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ILC6363 Datasheet(PDF) 6 Page - Fairchild Semiconductor

Part No. ILC6363
Description  Step-Up DC-DC Converter for One-Cell Lithium-Ion Batteries
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
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ILC6363 Datasheet(HTML) 6 Page - Fairchild Semiconductor

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ILC6363
PRODUCT SPECIFICATION
6
REV. 1.3.5 5/21/02
Figure 5. PFM Waveform
Other Considerations
The other limitation of PWM techniques is that, while the
fundamental switching frequency is easier to filter out since
it's constant, the higher order harmonics of PWM will be
present and may have to be filtered out, as well. Any filtering
requirements, though, will vary by application and by actual
system design and layout, so generalizations in this area are
difficult, at best.
However, PWM control for boost DC-DC conversion is
widely used, especially in audio-noise sensitive applications
or applications requiring strict filtering of the high frequency
components.
Low Battery Detector
The ILC6363's low battery detector is a based on a CMOS
comparator. The negative input of the comparator is tied to
an internal 1.25V (nominal) reference, VREF. The positive
input is the LBI/SD pin. It uses a simple potential divider
arrangement with two resistors to set the LBI threshold as
shown in Figure 6. The input bias current of the LBI pin is
only 200nA. This means that the resistor values R1 and R2
can be set quite high. The formula for setting the LBI
threshold is:
VLBI = VREF x (1+R5/R6)
Since the LBI input current is negligible (<200nA), this
equation is derived by applying voltage divider formula
across R6. A typical value for R6 is 100k
Ω.
R5 = 100k
Ω x [(VLBI/VREF) -1],
where VREF=1.25V (nom.)
The LBI detector has a built in delay of 120ms. In order to
get a valid low-battery-output (LBO) signal, the input volt-
age must be lower than the low-battery-input (LBI) threshold
for a duration greater than the low battery hold time
(thold(LBI)) of 120msec. This feature eliminates false trigger-
ing due to voltage transients at the battery terminal caused by
high frequency switching currents.
Figure 6. Low Battery Detector
The output of the low battery detector is an open drain
capable of sinking 2mA. A 10k
Ω pull-up resistor is
recommended on this output.
For VLBI < 1.25V
The low battery detector can also be configured for voltages
<1.25V by bootstrapping the LBI input from VOUT. The
circuitry for this is shown in Figure 7.
Figure 7. VLBI < 1.25V
The following equation is used when VIN is lower than
1.25V:
R1 = R2 x [(VREF – VIN) / (VOUT – VREF)],
where VREF = 1.25V (nom.)
This equation can also be derived using voltage divider
formula across R2. A typical value for R2 is 100k
Ω.
Shut Down
The LBI pin is shared with the shutdown pin. A low voltage
(<0.4V) will put the ILC6363 into a power down state. The
simplest way to implement this is with an FET across R6 as
shown in Figure 8. Note that when the device is not in PWM
mode or is in shutdown the low battery detector does not
operate.
VSET
VOUT
Switch Waveform
R6
R5
LBI/SD
3
2 VIN
ILC6363
Shutdown
DELAY
100ms
1.25V
Internal
Reference
GND
7
LBO
3.3V
RPU
6
+
-
VOUT
8
R2
3
LBI/SD
VIN
R1
ILC6363
+
-
1.25V
Internal
Reference
GND
7


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