DN252 - Current-Limited DC/DC Converter Simplifies USB Power Supplies

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Current-Limited DC/DC Converter Simplifies USB
Power Supplies – Design Note 252
Bryan Legates
Many portable universal serial bus (USB) devices power
themselves from the USB host or hub power supply
when plugged into the USB port. Several requirements
must be met to ensure the integrity of the bus: the
USB specification dictates that the input capacitance
of a device must be less than 10μF to minimize inrush
currents when the device is plugged into the USB port;
when first plugged in, the device must draw less than
100mA from the port, and for high power devices, the
current drawn from the port can increase to 500mA
only after it is given permission to do so by the USB
controller. These requirements can be easily met using
the LT1®618 DC/DC converter, which provides an accurate input current control ideal for USB applications.
The LT1618 combines a traditional voltage feedback
loop with a unique current feedback loop to operate as
a constant-current, constant-voltage source.
USB to 12V Boost Converter
Figure 1 shows a 5V to 12V boost converter ideal
for USB applications. The converter has a selectable
100mA/500mA input current limit, allowing the device to
be easily switched between the USB low and high power
modes. Efficiency, shown in Figure 2, exceeds 85%.
L1
10μH
C1
4.7μF
3
D1
VOUT
12V
2
7
ISN
SW
8
9
20k
3.3V
100mA 500mA
0V
1
FB
VIN
4
5
90
C2
4.7μF
R2
107k
SHDN
IADJ GND
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
R1
909k
ISP
LT1618
3.3V
OFF ON
0V
USB to 5V SEPIC DC/DC Converter with
Short-Circuit Protection
The single-ended primary inductance converter (SEPIC)
shown in Figure 3 is ideal for applications where the
output must reduce to zero during shutdown. The input
current limit not only helps soft-start the output but also
provides short-circuit protection, ensuring USB device
compliance even under output fault conditions. Figure 4
shows the start-up characteristic of the SEPIC with a
50mA load. By limiting the input current to 100mA,
the output is soft-started, smoothly increasing and not
overshooting its final 5V value. Figure 5 shows that the
input current does not exceed 100mA even with the
output shorted to ground (thus the flat output voltage
waveform in the picture). Efficiency for this SEPIC
converter is shown in Figure 6.
VC
85
EFFICIENCY (%)
0.1Ω
VIN
5V
If the load demands more current than the converter
can provide with the input current limited to 100mA (or
500mA), the output voltage will simply decrease and
the LT1618 will operate in constant-current mode. For
example, with an input current limit of 100mA, around
35mA can be provided to the 12V output. If the load
increases to 50mA, the output voltage will reduce to
around 8V to maintain a constant 100mA input current.
10
2k
13k
80
75
70
10nF
65
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK316BJ475
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-100
DN252 F01
Figure 1. USB to 12V Boost Converter
(with Selectable 100mA/500mA Input Current Limit)
03/01/252_conv
60
0
20
40 60 80 100 120 140 160
LOAD CURRENT (mA)
DN252 F02
Figure 2. USB to 12V Boost Efficiency
IIN
VIN
5V
C1
4.7μF
3
C3
0.47μF
L1
10μH
0.1Ω
VOUT
5V
2
7
ISN
SW
L2
10μH
ISP
LT1618
8
3.3V
OFF ON
0V
9
20k
D1
1
FB
VIN
C2
10μF
SHDN
IADJ GND
4
5
R1
316k
R2
107k
VC
10
2k
3.3V
100mA 500mA
0V
13k
10nF
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN JMK316BJ106
C3: TAIYO YUDEN EMK212BJ474
D1: ON SEMICONDUCTOR MBR0520
L1, L2: SUMIDA CR43-100
(408) 573-4150
(408) 573-4150
(408) 573-4150
(800) 282-9855
(847) 956-0666
DN252 F03
Figure 3. USB to 5V SEPIC Converter
VOUT
2V/DIV
IIN
50mA/DIV
DN254 F04
1ms/DIV
Figure 4. USB SEPIC During Start-Up
VOUT
2V/DIV
Efficiency for this circuit exceeds 70%, which is significantly higher than the 30% to 50% efficiency obtained
when using a charge pump for LED drive. When the
LT1618 is turned off, no current flows in the LEDs. Their
high forward voltages prevent them from turning on,
ensuring a true low current shutdown with no excess
battery leakage or light output.
L1
10μH
VIN
2.7V TO 4.2V
50mA/DIV
C1
4.7μF
DN254 F05
1ms/DIV
Figure 5. USB SEPIC Start-Up with Output Shorted
80
8
7
SW
SHDN
ISP
ISN
R3
5.1k
LT1618
4
IADJ
GND
2.49Ω
20mA
3
2
R1
2M
1
FB
VC
10
C2
1μF
C3
0.1μF
R2
100k
CC
0.1μF
70
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN TMK316BJ105
D1: ON SEMICONDUCTOR MBR0530
L1: SUMIDA CLQ4D10-100
65
60
9
10kHz TO 50kHz
PWM
BRIGHTNESS
ADJUST
D1
VIN
5
75
EFFICIENCY (%)
Li-Ion White LED Driver
In addition to providing an accurate input current limit,
the LT1618 can also be used to provide a regulated output current for current-source applications. White LED
drivers are one such application for which the LT1618
is ideally suited. With an input voltage range of 1.6V to
18V, the LT1618 can provide LED drive from a variety
of input sources, including two or more alkaline cells,
or one or more Li-Ion cells. The circuit in Figure 7 is
capable of driving six white LEDs from a single Li-Ion
cell. LED brightness can be adjusted using a pulse width
modulated (PWM) signal, as shown, or by using a DC
voltage to drive the IADJ pin directly, without the R3/
C3 lowpass filter. If brightness control is not needed,
simply tie the IADJ pin to ground. Typical output voltage with the LEDs shown is around 22V, and the R1,
R2 output divider sets the maximum output voltage to
around 26V to protect the LT1618 if the LEDs are disconnected. The LT1618’s constant-current loop regulates
50mV across the 2.49Ω sense resistor, setting the LED
current to 20mA.
0
50
100 150 200 250
LOAD CURRENT (mA)
300
350
(408) 573-4150
(408) 573-4150
(800) 282-9855
(847) 956-0666
DN254 F07
Figure 7. Li-Ion White LED Driver
DN252 F06
Figure 6. USB to 5V SEPIC Efficiency
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn252f_conv LT/TP 0301 345K • PRINTED IN THE USA
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