dm00056440

DN0007
Design note
STBB1 buck-boost converter used as a 500mA
LED driver with 1.8VDC-5.5VDC Vin
Designs from our labs describe tested circuit designs from ST labs which provide optimized solutions
for specific applications. For more information or support, visit www.st.com
By Patrick Jankowiak
Main components
STBB1-APUR
1 A, high efficiency adjustable single inductor dual mode buckboost DC-DC converter
Specification
•
Output current = 500mA (1.2A max)
•
Input Voltage Range 2.1VDC-5.5VDC
•
300mA LED current @ 1.8VDC Input Voltage.
•
800mA LED current @ 3.3VDC Input Voltage.
Circuit description
A buck-boost converter with a wide input voltage range is useful for current-limited or LED
power applications where it is desired to take advantage of low input voltages, to ensure
start-up under battery end-of-life conditions, or in a battery-powered application where
maintaining illumination in an emergency is more important than other considerations. The
STBB1 was designed as a buck-boost voltage regulator, but it also lends itself very well to
a current regulated voltage source.
The circuit shown in Figure 1 was developed to operate a 500mA white LED from a single
Li-ion or LiPo cell, two alkaline or NiCd cells, or any other power source within the
acceptable voltage range. At higher voltages, where the STBB1 operates in buck mode,
the LED current may be up to 1A, limited only by thermal factors and the switch current
rating. When Vin drops to 1.8V, the circuit still supplies 300mA. A 100uF capacitor, C1,
helps the circuit start at low voltages by applying voltage to the FB pin. A 1K resistor, R1,
serves to discharge C1 when the circuit is not operating. A 3.9V Zener diode, D2, limits the
output voltage to a safe value in case the LED opens or is removed.
The circuit shown in Figure 2 was developed to operate a 200mA LED and uses a slightly
different starting and protection scheme. When power is applied to Vin, a 100nF capacitor,
C2, provides an initial FB voltage of about 3.7V until sufficient output voltage is generated
to forward bias the LED. Resistor R4 prevents the FB voltage pulse from C2 from being
absorbed by current programming resistor R2. Once the LED is conducting, a feedback
voltage of 0.5V is provided by the voltage drop across R3. Diode D2 prevents the resistor
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DN0007 Rev 1
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network R1-R2 from influencing the FB voltage provided by R3. Should the LED open, D2
conducts and FB voltage is provided through the R1-R2 voltage divider and the output
voltage is limited to a safe value until the LED can be replaced.
Figure 1. Circuit diagram – 500mA
Table 1.
Reference
BOM for Figure 1
Manufacturer
Part Number/ Description
Value
Size
U1
STMicroelectronics STBB1-APUR
CIN
Murata
GRM21BR71A106KE51L
10 µF
0805
COUT1
Murata
GRM21BR60J226ME39L
22 µF
0805
COUT2
Murata
GRM21BR71A106KE51L
22 µF
0805
TDK
VLCF4020T-2R2N1R7
Coilcraft
XFL4020-222ME
C1
Any
Capacitor, 6VDC
100uF
R1
Any
Resistor, Chip, 1/16W, 1%
1K
0603
R2
Any
Resistor, Chip, 1W, 1%
1Ω
2512
D1
Any
LED White
D2
Any
Zener diode, 2W
Battery
Any
2 x Battery Lithium AA
L1
June 2012
DFN10 (3 x 3 mm)
2.2 µH 4 x 4 x 2 mm
DN0007 Rev 1
3.9V
2/5
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Figure 2. Circuit diagram – 200mA
Table 2.
Reference
BOM for Figure 1
Manufacturer
Part Number/ Description
Value
Size
U1
STMicroelectronics STBB1-APUR
CIN
Murata
GRM21BR71A106KE51L
10 µF
0805
COUT
Murata
GRM21BR60J226ME39L
22 µF
0805
TDK
VLCF4020T-2R2N1R7
Coilcraft
XFL4020-222ME
C1,C2
Murata
GRM188R71C104KA01D
100nF
0603
R1
Any
Resistor, Chip, 1/16W, 1%
470K
0603
R2
Any
Resistor, Chip, 1/16W, 1%
100K
0603
R3
Any
Resistor, Chip, 1/2W, 1%
2.2 Ω
2010
R4
Any
Resistor, Chip, 1/16W, 1%
1M
0603
R5
Any
Resistor, Chip, 1/16W, 1%
150K
0603
D1
Any
LED White
D2
FAIRCHILD
1N4148WS
Battery
Any
2 x Battery Lithium AA
L1
DFN10 (3 x 3 mm)
2.2 µH 4 x 4 x 2 mm
SOD-323F
Measurement results
Measurements were made of the input voltage and LED current in the basic circuit of
Figure 1, using a high power white LED with Vf of about 3.7V. These figures should be
considered guidelines and not absolute. Slightly more or less current may be available
depending on the individual circuit and layout, but ultimately the device will limit the current
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and internal temperature to a safe value while providing as much as possible under the
circumstances.
•
Vin=1.8V: ILED =300mA
•
Vin = 3.3V: ILED =800mA
•
Vin = 5.5V: ILED=1000mA
Current output capability of the STBB1 was also measured with a load requiring 2.5V. The
graph is the maximum output current at VIN = 1.8 V with VOUT = 2.5V. The maximum
output current at VIN = 1.8 V with VOUT = 3.3 V is around 400 mA.
Figure 3. Output current vs. Temperature at Vin=1.8VDC
Support material
Documentation
Datasheet STBB1-AXX, 1 A, high efficiency single inductor dual mode buck-boost DC-DC
converter
Revision history
Date
6-Jun-2012
June 2012
Version
1
Changes
Initial release
DN0007 Rev 1
4/5
www.st.com
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