BL8548
Max. 3MHz, Variable Frequency Step-up DC/DC Converter
DESCRIPTION
The BL8548 variable frequency step-up Converter
drives white LEDs with a constant current to
provide backlight in cell phones, PDAs, and other
hand-held devices. It features allowing series
connection of the white LEDs so that the LED
currents are identical for uniform brightness. An
enable input can be pulsed repeatedly to adjust
LEDs brightness. The fast 3MHz operation
frequency allows for smaller capacitor and
inductor. Fault condition protection uses cycle-by
cycle current limiting to sense maximum inductor
current and over-voltage protection. The 0.2V
low reference voltage minimized the power loss
across the current sense resistor.
The converter can operate from 2V to 6V, and
capable of delivering maximum 250mA output
current at 4-LEDs application with 3V input
voltage. Quiescent current drawn from power
source is as low as 120uA. All of these features
make BL8548 be suitable for the portable devices,
which are supplied by a single battery.
BL8548 is available in SOT23-6 package that is PB
free.
FEATURES
Up to 19V Output Voltage
Wide Operation Range: 2V to 6V
Maximum 3MHz Operating Frequency
PWM Dimming Control
Shutdown Current <1uA
Current Limit Cycle-by-Cycle
Low Current Sense Threshold: 200mV
19V Over Output Voltage Protection
Compact SOT23-6 Package
APPLICATIONS
Compact Back Light Module
Power Source for LED
Constant Current Source
TYPICAL APPLICATION
BL8548
Figure1. BL8548 Typical Application Circuit
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BL8548
ORDERING INFORMATION
BL8548
□
1
Code
1
2
3
2 3
□□
MARKING INFORMATION
Product Classification
Marking
LE: Product Code
LEZZ
ZZ: Date Code
1
Description
Temperature & Rohs:
C: -40~85°C, Pb Free Rohs Std.
Package Type:
B6: SOT-23-6
Packing Type:
TR: Tape & Reel (Standard)
BL8548CB6TR
6
5
4
LEZZ
2
3
PIN DESCRIPTION
Pin No.
1
2
3
4
5
6
Symbol
LX
GND
FB
CE
OVP
VIN
Switching node
Ground
Pin for Feedback Voltage
Description
Chip Enable Pin (Active with “H”), connect to VIN if not used.
Over Voltage Protection
Power Supply
BLOCK DIAGRAM
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BL8548
ABSOLUTE MAXIMUM RATING
Parameter
Max Input Voltage
CE Pin Voltage
LX Pin Output Current
LX Pin Voltage
Operating Junction Temperature (TJ)
Ambient Temperature (TA)
Power Dissipation
SOT-23-6
Storage Temperature (Ts)
Lead Temperature & Time
-0.3V to 8V
-0.3V to (Vin+0.3V)
1.8A
19.4V
125C
-40C to 85C
400mW
-40C to 150C
260C, 10Sec
Value
ELECTRICAL CHARACTERISTICS
(Ta=25ºC，VIN=3V，VCE=3V, unless otherwise noted)
Symbol
VIN
VFB
IFB
IQ
FMAX
ILIMIT
VCESAT
ILX
VCEH
VCEL
ICE
VOVP
Item
Operating Input Voltage
Feedback Voltage
FB Pin Bias Current
Quiescent Current
Maximum switching
Frequency
Switching Current Limit
Switching Saturation
Voltage
Switching pin Leak Current
CE Voltage High
CE Voltage Low
CE Pin Bias Current
Over Voltage Protection
Conditions
Min.
2
0.190
10
Typ.
0.200
45
93
0.4
3
1.8
Max.
6
0.210
100
120
1.0
Unit
V
V
nA
uA
uA
MHz
A
mV
uA
V
V
uA
V
VFB=0.3V
VCE=0V
VFB=0V, Floating OVP Pin
ILX=300mA
VLX=5V
VCE=0 to 3V
VCE=3 to 0V
VIN=VCE=3V
1.5
260
0.11
0.4
18
19
TYPICAL PERFORMANCE CHARACTERISTICS
CE Pin Bias Current vs. EN pin Voltage
(VIN=VCE)
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
VCE=2.5V
CE Pin Current vs. Temperature
(VIN=VCE))
50
CE Pin Current(uA)
40
30
20
10
0
-50
-25
0
25
Temperature
VCE=3V
VCE=6V
CE Pin Bias Curren(uA)
50
75
100
CE Pin Voltage(V)
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BL8548
Quiescent Current vs. Supply Voltage
(VCE=0V)
Quiescent Current(uA)
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
Supply Voltage(V)
Quiescent Current(uA)
120
100
80
60
40
20
0
0
1
2
3
4
5
6
Supply Voltage(V)
Quiescent Current vs. Supply Voltage
(VCE=VIN,VFB=0.3V)
FB Pin Voltage vs.Supply Voltage
FB Pin Voltage vs. Temperature
FB Pin Voltage(V)
0.22
0.20
0.18
0.16
0.14
0.12
FB Pin Voltage(V)
0.22
0.20
0.18
0.16
0.14
0.12
2
3
4
Supply Voltage(V)
5
6
-50
-25
0
25
50
75
100
Temperature(ºC)
Maximum Output Currenr(mA)
Efficiency vs. Output Current
(4LEDs)
100
80
60
40
20
0
0
20
40
60
80
100
120
Output Current(mA)
VIN=2.5V
VIN=3V
VIN=6V
Maximum Output Currenr vs. Output Voltage
600
500
400
300
200
100
0
4
8
12
16
Output Voltage(V)
Vin=3V
20
Vin=6V
24
Efficiency(%)
Vin=2.5V
Switch Frequency vs. Supply Voltage
(4LEDs*100mA)
Switch Frequency(MHz)
4
3
2
1
0
2
3
4
Supply Voltage(V)
5
6
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BL8548
APPLICATION INFORMATION
Dimming Control
1. Using a PWM Signal to CE Pin
When adding the PWM signal to CE pin, the
BL8548 is turned on or off by the PWM signal, so
the LEDs operate at either zero or full current.
The average LED current increase proportionally
with the duty cycle of the PWM signal.
2. Using a DC Voltage to FB Pin
From the Figure 2, we can add a DC voltage to FB
pin, we adjust the LED current by Changing the
DC voltage, which control the brightness, DC
voltage range is from 0V to 2V.
LED1
4. PCB Layout
Layout is critical to achieve clean and stable
operation. The switching power stage requires
particular attention. Follow these guidelines for
good PC board layout:
1) Place decoupling capacitors as close to the IC as
possible
2) Connect input and output capacitors to the
same power ground node with a star ground
configuration then to IC ground.
3) Keep the high-current paths as short and wide
as possible.
4) If possible, connect VIN, LX, and GND separately
to a large copper area to help cool the IC to
further improve efficiency and long-term
reliability.
5) Ensure all feedback connections are short and
direct. Place the feedback resistors as close to
the IC as possible.
6) Route high-speed switching nodes away from
sensitive analog areas
LED2
LC3320
BL8548
FB
VDC
R13
90K
R12
5K
R1
10
LED3
Figure 2. Dimming Control Using a DC Voltage
3. Using a Filtered PWM Signal to FB Pin
The filtered PWM signal can be considered as an
adjustable DC voltage. It can be used to replace
the variable DC voltage source in dimming control.
The circuit is shown in Figure 3.
LED1
LED2
LC3320
BL8548
FB
PWM R14
R13
90K
C12
0.1uF
R12
5K
R1
10
LED3
Figure 4. Recommended Component Placement
Figure 3. Dimming Control Using a Filtered PWM Signal
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