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參數資料
| 型號： | LM4804LQ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Low Voltage High Power Audio Power Amplifier |
| 中文描述： | 1.9 W, 1 CHANNEL, AUDIO AMPLIFIER, QCC28 |
| 封裝： | LLP-28 |
| 文件頁數： | 11/16頁 |
| 文件大小： | 820K |
| 代理商： | LM4804LQ |
Application Information
(Continued)
PULSE FREQUENCY MODULATION (PFM)
Pulse Frequency Modulation is typically accomplished by
switching continuously until the voltage limit is reached and
skipping cycles after that to just maintain it. This results in a
somewhat hysteretic mode of operation. The coil stores
more energy each cycle as the current ramps up to high
levels. When the voltage limit is reached, the system usually
overshoots to a higher voltage than required, due to the
stored energy in the coil (see Figure 3). The system will also
undershoot somewhat when it starts switching again be-
cause it has depleted all the stored energy in the coil and
needs to store more energy to reach equilibrium with the
load. Larger output capacitors and smaller inductors reduce
the ripple in these situations. The frequency being filtered,
however, is not the basic switching frequency. It is a lower
frequency determined by the load, the input/output voltage
and the circuit parameters. This mode of operation is useful
in situations where the load variation is significant. Power
managed computer systems, for instance, may vary from
zero to full load while the system is on and this is usually the
preferred regulation mode for such systems.
CYCLE TO CYCLE PFM
When the load doesn’t vary over a wide range (like zero to
full load), ratio adaptive circuit techniques can be used to
achieve cycle to cycle PFM regulation and lower ripple (or
smaller output capacitors). The key to success here is
matching the duty cycle of the circuit closely to what is
required by the input to output voltage ratio. This ratio then
needs to be dynamically adjusted for input voltage changes
(usually caused by batteries running down). The chosen
ratio should allow most of the energy in each switching cycle
to be delivered to the load and only a small amount to be
stored. When the regulation limit is reached, the overshoot
will be small and the system will settle at an equilibrium point
where it adjusts the off time in each switching cycle to meet
the current requirements of the load. The off time adjustment
is done by exceeding the regulation limit during each switch-
ing cycle and waiting until the voltage drops below the limit
again to start the next switching cycle. The current in the coil
never goes to zero like it frequently does in the hysteretic
operating mode of circuits with wide load variations or duty
cycles that aren’t matched to the input/output voltage ratio.
Optimizing the duty cycle for a given set of input/output
voltages conditions can be done by using the circuit values
in the Application Notes.
LOW VOLTAGE START-UP
The LM4804 can start-up from voltages as low as 1.1 volts.
On start-up, the control circuitry switches the N-channel
MOSFET continuously until the output reaches 3 volts. After
this output voltage is reached, the normal step-up regulator
feedback and gated oscillator control scheme take over.
Once the device is in regulation, it can operate down to
below .8V input, since the internal power for the IC can be
boot-strapped from the output using the Vdd pin.
SHUT DOWN
The LM4804 features a shutdown mode that reduces the
quiescent current to less than a guaranteed 2.5uA over
temperature. This extends the life of the battery in battery
powered applications. During shutdown, all feedback and
control circuitry is turned off. The regulator’s output voltage
drops to one diode drop below the input voltage. Entry into
the shutdown mode is controlled by the active-low logic input
pin EN (pin- 2). When the logic input to this pin is pulled
below 0.15V
, the device goes into shutdown mode. The
logic input to this pin should be above 0.7V
DD
for the device
to work in normal step-up mode.
SELECTING OUTPUT CAPACITOR (C
O
) FOR BOOST
CONVERTER
A single ceramic capacitor of value 4.7μF to 10μF will pro-
vide 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 due to 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, it is recommended that
they be paralleled with ceramic capacitors to reduce ringing,
switching losses, and output voltage ripple.
INTERNAL CURRENT LIMIT AND THERMAL
PROTECTION
An internal cycle-by-cycle current limit serves as a protection
feature. This is set high enough (2.85A typical, approxi-
mately 4A maximum) so as not to come into effect during
normal operating conditions. An internal thermal protection
circuit disables the MOSFET power switch when the junction
temperature (T
J
) exceeds about 160C. The switch is re-
enabled when T
J
drops below approximately 135C.
NON-LINEAR EFFECT
The LM4804 takes advantage of a non-linear effect that
allows for the duty cycle to be programmable. The C3 ca-
pacitor is used to dump charge on the FREQ pin in order to
manipulate the duty cycle of the internal oscillator. The part
is being tricked to behave in a certain manner, in the effort to
make this Pulse Frequency Modulated (PFM) boost switch-
ing regulator behave as a Pulse Width Modulated (PWM)
boost switching regulator.
CHOOSING THE CORRECT C3 CAPACITOR
The C3 capacitor allows for the duty cycle of the internal
oscillator to be programmable. Choosing the correct C3
capacitor to get the appropriate duty cycle for a particular
application circuit is a trial and error process. The non-linear
effect that C3 produces is dependent on the input voltage
and output voltage values. The correct C3 capacitor for
particular input and output voltage values cannot be calcu-
lated. Choosing the correct C3 capacitance is best done by
trial and error, in conjunction with the checking of the induc-
tor peak current to make sure your not too close to the
current limit of the device. As the C3 capacitor value in-
creases, so does the duty cycle. And conversely as the C3
capacitor value decreases, the duty cycle decreases. An
incorrect choice of the C3 capacitor can result in the part
prematurely tripping the current limit and/or double pulsing,
which could lead to the output voltage not being stable.
SETTING THE OUTPUT VOLTAGE
The output voltage of the step-up regulator can be set by
connecting a feedback resistive divider made of R
F1
and
R
F2
. The resistor values are selected as follows:
R
1
= R
2
[(V
OUT
/1.24) 1]
L
www.national.com
11
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相關代理商/技術參數 |
參數描述 |
|---|---|
| LM4804LQ/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4804LQX | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4804LQX/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4805 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:3V, 1W Boosted Boomer |
| LM4805_0508 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Voltage High Power Audio Power Amplifier |
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