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參數(shù)資料
| 型號： | LX1910-CDUT |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.8 A SWITCHING REGULATOR, 1325 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 602K |
| 代理商： | LX1910-CDUT |
LX1910
PRELIMINARY DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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High Frequency Step Down Regulator
I N T E GR A T ED
PRODUCT S
Copyright
2000
Rev. 0.9a, 2004-04-16
TH EORY OF O PERAT ION
The LX1910 is a step-down, pulse-width modulated
(PWM), DC-DC converter with an adjustable output
voltage range of 1.2V to 80% of the input voltage.
An internal 1MHz reference oscillator determines the
switching frequency when in the PWM mode of operation.
During a light load condition the PWM mode will switch
into a PFM mode of operation. In this PFM mode the
switched output has a fixed ON time and the frequency is
reduced to provide the lower output current demand. At a
complete NO load condition the output switching becomes
intermittent (Skip Mode) in order to maintain the output
voltage level. The device switches modes when the error
amplifier output goes below a voltage level that is set at the
minimum voltage of the saw-tooth reference signal at the
PWM comparator.
The LX1910 uses a low power, high gain bandwidth,
transconductance amplifier for its error amplifier.
The
bandgap voltage reference goes to the positive input of the
error amplifier. An external resistor divider feeds back the
output voltage to the negative input of the error amplifier.
The output of the transconductance error amplifier is a
current proportional to gm gain of the amplifier. This
output current is converted back to a voltage by the
impedance connected to the output of this amplifier.
The output of the error amplifier is connected to the
COMP pin. The impedance connected to this pin will
change the open loop response of this error amplifier. A
capacitor to ground on the COMP pin will establish a gain
setting for the error amplifier at a given frequency on the
Bode plot of the system response. The capacitor sets a pole
in the Bode plot response which establishes a -1 slope in the
response for a given frequency range. The output LC filter
also establishes another double pole (-2 slope) at the LC
frequency. To produce a stable system the slope of the
Bode plot has to be back to a -1 slope when the gain goes
through unity or 0db.
For systems utilizing output
capacitors with appreciable ESR, a resistor added in series
with the capacitor on the COMP pin can create a -1 slope
shelved at a lower frequency than the system cross over
frequency ensuring stability. The ESR of the output filter
capacitor will shelve the -2 slope of the LC filter back to a
-1 slope at the break frequency established by the RC
values.
This break frequency needs to be at a lower
frequency that the cross over frequency, usually a 5 to 1
ratio. For additional stability a capacitor can be connected
across the top resistor of the feedback divider network.
This will produce a zero, +1 slope, in the Bode plot
response at a frequency determined by the RC values. To
be effective this lead should start at a lower frequency than
the cross over frequency, again, at a ratio of 5 to 1.
For low ESR output capacitors a more complex
compensation scheme is laid out in the applications
section.
The LX1910 has an internal current limit established by
a comparator with a fixed voltage reference. The PMOS
transistor switch has a current mirror brought off the drain
to give a current reduction for sensing the output current.
The current from the current mirror is passed through a
resistor to ground that is also connected to the other input
of the current limit comparator.
The current limit is
processed on a cycle by cycle basis and when the current
limit comparator changes state it turns off the output
switch thereby reducing the PWM duty cycle. The output
pulse width can be reduced to less than 100ns when
confronted with a short circuit.
The LX1910 also has a thermal shutdown detector that
senses the chip temperature and shuts down operation
when the temperature hits 150°C.
This gives added
protection when there is a continuous short circuit on the
output.
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