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LR8501 Datasheet(PDF) 8 Page - Leshan Radio Company

Part No. LR8501
Description  1.5MHz 600mA Synchronous Step-Down Converter with Low Quiescent Current
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Maker  LRC [Leshan Radio Company]
Homepage  http://www.lrc.cn/
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LR8501 Datasheet(HTML) 8 Page - Leshan Radio Company

 
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DROPOUT OPERATION
As the input supply voltage decreases to a
value approaching the output voltage, the duty
cycle increases toward the maximum on-time.
Further reduction of the supply voltage forces the
main switch to remain on for more than one cycle
until reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus
the voltage drop across the P-channel MOSFET
and the inductor.
An important detail to remember is that at low
inputs supply voltages, the RDS(ON) of the
P-channel switch increases. Therefore, the user
should calculate the power dissipation when the
LR8501 is used at 100% duty cycle with low input
voltage.
APPLICATION INFORMATION
The basic LR8501 applic ation circuits are
shown in Figure 1.External component selection is
driven by the load requirement and begins with the
selection of L followed by CIN and COUT.
SETTING THE OUTPUT VOLTAGE
Figure1 shows the basic application circuit
with LR8501 adjustable output version. The
external resistor sets the output voltage according
to the following equation:
VOUT
0.6V
1
R2
R1
Table 2.Resistor select for output voltage setting
VOUT
R1
R2
1.2V
316K
316K
1.5V
316K
470K
1.8V
316K
634K
2.5V
316K
1M
INPUT CAPACITOR SELECTION
In continuous mode, the source current of the
top MOSFET is a square wave of duty cycle
VOUT/VIN. To prevent large voltage transients, a
low ESR input capacitor sized for the maximum
RMS current must be used. The maximum RMS
capacitor current is given by:
CIN required IRMS
IOMAX
VOUT VIN VOUT /
VIN
This formula has a maximum at VIN = 2VOUT,
where IRMS = IOUT/2. This simple worst-case
condition is commonly used for design because
even significant deviations do not offer much relief.
Ceramic capacitors with X5R or X7R dielectrics
are highly recommended because of their low
ESR and small temperature coefficients. A 4.7μF
ceramic capacitor for most application is sufficient.
INDUCTOR SELECTION
For most applications, the value of the
inductor will fall in the range of 1μH to 4.7μH. Its
value is chosen based on the desired ripple
current. Large value inductor lower ripple current
and small value inductor result in higher ripple
currents. Higher VIN or VOUT also increases the
ripple current as shown in the following equation:
∆IL
VOUT
VIN VOUT
VIN Lf
A reasonable starting point for setting ripple
current is
I
L=240mA (40% of 600mA). The DC
current rating of the inductor should be at least
equal to the maximum load current plus half the
ripple current to prevent core saturation.
Different core materials and shapes will
change
the
size/current
and
price/current
relationship of an inductor. The choice of which
LESHAN RADIO COMPANY, LTD.
8/11


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