FAN4855 Datasheet, PDF (9/12 Pages) Fairchild Semiconductor – 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect

The synchronous rectifier significantly improves effi-
ciency 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 kΩ, should be connected only to V_OUT.
The low-battery detector circuit is typically used to super-
vise 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 approxi-
mately 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 typi-
cal dependence 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 feed-
back circuit (see Test Circuit). The value of the R5 is rec-
ommended 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 volt-
age.
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 resis-
tor values R1 and R2 can be calculated using the follow-
ing 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 equa-
tion:
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 tanta-
lum capacitors are recommended. Ceramic capacitors
are recommended at input only; if connected at the out-
put 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
Vendor
MuRata
AVX
Sprague
Kemet
Description
X5R Ceramic
TAJ,TPS series tantalum
595D series tantalum
T494 series tantalum
Inductor Selection
The inductor parameters directly influencing the device
performance are the saturation current and the DC resis-
tance. The FAN4855 operates with a typical inductance
of 10µH. The lower the resistance, the higher the effi-
ciency. The saturation 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
Supplier
MuRata
Coilcraft
Coiltronics
Sumida
Manufacturer Part Number
LQ66C100M4
DT1608C-103
UP1B100
CDR63B-100
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FAN4855 Rev. 1.0.1
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