power
supply.
With the LM4801’s adjustable low-dropout (LDO) CMOS
linear regulator delivers an output current of up to 300mA,
has shutdown mode (1nA, typ) low quiescent current (90µA,
typ) and LDO voltage (120mV, typ). The regulator is stable
with small ceramic capacitive load (2.2µF, typ). The regulator
includes regulation fault detection, a bandgap voltage reference, and constant current limiting. It is designed for low
power, low current applications that can take advantage of
its 300mA output current capability.
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The LM4801 features an externally controlled micropower
shutdown mode and thermal shutdown protection. It also
utilizes circuitry that reduces "clicks and pops" during device
turn-on and return from shutdown.
Boomer audio power amplifiers are designed specifically to
use few external components and provide high quality output
power in a surface mount package.
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PO (BTL): VDD = 5V, THD+N ≤ 1%, RL = 8Ω 1.0W(typ)
Power supply range (amplifier)
3.0V to 5.0V
Power supply range (LDO)
2.5V to 6.0V
Shutdown current
0.07µA (typ)
LDO output current
300mA (min)
LDO dropout voltage (IOUT = 300mA)
120mV (typ)
LDO quiescent supply current
90µA (typ)
LDO shutdown supply current
1nA (typ)
LDO PSRR
60dB
LDO turn-on time
120ms (typ)
LDO ouput noise-voltage
37µVRMS (typ)
Features
Stereo BTL amplifier
Adjustable LDO regulator
“Click and pop” suppression circuitry
LDO is stable with small-value ceramic output capacitors
Unity-gain stable audio amplifiers
LDO has over-current protection
Thermal shutdown protection circuitry
TSSOP (MH) package
Applications
n Multimedia monitors
n Portable and desktop computers
n Portable televisions
Connection Diagram
20073629
Top View
Order Number LM4801MH
See NS Package Number MXA28A for TSSOP
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation
DS200736
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LM4801 1W Stereo Audio Amplifier Plus Adjustable LDO
September 2003
LM4801
Typical Application
20073601
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2
−0.3V to 6.5V
Fault Sink Current
Infrared (15 sec.)
220˚C
Thermal Resistance
6.0V
LDO-VCC, LDO-OUT, LDO-SHDN,
ADJ, CC, FAULT (pins 11-15, 17,19)
215˚C
See AN-450 “Surface Mounting and their Effects on
Product Reliablilty” for other methods of soldering
surface mount devices.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Amplifier Supply Voltage (pins 5, 24)
Vapor Phase (60 sec.)
θJC (typ) — MXA28A
20˚C/W
θJA (typ) — MXA28A (Note 2)
41˚C/W
20mA
Storage Temperature
Input Voltage
Operating Ratings
−65˚C to +150˚C
−0.3V to VDD +0.3V
Power Dissipation (Note 2)
Internally limited
ESD Susceptibility(Note 3)
2000V
ESD Susceptibility (Note 4)
Temperature Range
TMIN ≤ TA ≤ TMAX
200V
Junction Temperature
−40˚C ≤ TA ≤ 85˚C
Supply Voltage
150˚C
Solder Information
Pins 5, 24
3.0V ≤ VDD ≤ 5.5V
Pin 17
2.5V ≤ VCC ≤ 6.0V
Small Outline Package
Stereo Amplifier Electrical Characteristics for Entire IC
(Notes 1, 5)
The following specifications apply for VDD = 5V unless otherwise noted. Limits apply for TA = 25˚C.
Symbol
VDD
Parameter
Conditions
LM4801
Limit
(Note 6)
(Note 7)
Supply Voltage
IDD
Quiescent Power Supply Current
ISD
Shutdown Current
VDD applied to the SHUTDOWN pin
VOS
Output Offset Voltage
VIN = 0V
PO
Output Power (Note 13)
VIN = 0V, IO = 0A (Note 8)
Units
(Limits)
Typical
8
3
V (min)
5.5
V (max)
20
3.5
mA (max)
mA (min)
0.07
2
µA (min)
5
50
mV (max)
THD+N = 1%, f = 1kHz, RL = 8Ω
1.1
1.0
W (min)
THD+N = 10%, f = 1kHz, RL = 8Ω
1.5
W
0.13
%
THD+N
Total Harmonic Distortion+Noise
20Hz ≤ f ≤ 20kHz, AVD = 2
RL = 8Ω, PO = 1W
PSRR
Power Supply Rejection Ratio
VDD = 5V, VRIPPLE = 200mVRMS,
RL = 8Ω, CB = 1.0µF
Inputs Floating
Inputs terminated with 10Ω
67
43
dB
dB
XTALK
Channel Separation
f = 1kHz, CB = 1.0µF
90
dB
SNR
Signal To Noise Ratio
VDD = 5V, PO = 1W, RL = 8Ω
98
dB
LDO Electrical Characteristics
Unless otherwise specified, all limits guaranteed for VIN = VO +0.5V (Note 10), VSHDN = VIN, CIN = COUT = 2.2µF, CCC = 33nF,
TJ = 25˚C. Boldface limits apply for the operating temperature extremes: −40˚C and 85˚C.
Symbol
VIN
Parameter
Conditions
Min
(Note 7)
Input Voltage
∆VO
Output Voltage Tolerance
VO
Output Adjust Range
IO
Maximum Output Current
ILIMIT
Output Current Limit
100µA ≤ IOUT ≤ 300mA
VIN = VO + 0.5V (Note 7)
SET = OUT
Average DC Current Rating
Max
(Note 7)
Units
2.5
6.0
V
-2
+2
-3
+3
1.25
6
300
330
3
Typ
(Note 6)
% of
VOUT (NOM)
V
mA
770
mA
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LM4801
Absolute Maximum Ratings
Stereo Amplifier(Notes 1, 5)
LM4801
LDO Electrical Characteristics
(Continued)
Unless otherwise specified, all limits guaranteed for VIN = VO +0.5V (Note 10), VSHDN = VIN, CIN = COUT = 2.2µF, CCC = 33nF,
TJ = 25˚C. Boldface limits apply for the operating temperature extremes: −40˚C and 85˚C.
Symbol
IQ
VDO
∆VO
en
VSHDN
Parameter
Supply Current
Conditions
Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)
IOUT = 0mA
90
270
IOUT = 300mA
225
Shutdown Supply Current
VO = 0V, SHDN = GND
Dropout Voltage
(Note 10), (Note 11)
IOUT = 1mA
0.001
Units
µA
1
µA
220
mV
0.1
%/V
0.4
IOUT = 200mA
80
IOUT = 300mA
120
Line Regulation
IOUT = 1mA, (VO + 0.5V) ≤ VI ≤ 6V
(Note 10)
Load Regulation
100µA ≤ IOUT ≤ 300mA
Output Voltage Noise
IOUT = 10mA, 10Hz ≤ f ≤ 100kHz
37
Output Voltage Noise Density
10Hz ≤ f ≤ 100kHz, COUT = 10µF
190
SHDN Input Threshold
VIH, (VO + 0.5V) ≤ VI ≤ 6V
(Note 10)
-0.1
0.01
0.002
%/mA
µVRMS
nV/
2
V
VIL, (VO + 0.5V) ≤ VI ≤ 6V
0.4
ISHDN
SHDN Input Bias Current
SHDN = GND or IN
0.1
100
nA
ISET
SET Input Leakage
SET = 1.3V
0.1
2.5
nA
VFAULT
FAULTDetection Voltage
VO ≥ 2.5V, IOUT = 200mA (Note
12)
120
280
mV
0.115
0.25
V
0.1
100
nA
FAULT Output Low Voltage
ISINK = 2mA
IFAULT
FAULT Off-Leakage Current
FAULT = 3.6V, SHDN = 0V
TSD
Thermal Shutdown
Temperature
160
Thermal Shutdown Hysteresis
10
TON
Start-Up Time
˚C
COUT = 10µF, VO at 90% of Final
Value
120
µs
Note 1: 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 2: The maximum power dissipation is dictated by TJMAX, θJA, and the ambient temperature TA and must be derated at elevated temperatures. The maximum
allowable power dissipation is PDMAX = (TJMAX − TA)/θJA. For the LM4801, TJMAX = 150˚C. The θJAfor the LM4801 in the 28-pin MXA28A package, when board
mounted and its DAP is soldered to a 2in2 copper heatsink plane, is 41˚C/W.
Note 3: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 4: Machine model, 220pF–240pF discharged through all pins.
Note 5: All voltages are measured with respect to the ground (GND) pins unless otherwise specified.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Note 9: Output power is measured at the device terminals.
Note 10: Condition does not apply to input voltages below 2.5V since this is the minimum input operating voltage.
Note 11: Dropout voltage is measured by reducing VIN until VO drops 100mV from its nominal value at VIN -VO = 0.5V. Dropout Voltage does not apply to the 1.8
version.
Note 12: The FAULT detection voltage is specified for the input to output voltage differential at which the FAULT pin goes active low.
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LM4801
Stereo Amplifier Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
20073617
20073624
VDD = 3V, RL = 8Ω, POUT = 150mW
VDD = 5V, RL = 8Ω, POUT = 150mW
THD+N vs Frequency
THD+N vs Output Power
20073620
20073628
VDD = 5.5V, RL = 8Ω, POUT = 150mW
VDD = 3V, RL = 8Ω, fIN = 1kHz
THD+N vs Output Power
THD+N vs Output Power
20073633
VDD = 5V, RL = 8Ω, fIN = 1kHz
20073630
VDD = 5.5V, RL = 8Ω, fIN = 1kHz
5
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LM4801
Stereo Amplifier Typical Performance Characteristics
Output Power vs
Supply Voltage
(Continued)
Output Power vs
Load Resistance
20073602
20073637
RL = 8Ω, fIN = 1kHz, at (from top to bottom at 4.5V):
THD+N = 10%, THD+N = 1%
RL = 8Ω, fIN = 1kHz, at (from top to bottom at 24Ω):
THD+N = 10%, THD+N = 1%
Power Dissipation vs
Load Dissipation
Dropout Voltage vs
Supply Voltage
20073634
20073615
VDD = 5V, fIN = 1kHz, at (from top to bottom at 0.2W):
RL = 8Ω, RL = 16Ω, RL = 32Ω
Power Derating Curve
Cross Talk
20073604
VDD = 3V, RL = 8Ω, POUT= 150mW, at (from top to bottom
at 2kHz):
-IN A driven, VOUTB measured;
-IN B driven, VOUTA measured
20073616
VDD = 5V, RL = 8Ω, fIN = 1kHz
2in2 copper heatsink area
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Cross Talk
LM4801
Stereo Amplifier Typical Performance Characteristics
(Continued)
Cross Talk
20073607
20073605
VDD = 5V, RL = 8Ω, POUT= 150mW, at (from top to bottom
at 2kHz):
-IN A driven, VOUTB measured;
-IN B driven, VOUTA measured
VDD = 5.5V, RL = 8Ω, POUT= 150mW, at (from top to
bottom at 2kHz):
-IN A driven, VOUTB measured;
-IN B driven, VOUTA measured
PSRR vs Frequency
PSRR vs Frequency
20073610
VRIPPLE
20073608
VDD = 3V, RL = 8Ω, RSOURCE= 10Ω,
= 200mVP-P, at (from top to bottom at 500Hz):
CBYPASS = 0.1µF, CBYPASS = 1.0µF
VRIPPLE
PSRR vs Frequency
PSRR vs Frequency
20073613
VRIPPLE
VDD = 3V, RL = 8Ω, RSOURCE= ∞,
= 200mVP-P, at (from top to bottom at 500Hz):
CBYPASS = 0.1µF, CBYPASS = 1.0µF
20073611
VDD = 5V, RL = 8Ω, RSOURCE= 10Ω,
= 200mVP-P, at (from top to bottom at 500Hz):
CBYPASS = 0.1µF, CBYPASS = 1.0µF
VRIPPLE
7
VDD = 5V, RL = 8Ω, RSOURCE= ∞,
= 200mVP-P, at (from top to bottom at 500Hz):
CBYPASS = 0.1µF, CBYPASS = 1.0µF
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LM4801
Stereo Amplifier Typical Performance Characteristics
Open Loop
Frequency Response
(Continued)
Power Supply Current vs
Power Supply Voltage
20073635
RL = ∞, RSOURCE = 50Ω, VIN = 0V
20073622
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Unless otherwise specified, VIN = VOUT + 0.5V, CIN =
Dropout Voltage vs. Load Current
(For Different Output Voltages)
Dropout Voltage vs. Load Current
(For Different Output Temperatures)
200736A2
200736A3
FAULT Detect Threshold vs. Load Current
Supply Current vs. Input Voltage
200736A4
200736A5
Supply Current vs. Load Current
Power Supply Rejection Ratio vs. Frequency
200736A6
200736A7
9
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LM4801
LDO Typical Performance Characteristics
COUT = 2.2µF, CCC = 33nF, TJ = 25˚C, VSHDN = VIN.
LM4801
LDO Typical Performance Characteristics Unless otherwise specified, VIN = VOUT + 0.5V, CIN =
COUT = 2.2µF, CCC = 33nF, TJ = 25˚C, VSHDN = VIN. (Continued)
Output Noise Spectral Density
Output Noise (10Hz to 100kHz)
200736A9
200736A8
Output Impedance vs. Frequency
Line Transient Response
200736B1
200736B0
Load Transient
Shutdown Response
200736B3
200736B2
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Power-Up Response
Power-Down Response
200736B5
200736B4
External Components Description
(Refer to Figure 1.)
Components
Functional Description
1.
Ri
The Inverting input resistance, along with Rf, set the closed-loop gain. Ri, along with Ci, form a high pass
filter with fc = 1/(2πRiCi).
2.
Ci
The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. Ci, along with Ri, create a
highpass filter with fc = 1/(2πRiCi). Refer to the section, SELECTING PROPER EXTERNAL
COMPONENTS, for an explanation of determining the value of Ci.
3.
Rf
The feedback resistance, along with Ri, set the closed-loop gain.
4.
Cs
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of, this capacitor.
5.
CB
The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the SELECTING
PROPER EXTERNAL COMPONENTS section for information concerning proper placement and selecting
CB’s value.
6.
R1
Combined with R2, sets the LDO’s output voltage according to the following equation:
R1 = R2 ((VOUT / 1.25V) -1)
7.
R2
Combined with R1, sets the LDO’s output voltage according to the following equation:
R2 = (1.25V x R1) / (VOUT - 1.25V)
11
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LM4801
LDO Typical Performance Characteristics Unless otherwise specified, VIN = VOUT + 0.5V, CIN =
COUT = 2.2µF, CCC = 33nF, TJ = 25˚C, VSHDN = VIN. (Continued)
LM4801
Application Information
20073601
* Refer to the section Proper Selection of External Components, for a detailed discussion of CB size.
FIGURE 1. Typical Audio Amplifier Application Circuit
Pin out shown for the LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4801 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
Rf and Ri set the closed-loop gain of Amp1A, whereas two
internal 20kΩ resistors set Amp2A’s gain at -1. The LM4801
drives a load, such as a speaker, connected between the two
amplifier outputs, -OUTA and +OUTA.
Figure 1 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially (commonly referred to
as "bridge mode"). This results in a differential gain of
AVD = 2 x (Rf / Ri)
output signal is not clipped. To ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, singleended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when
successful single-ended or bridged amplifier.
states the maximum power dissipation point
ended amplifier operating at a given supply
driving a specified output load
(1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing
across the load. This produces four times the output power
when 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 that the
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designing a
Equation (2)
for a singlevoltage and
PDMAX = (VDD)2 / (2π2 RL) Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4801 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli12
POWER SUPPLY BYPASSING
(Continued)
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4801’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation in the output signal. Keep the length of leads and
traces that connect capacitors between the LM4801’s power
supply pin and ground as short as possible. Connecting a
1µF capacitor, CB, between the BYPASS pin and ground
improves the internal bias voltage’s stability and improves
the amplifier’s PSRR. The PSRR improvements increase as
the bypass pin capacitor value increases. Too large, however, increases turn-on time and can compromise amplifier’s
click and pop performance. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system
cost, and size constraints.
fier. From Equation (3), assuming a 5V power supply and an
8Ω load, the maximum single channel power dissipation is
0.633W or 1.27W for stereo operation.
PDMAX = 4 x (VDD)2 / (2π2 RL) Bridge Mode
(3)
The LM4801’s power dissipation is twice that given by Equation (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not exceed the power dissipation given by Equation (4):
PDMAX’ = (TJMAX − TA) / θJA
(4)
The LM4801’s TJMAX = 150˚C. In the MH package soldered
to a DAP pad that expands to a copper area of 2in2 on a PCB
, the LM4801’s θJA is 41˚C/W. At any given ambient temperature TJ\A, use Equation (4) to find the maximum internal
power dissipation supported by the IC packaging. Rearranging Equation (4) and substituting PDMAX for PDMAX’ results in
Equation (5). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipation without violating the LM4801’s maximum junction temperature.
TA = TJMAX − 2 x PDMAX θJA
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4801’s shutdown function. Activate micro-power shutdown by applying VDD to the SHUTDOWN pin. When active,
the LM4801’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically VDD/2. The low 0.7µA typical
shutdown current is achieved by applying a voltage that is as
near as VDD as possible to the SHUTDOWN pin. A voltage
thrat is less than VDD may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 10kΩ pull-up resistor between the
SHUTDOWN pin and VDD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier operation by closing the switch. Opening the switch connects the
SHUTDOWN pin to VDD through the pull-up resistor, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
(5)
For a typical application with a 5V power supply and an 8Ω
load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 98˚C for the MH
package.
TJMAX = PDMAX θJA + TA
(6)
Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4801’s 150˚C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
If twice the value given by Equation (3) exceeds the result of
Equation (4), then decrease the supply voltage, increase the
load impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce θJA. The heat sink can be created using additional
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the θJA is the sum of θJC, θCS, and θSA. (θJC is the
junction−to−case thermal impedance, CS is the case−to−sink
thermal impedance, and θSAis the sink−to−ambient thermal
impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output
power levels.
TABLE 1. LOGIC LEVEL TRUTH TABLE FOR
SHUTDOWN OPERATION
SHUTDOWN
OPERATIONAL MODE
Low
Full power, stereo BTL
amplifiers
High
Micro-power Shutdown
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4801’s performance requires properly selecting external components. Though the LM4801 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component values.
13
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LM4801
Application Information
LM4801
Application Information
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4801 contains circuitry to minimize turn-on and shutdown transients or "clicks and pop". For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4801’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the BYPASS pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches 1/2 VDD. As soon as the voltage on the
BYPASS pin is stable, the device becomes fully operational.
Although the bypass pin current cannot be modified, changing the size of CB alters the device’s turn-on time and the
magnitude of "clicks and pops". Increasing the value of CB
reduces the magnitude of turn-on pops. However, this presents a tradeoff: as the size of CB increases, the turn-on time
increases. There is a linear relationship between the size of
CB and the turn-on time. Here are some typical turn-on times
for various values of CB:
(Continued)
The LM4801 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ratio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the
Audio Power Amplifier Design section for more information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (Ci in Figure 1). A high value capacitor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, Ci has an affect on
the LM4801’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually VDD/2)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
Rf. Thus, pops can be minimized by selecting an input
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (RI) and the input
capacitor, CI produce a −3dB high pass filter cutoff frequency
that is found using Equation (7).
CB
20 ms
0.1µF
200 ms
0.22µF
440 ms
0.47µF
940 ms
1.0µF
2 Sec
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching VDD may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
NO LOAD STABILITY
The LM4801 may exhibit low level oscillation when the load
resistance is greater than 10kΩ. This oscillation only occurs
as the output signal swings near the supply voltages. Prevent this oscillation by connecting a 5kΩ between the output
pins and ground.
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, CI, using Equation (4), is 0.063µF. The 1.0µF
CI shown inFigure 1 allows the LM4801 to drive high efficiency, full range speaker whose response extends below
30Hz.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor connected to the BYPASS pin. Since CB determines how fast
the LM4801 settles to quiescent operation, its value is critical
when minimizing turn−on pops. The slower the LM4801’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 0.1µF to
0.39µF), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and
pops.
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TON
0.01µF
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
1WRMS
8Ω
1VRMS
20kΩ
100Hz−20 kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (4), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional volt-
14
1/(2π*20kΩ*20Hz) = 0.398µF
(Continued)
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (9).
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain, AVD, determines the
upper passband response limit. With AVD = 3 and fH =
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4801’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-lrestricting
bandwidth limitations.
(8)
VDD ≥ (VOUTPEAK + (VODTOP + VODBOT))
(15)
(9)
LDO General Information
Figure 2 shows the LM4801’s LDO functional block diagram.
A 1.25V bandgap reference, an error amplifier and a PMOS
pass transistor perform voltage regulation while being supported by shutdown, fault, and the usual Temperature and
current protection circuitry
The Output Power vs Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4801 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4801’s power dissipation requirements, the minimum differential gain is found using Equation
(10).
The regulator’s topology is the classic type with negative
feedback from the output to one of the inputs of the error
amplifier. Feedback resistors R1 and R2 are either internal or
external to the IC, depending on whether it is the fixed
voltage version or the adjustable version. The negative feedback and high open loop gain of the error amplifier cause the
two inputs of the error amplifier to be virtually equal in
voltage. If the output voltage changes due to load changes,
the error amplifier provides the appropriate drive to the pass
transistor to maintain the error amplifier’s inputs as virtually
equal. In short, the error amplifier keeps the output voltage
constant in order to keep its inputs equal.
(10)
Thus, a minimum gain of 2.83 allows the LM4801’s to reach
full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3.
The amplifier’s overall gain is set using the input (Ri) and
feedback (Rf) resistors. With the desired input impedance
set at 20kΩ, the feedback resistor is found using Equation
(11).
(11)
Rf/Ri = AVD/2
The value of Rf is 30kΩ.
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired ± 0.25dB
pass band magnitude variation limit, the low frequency response must extend to at least one−fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ± 0.25dB
desired limit. The results are an
200736A0
fL = 100Hz/5 = 20Hz
(12)
FH = 20kHzx5 = 100kHz
(13)
FIGURE 2. LDO Functional Block Diagram
and an
Output Voltage Setting
The output voltage is set according to the amount of negative feedback (Note that the pass transistor inverts the feedback signal). Figure 3 simplifies the LDO’s topology. This
type of regulator can be represented as an op amp configured as non-inverting amplifier and a fixed DC Voltage
(VREF) for its input signal. The special characteristic of this
op amp is its extra-large output transistor that only sources
current. In terms of its non-inverting configuration, the output
voltage equals VREF times the closed loop gain:
As mentioned in the External Components section, Ri
and Ci create a highpass filter that sets the amplifier’s lower
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (14).
(14)
the result is
15
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LM4801
Application Information
LM4801
Application Information
an important consideration. The X5R generally maintain their
capacitance value within ± 20%. The X7R type are desirable
for their tighter tolerance of 10% over temperature.
(Continued)
Ceramic capacitors pose a challenge because of their relatively low ESR. Like most other LDOs, the LDO relies on a
zero in the frequency response to compensate against excessive phase shift in the regulator’s feedback loop. If the
phase shift reaches 360˚ (i.e.; becomes positive), the regulator will oscillate. This compensation usually resides in the
zero generated by the combination of the output capacitor
with its equivalent series resistance (ESR). The zero is
intended to cancel the effects of the pole generated by the
load capacitance (CL) combined with the parallel combination of the load resistance (RL) and the output resistance
(RO) of the regulator. The challenge posed by low ESR
capacitors is that the zero it generates can be too high in
frequency for the pole that it’s intended to compensate. The
LM4801 overcomes this challenge by internally generating a
strategically placed zero.
Utilize the following equation for adjusting the output to a
particular voltage:
Choose R2 = 100k to optimize accuracy, power supply rejection, noise and power consumption.
200736B6
FIGURE 3. Regulator Topology Simplified
Similarity in the output capabilities exists between op amps
and linear regulators. Just as rail-to-rail output op amps
allow their output voltage to approach the supply voltage,
low dropout regulators (LDOs) allow their output voltage to
operate close to the input voltage. Both achieve this by the
configuration of their output transistors. Standard op amps
and regulator outputs are at the source (or emitter) of the
output transistor. Rail-to-rail op amp and LDO regulator outputs are at the drain (or collector) of the output transistor.
This replaces the threshold (or diode drop) limitations on the
output with the less restrictive source-to-drain (or VSAT) limitations. There is a trade-off, of course. The output impedance become significantly higher, thus providing a critically
lower pole when combined with the capacitive load. That’s
why rail-to-rail op amps are usually poor at driving capacitive
loads and recommend a series output resistor when doing
so. LDOs require the same series resistance except that the
internal resistance of the output capacitor will usually suffice.
Refer to the output capacitance section for more information.
200736B7
FIGURE 4. Simplified Model of Regulator
Loop Gain Components
Figure 4 shows a basic model for the linear regulator that
helps describe what happens to the output signal as it is
processed through its feedback loop; that is, describe its
loop gain (LG). The LG includes two main transfer functions:
the error amplifier and the load. The error amplifier provides
voltage gain and a dominant pole, while the load provides a
zero and a pole. The LG of the model in Figure 3 is described
by the following equation:
The first term of the above equation expresses the voltage
gain (numerator) and a single pole role-off (denominator) of
the error amplifier. The second term expresses the zero
(numerator) and pole (denominator) of the load in combination with the RO of the regulator.
Figure 5 shows a Bode plot that represents a case where the
zero contributed by the load is too high to cancel the effect of
the pole contributed by the load and RO. The solid line
illustrates the loop gain while the dashed line illustrates the
corresponding phase shift. Notice that the phase shift at
unity gain is a total 360˚ -the criteria for oscillation.
Output Capacitance
The LDO is specifically designed to employ ceramic output
capacitors as low as 2.2µF. Ceramic capacitors below 10µF
offer significant cost and space savings, along with high
frequency noise filtering. Higher values and other types and
of capacitor may be used, but their equivalent series resistance (ESR) should be maintained below 0.5Ω
Ceramic capacitor of the value required by the LDO are
available in the following dielectric types: Z5U, Y5V, X5R and
X7R. The Z5U and Y5V types exhibit a 50% or more drop in
capacitance value as their temperature increases from 25˚C,
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16
Power Dissipation
(Continued)
Power dissipation refers to the part’s ability to radiate heat
away from the silicon, with packaging being a key factor. A
reasonable analogy is the packaging a human being might
wear, a jacket for example. A jacket keeps a person comfortable on a cold day, but not so comfortable on a hot day. It
would be even worse if the person was exerting power
(exercising). This is because the jacket has resistance to
heat flow to the outside ambient air, like the IC package has
a thermal resistance from its junctions to the ambient (θJA).
θJA has a unit of temperature per power and can be used to
calculate the IC’s junction temperature as follows:
TJ = θJA (PD) + TA
TJ is the junction temperature of the IC. θJA is the thermal
resistance from the junction to the ambient air outside the
package. PD is the power exerted by the IC, and TA is the
ambient temperature.
PD is calculated as follows:
PD = IOUT (VIN -VO)
θJA for the LM4801 package (MSOP-8) is 223˚C/W with no
forced air flow, 182˚C/W with 225 linear feet per minute
(LFPM) of air flow, 163˚C/W with 500 LFPM of air flow, and
149˚C/W with 900 LFPM of air flow.
θJA can also be decreased (improved) by considering the
layout of the PC board: heavy traces (particularly at VIN and
the two VOUT pins), large planes, through-holes, etc.
Improvements and absolute measurements of the θJA can
be estimated by utilizing the thermal shutdown circuitry that
is internal to the IC. The thermal shutdown turns off the pass
transistor of the device when its junction temperature
reaches 160˚C (Typical). The pass transistor doesn’t turn on
again until the junction temperature drops about 10˚C (hysteresis).
Using the thermal shutdown circuit to estimate , θJA can be
done as follows: With a low input to output voltage differential, set the load current to 300mA. Increase the input voltage
until the thermal shutdown begins to cycle on and off. Then
slowly decrease VIN (100mV increments) until the part stays
on. Record the resulting voltage differential (VD) and use it in
the following equation:
200736B8
FIGURE 5. Loop Gain Bode Plot Illustrating
Inadequately High Zero for Stability Compensation
The LDO generates an internal zero that makes up for the
inadequately high zero of the low ESR ceramic output capacitor. This internally generated zero is strategically placed
to provide positive phase shift near unity gain, thus providing
a stable phase margin.
No-Load Stability
The LM4801 remains stable during no-load conditions, a
necessary feature for CMOS RAM keep-alive applications.
Input Capacitor
The LM4801 requires a minimum input capacitance of about
1µF. The value may be increased indefinitely. The type is not
critical to stability. However, instability may occur with bench
set-ups where long supply leads are used, particularly at
near dropout and high current conditions. This is attributed to
the lead inductance coupling to the output through the gate
oxide of the pass transistor; thus, forming a pseudo LCR
network within the Loop-gain. A 10µF tantalum input capacitor remedies this non-situ condition; its larger ESR acts to
dampen the pseudo LCR network. This may only be necessary for some bench setups. 1µF ceramic input capacitor are
fine for most end-use applications.
If a tantalum input capacitor is intended for the final application, it is important to consider their tendency to fail in short
circuit mode, thus potentially damaging the part.
Fault Detection
The LDO provides a FAULT pin that goes low during out of
regulation conditions like current limit and thermal shutdown,
or when it approaches dropout. The latter monitors the inputto-output voltage differential and compares it against a
threshold that is slightly above the dropout voltage. This
threshold also tracks the dropout voltage as it varies with
load current. Refer to Fault Detect vs. Load Current curve in
the typical characteristics section.
The FAULT pin requires a pull-up resistor since it is an
open-drain output. This resistor should be large in value to
reduce energy drain. A 100kΩ pull-up resistor works well for
most applications.
Figure 6 shows the LDO’s with delay added to the FAULT pin
for the reset pin of a microprocessor. The output of the
comparator stays low for a preset amount of time after the
regulator comes out of a fault condition.
Noise Bypass Capacitor
The noise bypass capacitor (CC) significantly reduces the
LDO’soutput noise. Connect the CC capacitor between pin 6
and ground. The optimum value for CC is 33nF.
Pin 6 directly connects to the high impedance output of the
bandgap. The DC leakage of the CC capacitor should be
considered; loading down the reference will reduce the output voltage. NPO and COG ceramic capacitors typically offer
very low leakage. Polypropylene and polycarbonate film carbonate capacitor offer even lower leakage currents.
CC does not affect the transient response; however, it does
affect turn-on time. The smaller the CC value, the quicker the
turn-on time.
17
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LM4801
Application Information
LM4801
Application Information
(Continued)
200736C0
FIGURE 7. Minimum Battery Detector that Disconnects
the Load Via the SHDN Pin of the LM4801
Resistor value for VUT and VLT are determined as follows:
200736B9
FIGURE 6. Power on Delayed Reset Application
The delay time for the application of Figure 5 is set as
follows:
(The application of Figure 6 used a GT of 5µ mho)
The application is set for a reset delay time of 8.8ms. Note
that the comparator should have high impedance inputs so
as to not load down the VREF at the CC pin of the LM4801.
Shutdown
The LM4801’s LDO goes into sleep mode when the SHDN
pin is in a logic low condition. During this condition, the pass
transistor, error amplifier, and bandgap are turned off, reducing the supply current to 1nA typical. The maximum guaranteed voltage for a logic low at the SHDN pin is 0.4V. A
minimum guaranteed voltage of 2V at the SHDN pin will turn
the LDO back on. The SHDN pin may be directly tied to VIN
to keep the part on. The SHDN pin may exceed VIN but not
the ABS MAX of 6.5V.
Figure 6 shows an application that uses the SHDN pin. It
detects when the battery is too low and disconnects the load
by turning off the regulator. A micropower comparator
(LMC7215) and reference (LM385) are combined with resistors to set the minimum battery voltage. At the minimum
battery voltage, the comparator output goes low and tuns off
the LDO and corresponding load. Hysteresis is added to the
minimum battery threshold to prevent the battery’s recovery
voltage from falsely indicating an above minimum condition.
When the load is disconnected from the battery, it automatically increases in terminal voltage because of the reduced IR
drop across its internal resistance. The Minimum battery
detector of figure 6 has a low detection threshold (VLT) of
3.6V that corresponds to the minimum battery voltage. The
upper threshold (VUT) is set for 4.6V in order to exceed the
recovery voltage of the battery.
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The above procedure assumes a rail-to-rail output comparator. Essentially, R2 is in parallel with R1 prior to reaching the
lower threshold, then R2 becomes parallel with R3 for the
upper threshold. Note that the application requires rail-to-rail
input as well.
The resistor values shown in Figure 7 are the closest practical to calculated values.
Fast Start-up
The LM4801’s LDO provides fast start-up time for better
system efficiency. The start-up speed is maintained when
using the optional noise bypass capacitor. An internal 500µA
current source charges the capacitor until it reaches about
90% of its final value.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 8 through 10 show the recommended two-layer PC
board layout that is optimized for the 28-pin MH-packaged
LM4801 and associated components. These circuits are
designed for use with an external 5V supply and 8Ω (or
greater) speakers.
This circuit board is easy to use. Apply 5V and ground to the
board’s VDD and GND pads, respectively. Connect speakers
between the board’s -OUTA and +OUTA and OUTB and
18
LM4801
Application Information
(Continued)
+OUTB pads. Apply the stereo input signal to the input pins
labeled "-INA" and "-INB." The stereo input signal’s ground
references are connected to the respective input channel’s
"GND" pin, adjacent to the input pins.
20073645
FIGURE 9. Recommended MHPC board layout:
component-side layout
20073644
FIGURE 8. Recommended MH board layout:
component-side silkscreen
20073648
FIGURE 10. Recommended MH board layout:
bottom-side layout
19
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LM4801 1W Stereo Audio Amplifier Plus Adjustable LDO
Physical Dimensions
inches (millimeters) unless otherwise noted
20-Lead Molded PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH
Order Number LM4801MH
NS Package Number MXA28A
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