Dec 2005 Constant Current from 3A DC/DC Converter with 2 Rail-to-Rail Current Sense Amplifiers

DESIGN FEATURES
Constant Current from 3A DC/DC
Converter with 2 Rail-to-Rail
by Daniel Chen
Current Sense Amplifiers
Introduction
Traditional DC/DC converters use
voltage feedback for constant output
voltage regulation. There are many
applications, however, that need to
regulate a constant output current.
Driving LEDs in series is one such
application. The LT3477 combines a
traditional voltage feedback loop and
two unique current feedback loops
to operate as a constant-current,
constant-voltage source. It is a current mode, 3A DC/DC converter with
dual rail-to-rail 100mV current sense
amplifiers that can be configured as a
buck mode or buck-boost mode LED
driver. It is versatile enough to also be
configured as an input-output current limited boost, SEPIC or inverting
converter. Both current sense voltages
can be adjusted independently using
the IADJ1 and IADJ2 pins.
With two identical precision current
sense amplifiers, the LT3477 can provide an accurate input current limit as
well as an accurately regulated output
current. With an input voltage range of
2.5V to 25V, the LT3477 works from
a variety of input sources. The 42V
switch rating allows an output voltage
of up to 41V to be generated, easily
The unique feature of
the three-feedback-loop
topology (two current and
one voltage) is that it can
support constant voltage
and/or constant current
applications.
VADJ
–
+
+
A1
VADJ
–
+
+
A2
IA1
–
+
IA2
–
IADJ2
FBP
+
FBN
–
VREF
A3
–
+
A4
R
S
VA
Σ
SLOPE
VREF
1.25V
Q1
Q
–
ISN2
SW
VC
+
IADJ1
ISP2
Figure 1 shows a block diagram of the
LT3477. The voltage error amplifier
has both FBP and FBN pins to allow a
positive or negative output configuration. With the addition of two current
feedback control loops, amplifier A3
becomes a summing point for three
feedback loops. Depending on configuration, any of the loops can take
over feedback control by sourcing or
sinking current at the VC node. The
unique feature of the three-feedbackloop topology (two current and one
voltage) is that it can support constant
voltage and/or constant current applications.
+
ISN1
How It Works
driving up to ten white LEDs in series.
The buck mode LED configuration is
capable of driving multiple ten-LED
strings in parallel if external current
mirroring circuitry is added.
The switching fequency is adjustable from 200kHz to 3.5Mhz, set by
SS
ISP1
a single resistor. The available high
operating frequencies allow the use
of low profile inductors and capacitors—important in applications where
space is a premium. The wide available
range makes it possible to optimize size
and efficiency for your application.
OSCILLATOR
SHDN VIN
RT
Figure 1. LT3477 block diagram
Linear Technology Magazine • December 2005
25
DESIGN FEATURES
120
PVIN
32V
VCM = 10V
C1
2.2µF
R1
0.1Ω
VOLTAGE SENSE (mV)
100
80
D2
1A
60
D5
•
•
•
LED
STRING C2
1µF
40
L1
20
0
0
100 200 300 400 500 600 700 800
IADJ VOLTAGE (mV)
VIN
3.3V
Figure 2. Current sense amplifier voltage
sense level vs IADJ pin voltage
R2
1k
D2
ISN1
R1
0.1Ω
D1
R5
200k
SW1
ISN2
FBP
RT
GND
SS
C4
33nF
R3
22k
Schottky diode is connected between
the SW and PVIN nodes. With high side
current sense, the boost converter is
effectively converted into a buck LED
converter, which increases the part’s
power handling capability. In addition, the VIN pin, which provides the
chip operating current, can be tied to
a lower voltage level such as 3.3V. As
a result, the power consumption on
the chip itself is also reduced, thus
improving overall efficiency. Over 90%
efficiency can be readily achieved with
a wide range of inductor and frequency
selections.
FBN
90
85
LT3477
ISP2
ISN2
VREF
RT
GND
C3
10nF
SS
C4
33nF
VIN = 8V
80
EFFICIENCY (%)
VC
FBP
ISP2
VREF
LED drivers use a grounded current
sense resistor to regulate current, but
the LT3477 current sense amplifiers
work in a high side sense scheme, so
the sensed voltage for current feedback
no longer needs to be ground referred.
In buck mode configuration, the sense
resistor is placed right at the input supply. The LEDs are placed between the
sense resistor and the inductor and the
VIN
IADJ1
IADJ2
SHDN
LT3477
Figure 3. Buck mode high current LED driver
L1
4.7µH
SHDN
R6
10k
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO D1OFA814AY-330M
D1: DIODES INC DFLS140
D3
LED BRIGHTNESS
CONTROL
0mV TO 650mV
FBN
C5
4.7nF
Buck Mode
High Current LED Driver
Figure 3 shows a typical application to
drive high current LEDs. Traditionally,
ISP1
VIN
IADJ1
IADJ2
SHDN
R5
309k
SW
ISN1
VC
Applications
C1
3.3µF
C3
3.3µF
SHDN
Current sense levels are adjustable
via sense resistors at the IADJ1 and
IADJ2 pins. The default sense voltage is
100mV for each current sense amplifier if the IADJ1 and IADJ2 pins are tied to
a potential higher than 650mV. If the
potentials at the IADJ1 and IADJ2 pins
are lower than 625mV, the LT3477
linearly adjusts the current sense level.
Figure 2 shows the voltage sense level
vs the IADJ pin voltage. For LED drivers, IADJ1 and IADJ2 pins can be used to
adjust LED current levels. Rail-to-rail
current sense amplifiers allow flexible
current sense schemes.
VIN
2.7V TO 16V
ISP1
D1
R3
18k
R6
10k
C2
4.7µF
75
VIN = 4.2V
70
65
60
55
C1: TAIYO YUDEN LMK316BJ335ML
C2: MURATA GRM31CR71E475KA88L
D1: DIODES, INC. B320A
L1: TOKO FDV0630-4R7M
Figure 4. Buck-boost LED driver
26
50
0
0.2
0.4
0.6
IOUT (A)
0.8
1.0
Figure 5. Buck-boost LED driver efficiency
Linear Technology Magazine • December 2005
DESIGN FEATURES
L2
10µH
C1
3.3µF
D1
VIN
IADJ1
IADJ2
R1
10k
LT3477
ISP2
VC
RT
GND
FBP
SS
C3
33nF
C4
4.7nF
330mA
R6
0.3Ω
ISN2
VREF
R4
1k
80
FBN
SHDN
SHDN
85
C2
3.3µF
R2
200k
SW
ISN1
ISP1
90
EFFICIENCY (%)
VIN
5V
R3
22k
LED1
55
LED2
50
LED3
LED4
VIN
3V TO 16V
C1
3.3µF
ISP1
ISN1
VIN
IADJ1
IADJ2
SHDN
SHDN
0.4
0.3
R4
0.15Ω
5.5V
670mA
R5
34.8k
L2
4.7µH
5.5V SEPIC Converter
with Short-Circuit Protection
Certain applications demand a converter output that is DC-isolated from
the input. SEPICs (single-ended primary inductance converters) provide
the solution. Figure 8 is an implementation which provides a 5.5V output
with complete short-circuit protection.
The current sense amplifier used for
current sense not only provides excellent short-circuit protection, but
also helps soft start the output. The
accurate output current limit ensures
the maximum current is set at 670mA.
When the load demands more, the
output voltage will droop while the
670mA output current is maintained.
Efficiency is shown in Figure 9.
Cuk Converter
The LT3477 provides pins for both
inputs to the voltage error amplifier,
which enables negative output voltages. Figure 10 is an implementation
continued on page 40
FBN
LT3477
0.2
Voltage feedback is used for open LED
protection.
D1
SW
0.1
Figure 7. 4W LED driver efficiency
330mA LED Driver
with Open LED Protection
LT3477 can also be used for LED
driver applications using a conventional boost topology with the
current sense amplifier for current
regulation. Figure 6 shows a typical
application circuit, and Figure 7 shows
the efficiency. Figure 6 uses a high
side current sense configuration for
feedback control. The current sense
amplifier could also be used for a
grounded current sense for this application, if desired, so the output can
be tied to the LED string directly. ISP2
would be tied to the cathode side of
the LEDs, and ISN2 is tied to ground.
C2
10µF
0
IOUT (A)
Figure 6. 4W LED driver
L1
4.7µH
70
65
60
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN TMK325BJ335MN
D1: DIODES INC. DFLS120L
L1: TOKO A915AY-100M
Buck-Boost LED Driver
In some applications, the input voltage might be comparable to the total
LED voltage drop or the input voltage
might fluctuate to higher or lower than
the total LED voltage drop. A buckboost LED driver works well in this
type of application. Figure 4 shows
the LT3477 buck-boost LED driver.
The cathode end of the LED string is
tied back to the input voltage, which
allows it to operate from a wide input
voltage range. R5 and R6 in Figure 4
are used for open LED protection.
Figure 5 is the efficiency measured
for this circuit.
75
90
VIN = 3V
85
ISP2
ISN2
VREF
R2
1k
FBP
RT
GND
SS
C5
4.7nF
C4
33nF
R3
18.2k
C3
10µF
C1: TAIYO YUDEN LMK316BJ335ML
C2: TAIYO YUDEN LMK325BJ106MN
C3: TAIYO YUDEN LMK316BJ106ZL
D1: DIODES INC. DFL5120L
L1, L2: TOKO FDV0630-4R7M
Figure 8. 5.5V SEPIC converter with short-circuit protection
Linear Technology Magazine • December 2005
R6
10k
EFFICIENCY (%)
80
VC
75
70
65
60
55
50
0
0.1
0.2
0.3 0.4
IOUT (A)
0.5
0.6
0.7
Figure 9. 5.5V SEPIC converter with
short-circuit protection efficiency
27
DESIGN IDEAS
Dual Display Power Supply
for Cell Phones
A typical application for the LT3466-1
is as a driver for dual displays in cell
phones. Present day, clam-shell cell
phones typically use a color TFT-LCD
main display and a secondary OLED
display. Figure 1 shows the LT34661 powering the main LCD backlight
and the secondary OLED display. The
86
VIN = 3.6V
8 LEDs
+15V/10mA
–15V/10mA
EFFICIENCY (%)
84
Low Cost, Complete
LCD Bias and White LED
Backlighting Solution for
Small TFT Displays
82
80
Small, active-matrix, TFT-LCD displays, used in cell phones, PDAs
and other handheld devices generally require four to ten white LEDs
for providing the backlight and fixed
+15V and –15V supply voltages to bias
the LCD. Figure 3 shows LT3466-1
powered complete TFT-LCD supply
with minimal external components
78
76
74
0
2.5
LT3466-1 drives 6 white LEDs at 20mA
for backlighting the main LCD panel
and generates 16V output for powering the OLED. The LT3466-1 allows
for independent dimming control of
the main and secondary displays via
the respective CTRL1 and CTRL2 pins.
Figure 2 shows the efficiency versus
output current for both the LED driver
and the boost converter. The typical
efficiency at 3.6V input supply is 84%
with the white LEDs and the OLED
driven at 20mA.
5
7.5
10
LED CURRENT (mA)
12.5
15
Figure 4. Efficiency versus LED current for
the circuit in Figure 3. The circuit achieves
greater than 83% efficiency driving eight
LEDs at 15mA from 3.6V input.
and high efficiency. The LT3466-1
drives eight white LEDs at 15mA and
generates 15V boost output powered
from a single Li-Ion supply. A discrete
charge pump produces the secondary
output of –15V. As seen in Figure 4,
the circuit achieves greater than 83%
efficiency driving eight LEDs at 15mA
from 3.6V input.
Conclusion
The LT3466-1 integrates a full featured
white LED driver and a boost converter
in a space saving 3mm × 3mm DFN
package. Integrated power switches
and Schottky diodes reduce the overall system cost and size making it an
excellent fit for handheld applications.
Features like internal compensation,
soft-start, Open LED protection enables LT3466-1 to provide complete
TFT-LCD supply (bias and white LED
backlight) for handheld devices with
minimal external components and
high efficiency.
LT3477, continued from page 27
C1
3.3µF
ISP1
ISN1
SW
VIN
IADJ1
IADJ2
SHDN
SHDN
D1
R2
402k
LT3477
ISP2
75
RLOAD
R4
0.2Ω
ISN2
VREF
FBP
–5V
FBN
VC
4.75k
L2
10µH
EF(FICIENCY (%)
R1
0.05Ω
VIN
5V
85
C2
0.47µF
L1
10µH
C5
3.3µF
55
45
RT
GND
65
SS
0
100
200
300
400
LOAD CURRENT (mA)
500
Figure 11. Efficiency of the Cuk converter.
C3
22nF
R3
100k
100pF
C4
33nF
R5
18.2k
R6
10k
C1, C5: TAIYO YUDEN LMK316BJ335ML
D1: DIODES INC. DFL5120L
L1, L2: TOKO A915AY-100M (D53LC SERIES)
Figure 10. Negative output voltage Cuk converter.
using a Cuk topology for 5V to –5V
conversion. The first current sense
amplifier is used for input current
limit, and the second current sense
amplifier is used for ground rail current sense to accurately limit the load
current at 500mA. Even though the
two current sense amplifiers are used,
40
efficiency up to 81% at 500mA output
load can still be achieved. Figure 11
shows the efficiency.
Conclusion
The rail-to-rail constant-current/constant-voltage operation of the LT3477
makes the device an ideal choice for
a variety of constant-current designs,
including negative outputs. The dual
current-sense amplifiers allow flexible
configuration for input current limit,
constant output current and fail-safe
protection, along with excellent output
voltage regulation. A wide input voltage range and the ability to produce
outputs up to 42V make the LT3477
extremely versatile.
Authors can be contacted
at (408) 432-1900
Linear Technology Magazine • December 2005
Similar pages