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參數資料
| 型號： | FAN4855MTC |
| 廠商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分類： | 穩壓器 |
| 英文描述： | 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect |
| 中文描述： | 0.5 A SWITCHING REGULATOR, PDSO8 |
| 封裝： | TSSOP-8 |
| 文件頁數： | 9/13頁 |
| 文件大小： | 148K |
| 代理商： | FAN4855MTC |
FAN4855
PRODUCT SPECIFICATION
REV. 1.0.0 3/17/04
9
The synchronous rectifier significantly improves efficiency
without the addition of an external component, so that
conversion efficiency can be as high as 94% over a large load
range, as shown in the Typical Operating Characteristics.
Even at light loads, the efficiency stays high because the
switching losses of the converter are minimized by reducing
the switching frequency.
Error Detection Comparator (LBI – LBO)
An additional comparator A3 is provided to detect low V
IN
or any other error conditions that is important to the user.
The non-inverting input of the comparator is internally
connected to a reference threshold voltage V
th
while the
inverting input is connected to the LBI pin. The output of the
low battery comparator is a simple open-drain output
that goes active low if the battery voltage drops below the
programmed threshold voltage on LBI. The output requires a
pull-up resistor having a recommended value of 100 k
,
should be connected only to V
OUT
.
The low-battery detector circuit is typically used to supervise
the battery voltage and to generate an error flag or a RESET
command when the battery voltage drops below a user-set
threshold voltage. The function is active only when the
device is enabled. When the device is disabled, the LBO-pin
is high impedance.
Shutdown
The device enters shutdown when V
SHDN
is approximately
less than 0.5V
IN
. During shutdown the regulator stops
switching, all internal control circuitry including the low-
battery comparator is switched off and the load is
disconnected from the input. The output voltage may drop
below the input voltage during shutdown. The typical depen-
dence shutdown voltage versus input voltage and the timing
process of the exiting shutdown are shown on the Diagrams.
For normal operation V
SHDN
should be driven up 0.8V
IN
or
connected to the V
IN
.
Application Information
Selecting the Output Voltage
The output voltage V
OUT
can be adjusted from 3V to 5V,
choosing resistors R4 and R5 of the divider in the feedback
circuit (see Test Circuit). The value of the R5 is recom-
mended to be less than 270k. R4 can be calculated using
the following equation:
R4 = R5[(V
OUT
/V
REF
) – 1]
where V
REF
= 1.24V
Setting the LBI Threshold of Low-Battery
Detector Circuit
The LBO-pin goes active low when the voltage on the
LBI-pin decreases below the set threshold typical voltage of
390 mV, which is set by to the internal reference voltage.
The battery voltage, at which the detection circuit switches,
can be programmed with a resistive divider connected to
the LBI-pin. The resistive divider scales down the battery
voltage to a voltage level of tenths of volt, which is then
compared to the LBI threshold voltage. The LBI-pin has a
built-in hysteresis of 25 mV. The resistor values R1 and R2
can be calculated using the following equation:
V
IN_MIN
= 0.39 x (R1+R2)/R2
The value of R2 should be 270k or less to minimize bias
current errors. R1 is then found by rearranging the equation:
R1 = R2 x (V
IN_MIN
/0.39 – 1)
If the low-battery detection circuit is not used, the LBI-pin
should be connected to GND (or to V
IN
) and the LBO-pin
can be left unconnected or tied to GND. Do not let the
LBI-pin float.
Component Selection
Input and Output Capacitors Selection
For common general purpose applications, 47μF tantalum
capacitors are recommended. Ceramic capacitors are recom-
mended at input only; if connected at the output they cannot
improve significantly the voltage ripple. More effective in
reducing the output ripple at light load is to connect a small
capacitor of 18 to 100pF between V
OUT
and FB pin.
Table 1. Recommended capacitors
Inductor Selection
The inductor parameters directly influencing the device per-
formance are the saturation current and the DC resistance.
The FAN4855 operates with a typical inductance of 10μH.
The lower the resistance, the higher the efficiency. The satu-
ration current should be rated higher than 0.8A, which is the
typical threshold to switch off the N-channel power FET.
Table 2. Recommended Inductors
Vendor
MuRata
AVX
Sprague
Kemet
Description
X5R Ceramic
TAJ,TPS series tantalum
595D series tantalum
T494 series tantalum
Supplier
MuRata
Coilcraft
Coiltronics
Sumida
Manufacturer Part Number
LQ66C100M4
DT1608C-103
UP1B100
CDR63B-100
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