www.fairchildsemi.com
REV. 1.0.0 3/17/04
Features
•Low power PFM boost regulator
• Input voltage range is from 1.6V to 4.5V
• Output voltage range is from 3V to 5V
• 500mA maximum load current capability
• 95% efficient power conversion
• 2-3 cell and single cell Li-Ion systems
•Variable on-time Pulse Frequency Modulation (PFM)
• Internal synchronous rectifier (no need for external diode)
•Low-battery detection
• Logic controlled shutdown with true-load disconnect
•Low (80µA) quiescent current
• MSOP-8 and TSSOP-8 Packages
Applications
• DSCs
•PDAs
• Cell phones, smart phones
• Portable instrumentations
• 2-3 AA / AAA cells operated devices
• Single cell Li-Ion operated devices
General Description
The FAN4855 is a low power boost regulator designed for
low voltage DC to DC conversion in 2-cell battery powered
systems such as digital cameras, cell phones and PDAs.
The converter starts-up at 1.3V and operates after the start at
an input voltage as low as 1V. Output voltage can be adjusted
by external resistors from 3.3V to 5V with a maximum load
current of 0.5A. Quiescent current in shut-down mode is
less than 10µA, which maximizes the battery live time. The
ON time changes with the input voltage to maintain the
ripple current constant and to provide the highest efficiency
over a wide load range—while maintaining low peak
currents in the boost inductor. The combination of built-in
power transistors, synchronous rectification and low supply
current, make the FAN4855 ideal for portable applications.
The FAN4855 is available in 8-lead MSOP and TSSOP
packages.
FAN4855
500mA High Efficiency Boost Regulator with
Adjustable Output, Shutdown and Low Battery Detect
Typical Application
VIN
Input 1.6V
to 4.5V
On
Off
Low Battery
Detect In
Output 3.3V to 5V
up to 0.5A
FAN4855
Low Battery
Detect Out
SHDN
LBI
LB0
GND
VL
VOUT
FB
1
2
3
4
8
7
6
5
PRODUCT SPECIFICATION FAN4855
2
REV. 1.0.0 3/17/04
Pin Configuration
Pin Description
Absolute Maximum Ratings
Absolute Maximum Ratings are those values, beyond which the device could be permanently damaged. Absolute maximum
ratings are stress ratings only and functional device operation is not implied.
PIN NAME FUNCTION
1 V
IN
Battery Input Voltage
. Supplies the IC during start-up. After the output is running, the IC draws
power from V
OUT
.
2 SHDN
Shut Down
. Pulling this pin low shuts down the regulator, isolating the load from the input.
3 LBI
Low-Battery Input
. Pulling this pin below 0.39V causes the LBO pin to go low.
4 LBO
Low-Battery Output
. This pin provides an active low signal to alert the user when the LBI
voltage falls below its targeted value. The open-drain output can be used to reset a
microcontroller.
5FB
Feedback Input
. For setting the output voltage. Connect this pin to the resistor divider.
6V
OUT
Boost regulator output
. Output voltage can be set to be in the 3 to 5V range. Startup at
moderate load is achievable at input voltages around 1.35V.
7V
L
Boost inductor connection
. Connect an inductor between this pin and V
IN
. When servicing the
output supply, this pin pulls low, charging the inductor, then shuts off dumping the energy through
the synchronous rectifier to the output.
8 GND
Ground of the IC
.
Parameter Min. Max. Units
V
IN
,
V
OUT
Voltages (Relative to GND) -0.3 6.5 V
Switch Voltage (V
L
to GND) -0.3 V
OUT
+ 0.3
V
Voltage on any other Pin -0.3 V
OUT
+ 0.3
V
Peak Switch Current (Ipeak) — Internally Limited —
Output Current (I
OUT
) 500 mA
Continuous Power Dissipation TSSOP Package 525 mW
MSOP Package 315
Thermal Resistance (
θ
JA
) TSSOP Package 124 °C/W
MSOP Package 206
Junction Temperature 150 °C
Storage Temperature Range -65 150 °C
Lead Temperature (soldering, 10s) 300 °C
VIN GND
VL
VOUT
FB
1
2
3
4
8
7
TOP VIEW
8-Pin TSSOP and MSOP
6
5
SHDN
LBI
LB0
FAN4855 PRODUCT SPECIFICATION
REV. 1.0.0 3/17/04
3
Recommended Operating Conditions
Electrical Characteristics
Unless otherwise specified, V
IN
= 1.6V to 3V, I
LOAD
= 1mA, T
A
= -40°C to +85°C. Test Circuit Fig.1. Typical values
are at T
A
= +25°C
Notes:
1. R4, R5, R6 tolerance
≤
0.1%.
2. Current through R1, R2 is not taken into account.
Parameter Min. Max. Units
Temperature Range -40 +85 °C
V
IN
Operating Range
1.6 0.9 V
OUT
V
V
OUT
Operating Range
3.0 5.0 V
Parameter Conditions Min. Typ. Max. Units
Start Up Voltage I
LOAD
< 1mA 1.35 1.6 V
Operating Voltage After start I
LOAD
=10mA, V
OUT
= 3.3V or
5V
1.0 V
Output Voltage V
OUT
(nom.) = 3.3V (Note 1)
V
OUT
(nom.) = 5V
3.15
4.775
3.3
5
3.45
5.225
V
V
Output Voltage Adjust Range 3 5 V
Steady State Output Current V
OUT
= 3.3V, V
IN
= 2.5V 300 500 mA
V
OUT
= 5V, V
IN
= 2.5V 200 330 mA
Pulse Width V
IN
= 3V 0.8 1.4 2
µ
s
V
IN
= 2.4V 1.2 1.7 2.5
µ
s
V
IN
= 1.8V 1.6 2.2 3.3
µ
s
V
IN
= 1.6V 1.7 2.5 4.0
µ
s
Minimum Off-Time 1
µ
s
Line Regulation I
OUT
= 2mA, V
OUT
= 3.3V 0.5 2 %
V
OUT
= 5V 0.5 2 %
Load Regulation 0 to 250mA, V
IN
= 2.4V, V
OUT
= 3.3V 0.5 %
0 to 150mA, V
IN
= 2.4V, V
OUT
= 5V 1 %
Feedback Voltage (VFB) 1.243 V
LBI Threshold Voltage 0.390 V
LBI Hysteresys 25 mV
Internal NFET, PFET ON Resist. I
LOAD
= 100mA 0.35
Ω
Power Efficiency I
LOAD
= 200mA, V
IN
= 3V, V
OUT
= 3.3V 95 %
Input Current in Shut Down
Mode
SHDN = 0V, V
IN
= 3V (Note 2) 8 50
µ
A
Quiescent Current SHDN = 3V, V
IN
= 3V, V
OUT
= 3.3V
(Note 2)
80 160
µ
A
LBO Output Voltage Low V
LBI
= 0, I
SINK
= 1mA 0.2 V
SHDN Input Threshold Voltage V
IN
= 3V, V
OUT
= 3.3V/5V 1.5 V
SHDN Input Threshold Voltage V
IN
= 1.6V, V
OUT
= 3.3V/5V 0.8 V
PRODUCT SPECIFICATION FAN4855
4
REV. 1.0.0 3/17/04
Figure 1. Test Circuit
SHDN
C5
0.
1µF
JP3
R6
287K
1
2
1
2
GND1
1
2
1
2
1
2
U1 FAN4855
1
2
3
4 5
6
7
8
Vin
SHDN
LBI
LBO
FB
Vout
VL
Gnd
+
C3
18pF
R1
750K
R2
240K R3
100K
C1
47µF
+C2
47µF
VOUT
3.3V or 5V
1
2
R5
240K
JP1
1
2
JP2
R4
402K
J1
SCOPE JACK
10uH
Reset
VIN
1.6V to 3.0V
Ext
Pull Up
GND
FAN4855 PRODUCT SPECIFICATION
REV. 1.0.0 3/17/04
5
Typical Operating Characteristics
(L = 10µH, C
IN
= 47µF, C
OUT
= 47µF/1.0µF, T = 25°C)
Load Current vs. Start-Up Voltage
(Resistive Load)
Input Voltage, V
Load Current, mA
Efficiency vs. Load Current
Vout = 3.3V
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
0.1 1 10 100 1000
Output Current, mA
Efficiency, %
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
0.1 1 10 100 1000
Output Current, mA
Efficiency, %
Efficiency vs. Load Current
Vout = 5V SHDN Threshold Voltage
Input Voltage, V
SHDN Voltage, V
Vin=2.0V
Vin=3V
Vin=1.5V
Vin=3V
Vin=2.0V Vin=1.5V
0
100
200
300
400
500
600
1.5 2 2.5 3 3.5 4
VOUT = 5V
VOUT = 3.3V
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
1.5 2 2.5 3 3.5 4 4.5
Starting Up and Turning Off
VOUT = 3.3V, Iloads = 10mA to 50mA
Input Voltage, V
Output Voltage, V
Starting Up and Turning Off
VOUT = 5V, Iloads = 10mA to 50mA
Output Voltage, V
0
0.5
1
1.5
2
2.5
3
3.5
4
0.6 0.8 1.01.21.41.6 1.8
Input Voltage, V
0.6 0.8 1.0 1.2 1.4 1.6 1.8
START UP
START UP
TURN OFF:
TURN OFF:
Iload=50mA
Iload=50mA
Iload=10mA Iload=10mA
Iload=10mA to 50mA
1
0
2
3
4
5
PRODUCT SPECIFICATION FAN4855
6
REV. 1.0.0 3/17/04
Typical Operating Characteristics
(L = 10µH, C
IN
= 47µF, C
OUT
= 47µF/1.0µF, T = 25°C)
Output Voltage vs. Temperature
-50 -25 0 25 50 75 100
Temperature (°C)
-0.6
-0.4
-0.2
0
0.2
VOUT Relative Change (%)
No Load Supply Current vs. Input Voltage
0.1
1
012345
10
100
1000
Input Voltage (V)
Input Current (µA)
VOUT = 3.3V
VOUT = 5V
“OFF”
“ON”
PRODUCT SPECIFICATION FAN4855
7
REV. 1.0.0 3/17/04
Typical Operating Characteristics
(Continued)
Line Transient Response @100mA Load Exiting Shutdown
Load Transient Response Load Transient Response
Heavy-Load Switching Waveforms Inductor Current and Switching Node Voltage
Inductor
Current
VL
VOUT
VOUT
VL
IL
VSHDN
PRODUCT SPECIFICATION FAN4855
8
REV. 1.0.0 3/17/04
Block Diagram
Functional Description
Boost Regulator
FAN4855 is an adjustable boost regulator that combines
variable ON and minimum OFF architecture with synchro-
nous rectification. Unique control circuitry provides high-
efficiency power conversion for both light and heavy loads
by transitioning between discontinuous and continuous
conduction mode based on load conditions. There is no
oscillator; a constant-peak-current limit of 0.8A in the
inductor allows the inductor current to vary between this
peak limit and some lesser value. The switching frequency
depends upon the load, the input and output voltage ranging
up to 430kHz.
The input voltage VIN comes to VIN pin and through the
external inductor to the VL pin of the device. The loop from
VOUT closes through the external resistive voltage divider to
the feedback pin VFB. The transfer ratio of this divider deter-
mines the output voltage. When VFB voltage drops below the
VREF = 1.24V the error amplifier A1 signals to regulator to
deliver charge to the output by triggering the Variable On-
Time One Shot. One Shot generates a pulse at the gate of the
Power NMOS transistor Q1. This transistor will charge the
Inductor L1 for the time interval TON resulting in a peak
current given by:
When the one–shot times out, the Q1 transistor releases the
VL pin, allowing the inductor to fly-back and momentarly
charge the output through the body diode of the transistor
Q2. But, as the voltage across the Q2 changes polarity, its
gate will be driven low by the Synchronous Rectifier Control
Circuit (SRC), causing Q2 to short out its body diode. The
inductor then delivers the charge to the load by discharging
into it through Q2.
Under light load conditions, the amount of energy delivered
in this single pulse satisfies the voltage-control loop, and the
converter does not command any more energy pulses until
the output drops again below the lower-voltage threshold.
Under medium and heavy loads, a single energy pulse is not
sufficient to force the output voltage above its upper thresh-
old before the minimum off time has expired and a second
charge cycle is commanded. Since the inductor current has
not reached zero in this case, the peak current is greater than
the previous value at the end of the second cycle. The result
is a ratcheting of inductor current until either the output volt-
age is satisfied, or the converter reaches its set current limit.
After a period of time TOFF > 1µS, determined by Minimum
Off–Time Logic and if VOUT is low (VFB < VREF), the
Variable On-Time One Shot will be turned ON again and
the process repeats.
The output capacitor of the converter filters the variable
component, limiting the output voltage ripple to a value
determined by its capacitance and its ESR.
LBO
SHDN
Start-Up
Minimum
Off-Time
Logic
Variable
On-Time
One Shot
Current
Limit
Control
Synchronous
Rectifier
Control
47
2
6
5
8
3
1
VIN
VL
VOUT
ILIMIT
ILIMIT
VREF
VOUT
GND
VFB
ILIMIT SHDN
N1
LBI
Q1
Q2
0.39V
A3
Control
Logic
A2
A1
–
+
–
+
–
+
SHDN
ILPEAK()
TON VIN
×
L1
---------------------------=
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 VIN
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 Vth 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 VOUT.
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 VSHDN is approximately
less than 0.5VIN. 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 VSHDN should be driven up 0.8VIN or
connected to the VIN.
Application Information
Selecting the Output Voltage
The output voltage VOUT 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[(VOUT/VREF) – 1]
where VREF = 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:
VIN_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 (VIN_MIN/0.39 – 1)
If the low-battery detection circuit is not used, the LBI-pin
should be connected to GND (or to VIN) 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 VOUT 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 Description
MuRata X5R Ceramic
AVX TAJ,TPS series tantalum
Sprague 595D series tantalum
Kemet T494 series tantalum
Supplier Manufacturer Part Number
MuRata LQ66C100M4
Coilcraft DT1608C-103
Coiltronics UP1B100
Sumida CDR63B-100
PRODUCT SPECIFICATION FAN4855
10 REV. 1.0.0 3/17/04
Layout and Grounding
Considerations
Careful design of printed circuit board is recommended since
high frequency switching and high peak currents are present
in DC/DC converters applications. A general rule is to place
the converter circuitry well away from any sensitive analog
components. The printed circuit board layout should be
based on some simple rules to minimize EMI and to ensure
good regulation performances:
1. Place the IC, inductor, input and output capacitor as
close together as possible.
2. Keep the output capacitor as close to the FAN4855 as
possible with very short traces to VOUT and GND pins.
Typically it should be within 0.25 inches or 6 mm.
3. Keep the traces for the power components wide,
typically > 50 mils or 1.25 mm.
4. Place the external networks for LBI and FB close to
FAN4855, but away from the power components as far
as possible to prevent voltage transient from coupling
into sensitive nodes.
5. On multilayer boards use component side copper for
grounding around the IC and connect back to a quiet
ground plane using vias. The ground planes act as
electromagnetic shields for some of the RF energy
radiated.
6. The connection of the GND pin of the IC (pin 8) to the
overall grounding system should be directly to the
bottom of the output filter capacitor. A star grounding
system radiating from where the power enters the PCB,
is a recommended practice.
Application Example
The FAN4855 can be used as a constant current source to
drive white LEDs like QTLP670C-IW. As shown in the
diagram below, the current is maintained constant over a
wide range of input voltages.
L = 10µH D1
FAN4855
1
2
3
4
8
7
6
5
D2
Cout
10µF
Cin
10µF
+
+
+D3
D4 62Ω
62Ω
62Ω
62Ω
20
Input Voltage (V)
LED Current (mA)
19.8
19.6
19.4
19.2
19
18.8
18.6
18.4
18.2
18
01234 5
PRODUCT SPECIFICATION FAN4855
11 REV. 1.0.0 3/17/04
Mechanical Dimensions
Package: T08, 8-Pin TSSOP
0.113 - 0.123
8
(2.87 - 3.12)
0.169 - 0.177
(4.29 - 4.50)
0.246 - 0.258
(6.25 - 6.55)
0.026 BSC
PIN 1 ID
1
(0.65 BSC) 0.043 MAX
(1.10 MAX)
0.033 - 0.037
(0.84 - 0.94)
0.008 - 0.012
SEATING PLANE
(0.20 - 0.30) 0.002 - 0.006
0°-8°
(0.05 - 0.71)
0.020 - 0.028
(0.51 - 0.71) 0.004 - 0.008
(0.10 - 0.20)
FAN4855 PRODUCT SPECIFICATION
REV. 1.0.0 3/17/04 12
Mechanical Dimensions
Package: 8-Pin MSOP
C_
_
D
C0.002[0.05]
0.002[0.05] M S
A
SE
LAND PATTERN RECOMMENDATION
C
L
SYMM
0.002 - 0.006
[0.06 - 0.15]
0.034
[0.84]
TYP
TYP
0.030 - 0.037
[0.76 - 0.94]
0.012+0.002
[0.3 ± 0.05]
TYP
TYP
TYP
R
0.007±0.002
0.375
[0.953]
[0.18±0.05]
0.021±0.005
[0.53±0.12]
0.005
[0.13]
TYP
R0.005
[0.13]
GAGE PLANE
SEATING PLANE
0°-6° TYP
(0.010)
[0.25]
0.118±0.004
-B-
-A-
[3±0.1]
0.189
[4.8]
0.040
[1.02]
TYP
0.016
[0.41] TYP
0.0256
[0.65]
0.193±0.004
0.118 ± 0.004
85
14
[3 ± 0.1]
[4.9±0.1]
TYP
PIN 1
IDENT
(0.0256)
[0.65]
PRODUCT SPECIFICATION FAN4855
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in a significant injury of the user.
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
www.fairchildsemi.com
3/17/04 0.0m 005
Stock#DS30004855
2004 Fairchild Semiconductor Corporation
Ordering Information (TA = -40°C to +85°C)
Part Number Package Packing
FAN4855MTC 8 Pin TSSOP Rails
FAN4855MTCX 8 Pin TSSOP Tape and Reel
FAN4855MU 8 Pin MSOP Rails
FAN4855MUX 8 Pin MSOP Tape and Reel
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
