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參數(shù)資料
| 型號： | LM4802BLQ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Audio Power Amplifier with Boost Converter to Drive Ceramic Speakers |
| 中文描述： | 1 CHANNEL, AUDIO AMPLIFIER, QCC28 |
| 封裝： | LLP-28 |
| 文件頁數(shù)： | 12/17頁 |
| 文件大?。?/td> | 1224K |
| 代理商： | LM4802BLQ |
Application Information
(Continued)
SETTING THE OUTPUT VOLTAGE (V
1
) OF BOOST
CONVERTER
The output voltage is set using the external resistors R1 and
R2 (see Figure 1). A value of approximately 13.3k
is rec-
ommended for R2 to establish a divider current of approxi-
mately 92μA. R1 is calculated using the formula:
R1 = R2 X (V
2
/1.23 1)
(5)
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM4802B’s internal Boost converter is inter-
nally compensated, the external feed-forward capacitor C
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 approxi-
mately 6kHz. C
f
1 can be calculated using the formula:
C
f
1 = 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 exceed-
ing 0.5A average but less than 1A, a Microsemi UPS5817
can be used.
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 = V
OUT
+ V
DIODE
- V
IN
/
V
OUT
+ V
DIODE
- V
SW
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 induc-
tors mean less inductor ripple current, which typically means
less output voltage ripple (for a given size of output capaci-
tor). 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
Where “l(fā)p” is the peak inductor current. An important point to
observe is that the LM4802B 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 in-
ductance 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 out-
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.
To better understand these trade-offs, a typical application
circuit (5V to 12V boost with a 10μH inductor) will be ana-
lyzed. We will assume:
V
IN
= 5V, V
OUT
= 12V, V
DIODE
= 0.5V, V
SW
= 0.5V
Since the frequency is 1.6MHz (nominal), the period is ap-
proximately 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:
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:
During the 0.390μs ON-time, the inductor current ramps up
0.176A and ramps down an equal amount during the OFF-
time. 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.
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the cur-
rent limiter cuts in is dependent on duty cycle of the appli-
cation. This is illustrated in a graph in the typical perfor-
mance characterization section which shows typical values
of switch current as a function of effective (actual) duty cycle.
20094198
FIGURE 2. 10μH Inductor Current
5V - 12V Boost (LM4802B)
L
www.national.com
12
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4802BLQ/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4802BLQBD | 功能描述:音頻 IC 開發(fā)工具 LM4802BLQ EVAL BOARD RoHS:否 制造商:Texas Instruments 產(chǎn)品:Evaluation Kits 類型:Audio Amplifiers 工具用于評估:TAS5614L 工作電源電壓:12 V to 38 V |
| LM4802BLQBD/NOPB | 制造商:Texas Instruments 功能描述:Evaluation Kit For Audio Power Amplifier With Boost Converter To Drive Ceramic Speakers |
| LM4802BLQX | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4802BLQX/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
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