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參數(shù)資料
| 型號： | LQA28A |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3V, 1W Boosted Boomer |
| 中文描述： | 3V的，1W的，細(xì)看布瑪 |
| 文件頁數(shù)： | 13/18頁 |
| 文件大?。?/td> | 1063K |
| 代理商： | LQA28A |
Application Information
(Continued)
tor, C
i
. A high value input coupling capacitor requires more
charge to reach its quiescent DC voltage (nominally 1/2
V
DD
). 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 fre-
quency response, turn-on pops can be minimized.
SELECTING BYPASS CAPACITOR FOR AUDIO
AMPLIFIER
Besides minimizing the input capacitor value, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
, 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 quies-
cent DC voltage (nominally 1/2 V
), the smaller the turn-on
pop. Choosing C
equal to 1.0μF along with a small value of
C
(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 motor-
boating), with C
equal to 0.1μF, the device will be much
more susceptible to turn-on clicks and pops. Thus, a value of
C
equal to 1.0μF is recommended in all but the most cost
sensitive designs.
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 (C
f
2) can be used as shown in Figure 2
to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that elimi-
nates possible high frequency oscillations. Care should be
taken when calculating the -3dB frequency because an in-
correct combination of R
f
and C
f
2 will cause rolloff before the
desired frequency
SELECTING OUTPUT CAPACITOR (C
O
) FOR BOOST
CONVERTER
A single 4.7μF to 10μF ceramic capacitor will provide suffi-
cient output capacitance for most applications. If larger
amounts of capacitance are desired for improved line sup-
port 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 signifi-
cant 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.
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.
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 15k is recom-
mended for R2 to establish a divider current of approxi-
mately 92μA. R1 is calculated using the formula:
R1 = R2 X (V
1
/1.23 1)
(5)
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM4805’s internal Boost converter is internally
compensated, the external feed-forward capacitor C
f
1 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 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 out-
L
www.national.com
13
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