©2005 Fairchild Semiconductor Corporation
1
www.fairchildsemi.com
December 2005
FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
FAN4855
500mA High Efficiency Boost Regulator with Adjustable
Output, Shutdown and Low Battery Detect
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
■
TSSOP-8 Package
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 con-
stant 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 transis-
tors, synchronous rectification and low supply current,
make the FAN4855 ideal for portable applications. The
FAN4855 is available in 8-lead TSSOP package.
Ordering Information
(T
A
= -40°C to +85°C)
Typical Application
Part Number Package Packing
FAN4855MTC 8 Pin TSSOP Rails
FAN4855MTCX 8 Pin TSSOP Tape and Reel
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
2
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
Pin Configuration
Pin Description
Absolute Maximum Ratings
Absolute Maximum Ratings are those values, beyond which the device could be permanently damaged. Absolute max-
imum 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 525 mW
Thermal Resistance (
θ
JA
) 124 °C/W
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
6
5
SHDN
LBI
LB0
3
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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
Ambient 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
4
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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
Tantalum
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
5
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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 1.01.21.41.61.8
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
6
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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”
7
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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
8
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FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
Block Diagram
Functional Description
Boost Regulator
FAN4855 is an adjustable boost regulator that combines
variable ON and minimum OFF architecture with syn-
chronous rectification. Unique control circuitry provides
high-efficiency power conversion for both light and heavy
loads by transitioning between discontinuous and contin-
uous 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 fre-
quency depends upon the load, the input and output volt-
age ranging up to 430kHz.
The input voltage V
IN
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 determines the output voltage. When VFB voltage
drops below the VREF = 1.24V the error amplifier A1 sig-
nals to regulator to deliver charge to the output by trig-
gering the Variable On-Time One Shot. One Shot
generates a pulse at the gate of the Power NMOS tran-
sistor 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 momen-
tarly 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 deliv-
ered 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-volt-
age threshold. Under medium and heavy loads, a single
energy pulse is not sufficient to force the output voltage
above its upper threshold 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 voltage is satis-
fied, or the converter reaches its set current limit.
After a period of time TOFF > 1µS, determined by Mini-
mum 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
--------------------------=
9www.fairchildsemi.com
FAN4855 Rev. 1.0.1
FAN4855 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 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 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 VSHDN is approxi-
mately 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 typi-
cal dependence 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 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[(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 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:
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 equa-
tion:
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 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
VOUT and FB pin.
Table 1. Recommended capacitors
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
Vendor Description
MuRata X5R Ceramic
AVXTAJ,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
10 www.fairchildsemi.com
FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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 cou-
pling 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
11 www.fairchildsemi.com
FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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)
12 www.fairchildsemi.com
FAN4855 Rev. 1.0.1
FAN4855 500mA High Efficiency Boost Regulator with Adjustable Output, Shutdown and Low Battery Detect
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.
The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is
not intended to be an exhaustive list of all such trademarks.
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 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, or (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 significant injury to the
user.
2. A critical component is 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.
PRODUCT STATUS DEFINITIONS
Definition of Terms
Datasheet Identification Product Status Definition
Advance Information
Preliminary
No Identification Needed
Obsolete
This datasheet contains the design specifications for
product development. Specifications may change in
any manner without notice.
This datasheet contains preliminary data, and
supplementary data will be published at a later date.
Fairchild Semiconductor reserves the right to make
changes at any time without notice in order to improve
design.
This datasheet contains final specifications. Fairchild
Semiconductor reserves the right to make changes at
any time without notice in order to improve design.
This datasheet contains specifications on a product
that has been discontinued by Fairchild semiconductor.
The datasheet is printed for reference information only.
Formative or
In Design
First Production
Full Production
Not In Production
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