BL8531
High Efficiency Low Noise PFM Step-up DC/DC Converter
DESCRIPTION
BL8531 series are CMOS-based PFM step-up DC-
DC Converter. The converter can start up by
supply voltage as low as 0.8V, and capable of
delivering maximum 200mA output current at
3.3V output with 1.8V input Voltage. Quiescent
current drawn from power source is as low as
5.5uA. All of these features make BL8531 series
be suitable for the portable devices, which are
supplied by a single battery to four-cell batteries.
To reduce the noise caused by the switch
regulator, BL8531 is well considerate in circuit
design and manufacture, so that the interferer to
other circuits by the device is reduced greatly.
BL8531 integrates stable reference circuits and
trimming technology, so it can afford high
precision and low temperature-drift coefficient of
the output voltage.
BL8531 is available in SOT-89-3, SOT-23-3, SOT-
23-5 and TO-92 packages, which is PB free. And in
5-pin packages, such as SOT-23-5, the device can
be switch on or off easily by CE pin, to minimize
the standby supply current.
FEATURES
•
Deliver 200mA at 3.3V Output voltage with
1.8V input Voltage
Low start-up voltage (when the output
current is 1mA)-----------------------------0.8V
Output voltage can be adjusted from 2.5V½
6.0V (In 0.1V step)
Output voltage accuracy ---------------±2％
Low temperature-drift coefficient of the
output voltage--------------------±100ppm/℃
Only three external components are
necessary: An inductor, a Schottky diode and
an output filter capacitor
High power conversion efficiency-----85％
Low quiescent current drawn from power
source-------------------------------------<5.5uA
•
•
•
•
•
•
•
APPLICATIONS
•
•
•
Power Source for PDA, DSC, MP3 Player,
Electronic toy and wireless mouse
Power Source for a Single or Dual-cell
Battery-Powered Equipments
Power Source for LED
TYPICAL APPLICATION
ELECTRICAL CHARACTERISTICS
Output Voltage VS. Output Current
(Vout=3.3V)
4.0
3.5
BL8531
Vout (V)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
0.1
0.2
0.3
Iout (A)
0.4
0.5
Vin=0.9V
Vin=1.0V
Vin=1.2V
Vin=1.5V
Vin=1.8V
Vin=2.0V
Vin=2.5V
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BL8531
ORDERING INFORMATION
BL8531
□□□□
1 2 3 4
Code
1
□
2
□
MARKING INFORMATION
Product Classification
Marking
30:Product Code
30XX
XX: Output Voltage
Product Classification
Marking
30:Product Code
30XX
XX: Output Voltage
Product Classification
Marking
LA: Product Code
XX: Output Voltage
LAXX
YYBZZ
YY: LOT NO.
B: FAB Code
ZZ: Date Code
Product Classification
Marking
LA: Product Code
XX: Output Voltage
LAXX
YYBZZ
YY: LOT NO.
B: FAB Code
ZZ: Date Code
BL8531CHBG□□
BL8531CC3TR□□
BL8531CB5TR□□
BL8531CB3TR□□
Description
3
□
4
□
Temperature&Rohs:
C: -40~85°C, Pb Free Rohs Std.
Package type:
B3: SOT-23-3
B5: SOT-23-5
C3: SOT-89-3
H: TO-92
Packing type:
TR: Tape&Reel (Standard)
BG: Bag (TO-92)
Output voltage:
e.g. 25=2.5V
33=3.3V
60=6.0V
ABSOLUTE MAXIMUM RATING
Parameter
Value
output Voltage Range
-0.3V-12V
LX Voltage
-0.3V-6.5V
CE Pin Voltage
-0.3V-(Vout+0.3)
Lx Pin Output Current
0.7A
Operating Junction
125°C
Temperature (Tj)
Ambient Temperature (Ta)
-40°C -85°C
Power
SOT-23-3
250mW
Dissipation
SOT-23-5
250mW
SOT-89-3
500mW
TO-92
500mW
Storage Temperature (Ts)
-40°C -150°C
Lead Temperature & Time
260°C, 10S
Note:
Exceed these limits to damage to the device.
Exposure to absolute maximum rating conditions may
affect device reliability.
PIN CONFIGURATION
CE
GND
OUT
LX
NC
Chip Enable (Active high)
Ground
Output Feedback Pin, Power supply for
internal
Switching Pin
No Connection
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BL8531
RECOMMENDED WORK CONDITIONS
Item
Input Voltage Range
Inductor
Input Capacitor
Output Capacitor
Ambient Temperature
Min
0.8
10
0
47
-40
Recommended
27
≥10
100
Max.
Vout
100
220
85
Unit
V
μH
μF
μF
°C
ELECTRICAL CHARACTERISTICS
SYMBOL
ITEM
TEST CONDITIONS
REFERENCE DATA
Min
Typ
Max
2.45
2.646
2.94
3.234
3.528
3.92
4.9
5.88
Iout=0mA，
Vin=Vout*0.6
Iout=1mA，
Vin：0→2V
Iout=1mA，
Vin：2→0V
Without external
components, Vout
=Vout×1.05
2.5
2.7
3.0
3.3
3.6
4.0
5.0
6.0
12
0.8
0.6
0.7
4
0.5
Vout=Vlx=6.5V
V
CE
：0→2V
V
CE
：
2→0V
LX on “L” side
Vout=Vout*0.96
On（Vlx“L”）side
70
400
75
85
80
0.8
0.3
0.5
5
7
2.55
2.754
3.06
3.366
3.672
4.08
5.1
6.12
Vout
15
0.9
UNIT
Vout
Output Voltage
V
Vin
Iin
Vstart
Vhold
IDD
Rswon
ILXleak
VCEH
VCEL
Fosc
Maxdty
η
Input Voltage
Input Current
Start-up voltage
Hold-on voltage
Quiescent current
drawn from power
source
Switch ON
Resistance
LX leakage current
CE “H” threshold
voltage
CE “H” threshold
voltage
Oscillator
frequency
Oscillator duty
cycle
Efficiency
V
uA
V
V
uA
Ω
uA
V
V
Khz
％
％
Note
：
1. Diode: Schottky type, such as: 1N5817, 1N5819, 1N5822
2. Inductor: 27uH(R<0.5
Ω
)
3. Capacitor: 100uF(Tantalum type)
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BL8531
BLOCK DIAGRAM
Note: CE pin is only available on 5 pins packages.
DETAILED DESCRIPTION
The BL8531 series are boost structure, voltage-
type Pulse-Frequency Modulation (PFM) step-up
DC-DC converter.
Only
three
external
components are necessary: an inductor, an
output filter capacitor and a schottky diode. And
the converter’s low noise and low ripple output
voltage can be adjusted from 2.5V to 5.0V, 0.1V
step. By using the depletion technics, the
quiescent current drawn from power source is
lower than 7uA. The high efficiency device
consists of resistors for output voltage detection
and trimming, a start-up voltage circuit, an
oscillator, a reference circuit, a PFM control
circuit, a switch protection circuit and a driver
transistor.
The PFM control circuit is the core of the BL8531
IC. This block controls power switch on duty cycle
to stabilize output voltage by calculating results
of other blocks which sense input voltage, output
voltage, output current and load conditions. In
PFM modulation system, the frequency and pulse
width is fixed. The duty cycle is adjusted by
skipping pulses, so that switch on-time is changed
based on the conditions such as input voltage,
output current and load. The oscillate block
inside BL8531 provides fixed frequency and pulse
width wave.
The reference circuit provides stable reference
voltage to output stable output voltage. Because
internal trimming technology is used, The chip
output change less than ±2%. At the same time，
the problem of temperature-drift coefficient of
output voltage is considered in design, so
temperature-drift coefficient of output voltage is
less than 100ppm/℃。
High-gain differential error amplifier guarantees
stable output voltage at difference input voltage
and load. In order to reduce ripple and noise, the
error amplifier is designed with high band-with.
Though at very low load condition, the quiescent
current of chip do effect efficiency certainly. The
four main energy loss of Boost structure DC-DC
converter in full load are the ESR of inductor, the
voltage of Schottky diode, on resistor of internal
N-channel MOSFET and its driver. In order to
improve the efficiency, BL8531 integrates low on-
resistor N-channel MOSFET and well design driver
circuits. The switch energy loss is limited at very
low level.
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BL8531
SELECTION THE EXTERNAL COMPONENTS
Thus it can be seen, the inductor and schottky
diode affect the conversion efficiency greatly. The
inductor and the capacitor also have great
influence on the output voltage ripple of the
converter. So it is necessary to choose a suitable
inductor, a capacitor and a right schottky diode,
to obtain high efficiency, low ripple and low noise.
Before discussion，we define
An inductor value of 3uH to 1mH works well in
most applications. If DC-DC converter delivers
large output current (for example: output current
is great than 50mA), large inductor value is
recommended in order to improve efficiency. If
DC-DC must output very large current at low
input supply voltage, small inductor value is
recommended.
The ESR of inductor will affect efficiency greatly.
Suppose ESR value of inductor is rL，Rload is load
resistor，then the energy can be calculated by
following expression:
D
≡
Vout
−
Vin
.
Vout
INDUCTOR SELECTION
Above all, we should define the minimum value
of the inductor that can ensure the boost DC-DC
to operate in the continuous current-mode
condition.
∆
η
≈
r
L
R
load
(1
−
D
)
2
D
(1
−
D
)
2
R
L
L
min
≥
2
f
The above expression is got under conditions of
continuous current mode, neglect Schottky
diode’s voltage, ESR of both inductor and
capacitor. The actual value is greater that it. If
inductor’s value is less than Lmin，the efficiency
of DC-DC converter will drop greatly, and the DC-
DC circuit will not be stable.
Secondly, consider the ripple of the output
voltage,
For example: input 1.5V, output is 3.0V,
Rload=20Ω, rL=0.5Ω, The energy loss is 10%.
Consider all above，inductor value of 47uH、
ESR<0.5Ω is recommended in most applications.
Large value is recommended in high efficiency
applications and smaller value is recommended.
CAPACITOR SELECTION
Ignore ESR of capacitor，the ripple of output
voltage is:
r
=
∆
Vout
D
=
Vout
R
load
Cf
D
•
Vin
∆
I
=
Lf
Vin
DVin
Im
ax
=
+
2
Lf
(1
−
D
)
2
R
L
If inductor value is too small, the current ripple
through it will be great. Then the current through
diode and power switch will be great. Because
the power switch on chip is not ideal switch, the
energy of switch will improve. The efficiency will
fall.
Thirdly
，
in general, smaller inductor values
supply more output current while larger values
start up with lower input voltage and acquire
high efficiency.
So large value capacitor is needed to reduce
ripple. But too large capacitor value will slow
down system reaction and cost will improve. So
100uF capacitor is recommended. Larger
capacitor value will be used in large output
current system. If output current is small (<10mA),
small value is needed.
Consider ESR of capacitor，ripple will increase：
r
'
=
r
+
Im
ax
•
R
ESR
Vout
When current is large, ripple caused by ESR will
be main factor. It may be greater than 100mV。
The ESR will affects efficiency and increase
energy loss. So low-ESR capacitor (for example:
tantalum capacitor) is recommend or connect
two or more filter capacitors in parallel.
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