User's Guide
SNAA027A – April 2006 – Revised May 2013
AN-1441 LM4962 Evaluation Board
1
Introduction
The LM4962 Evaluation Board is designed to evaluate both mono and stereo applications. The LM4962
integrates a switching boost converter with an audio power amplifier. For stereo applications an external
audio amplifier is needed. The LM4962 Evaluation Board presents the LM4962 along with the LM4951
audio power amplifier. For more information on the LM4962 or LM4951, see the device-specific data
sheet.
The LM4962 is an audio power amplifier primarily designed for driving ceramic speaker in cell phones,
Smart Phones, PDA’s and other portable applications. It is capable of driving 15Vpp (typ) BTL, per
channel, to a 2 μF + 30 Ω load with less than 1% THD + N from a 3.2VDC power supply. The LM4962
features a low power consumption shutdown mode, an internal thermal shutdown protection mechanism,
over current protection (OCP) and over voltage protection (OVP).
2
Operating Conditions
• Temperature Range
-40°C ≤ TA ≤ +85°C
• Boost Converter Supply Voltage (VDD)
3.0V < VDD < 5.0V
• Amplifier Supply Voltage
2.7V < VAMP < 8.5V
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1
Demo Board Schematic
3
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Demo Board Schematic
D2
L1
J8
V1=1.23(1+R2/R3)
Vdd
R3
Cs1
Vdd
SW
C3
C2
R2
FB
GND (sw)
R5
Soft-Start
Css
SW-Leak
VDD
Bootstrap
RPU4
OC/OV Detect
Flagout
J10
Rs
LM4962
Cf3
Vamp
Cs2
J3
Vdd R
PU
Rf3
SD Boost
Vdd
J5
RPU2
Cs3
GND
SD Amp
J6
Vo1
CinB RinB
Bypass
Vin
Cb1
Vo1
Rchg1
CinA
J7
Rchg2
OUTB
J2
Cchg
J1
Vo1
Cchg
Vo2
OUTA
Bypass
Rb2
Vo2
RinA
Vin
LM4951
VDD
RPU3
BW1
BW2
Shutdown
J4
Rf2
Rf1
Cf1
Figure 1. Demo Board Schematic
Table 1. Demo Board Connections
Designator
Label
J1
IN A
J2
OUTA
J3
SD Boost
Boost Converter Shutdown
J3 open = Shutdown
J3 shorted = Active
J4
SD AmpB
Amplifier B Shutdown (LM4951)
J4 open = Shutdown
J4 shorted = Active
J5
SD AmpA
Amplifier A Shutdown (LM4962)
J5 open = Shutdown
J5 shorted = Active
J6
IN B
J7
OUTB
J8
GND VDD
J9
J10
2
Function
Single Ended Audio Input Signal A (LM4962)
BTL Audio Output Signal A (LM4962)
Single Ended Audio Input Signal B (LM4951)
BTL Audio Output Signal B (LM4951)
Power Supply Connection
Stereo/Mono select
J9 Shorted = Stereo
J9 Open = Mono
Flagout
Flagout pin to monitor Over Voltage or Over Current conditions
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Start-Up Sequence
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4
Start-Up Sequence
For the LM4962 correct start-up sequencing is important for optimal device performance. Using the correct
start up sequence will improve click/pop performance as well as avoid transients that could reduce battery
life. For ringer/loudspeaker mode, the supply voltage should be applied first and both the boost and the
amplifier should be in shutdown (J3 and J5 open). Then the boost converter can be activated (J3 shorted)
followed by the amplifier (J5 shorted). If the boost converter shutdown is toggled while the amplifier is
active an audible pop will be heard.
5
Setting the Output Voltage (V1) of Boost Converter
The output voltage is set using the external resistors R2 and R5 (see Figure 1). A value of approximately
25 kΩ is recommended for R2 to establish the open loop gain of the boost converter. The output voltage
of the boost converter can be calculated using Equation 1.
V1 = VFB [1 + (R2 / R5)]
6
(1)
Feed-Forward Compensation For Boost Converter
Although the LM4962’s internal Boost converter is internally compensated, an external feed-forward
capacitor, Cf, 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's –3dB point should be approximately 60 kHz.
C3 can be calculated using the formula in Equation 2.
C3 = 1 / (2π x R2 x fz)
7
(2)
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 LM4962 device. It is recommended
that a 4-layer PCB be used so that internal ground planes are available. See Figure 1 for demo board
reference schematic and layout. Some additional guidelines to be observed:
• Keep the path between L1, D2, and C2 extremely short. Parasitic trace inductance in series with D2
and C2 will increase noise and ringing.
• If internal ground planes are available (recommended) use vias to connect directly to ground at the
GND (SW) and GND pins of U1, as well as the negative sides of capacitors Cs1 and C2
• To ensure correct operation of this device, it is essential that the GND (SW) pin (A3), GND pin (D1),
and the negative side of Cs2 be connected to the same GND plane. Cs2 should be placed as close as
possible to these two GND planes
8
General Layout Recommendations
This section provides practical guidelines for PCB layouts. Designers should note that these are only "ruleof-thumb" recommendations and the actual results will depend heavily on the final layout.
8.1
Power and Ground Circuits
For multi-layer boards, it is important to isolate the switching power and ground trace paths from the
amplifier 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 requires a greater amount of design time but will not
increase the final price of the board.
8.2
Avoiding Typical Design and 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°. Running digital and analog traces at 90°
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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PCB Layout
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Table 2. LM4962 Evaluation Board Bill of Materials
9
Item
Description
Package
U1
Ceramic Speaker Driver
DSBGA
U2
Audio Power Amplifier
DSBGA
Cf1, Cf3
Ceramic Capacitor
1210
Value
Notes
82 pF, 50V
50V
50V
C3
Ceramic Capacitor
1210
100 pF, 50V
CinA, CinB
Ceramic Capacitor
1210
0.39 μF, 50V
C2, Cs1, Cs2, Cs3
Ceramic Capacitor
1210
4.7 μF, 16V
Css
Ceramic Capacitor
1210
10 nF, 16V
Cb1, Cb2
Tantalum Capacitor
1210
1 μF, 16V
D2
Schottky Diode
SOD-123
20V, 0.5A
On Semi MBR0520
L1
Inductor
4.3mm X 4.8mm X
3.5mm
10 μH, 1.04A
Sumida CR43–100
R3
Resistor
0603
1.6 kΩ
1/10W, 5%
R2
Resistor
0603
25 kΩ
1/10W, 5%
Rpu, Rpu2, Rpu3,
Rchg1, Rchg2,
Resistor
0603
1 kΩ
1/10W, 5%
low ESR, ESR < 50 Ω
Rpu4
Resistor
0603
100 kΩ
1/10W, 5%
RinA, RinB, Rf2
Resistor
0603
20 kΩ
1/10W, 5%
Rf1, Rf3
Resistor
0603
200 kΩ
1/10W, 5%
Rchg1, Rchg2
Resistor
0603
1 kΩ
1/10W, 5%
R4
Resistor
0603
4.9 kΩ
1/10W, 5%
R3
Resistor
0603
100 mΩ
1/10W, 5%
PCB Layout
Figure 2. Top Layer
(Shown With Top Silkscreen visible)
4
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PCB Layout
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Figure 3. Mid Layer 1
(Shown With Top Silkscreen visible)
Figure 4. Mid Layer 2
(Shown With Top Silkscreen visible)
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5
PCB Layout
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Figure 5. Bottom Layer
(Shown With Top Silkscreen visible)
Figure 6. Top Silkscreen
6
AN-1441 LM4962 Evaluation Board
SNAA027A – April 2006 – Revised May 2013
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Revision Table
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10
Revision Table
Rev
Date
Description
0.1
04/20/06
Initial release.
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7
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