Charge Pump Based Multiple LED Driver

AND8192/D
Charge Pump Based
Multiple LED Driver
Prepared by: Michael Bairanzade
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
Abstract
On the other hand, combining three functions in the same
system creates a special case since the converter must be
capable of driving the wide current load needed for the
different functions. The typical currents used to drive the
LED, summarized in Table 1, range from a low 1 mA to
350 mA when the flash is activated. Moreover, unlike the
xenon photo flash, the LED system must have a relatively
long pulse of light to properly illuminate the scene.
Typically, a xenon pulse has a 1 ms flash duration, the LED
system being in the 100 ms to 200 ms range. Consequently,
the converter must be designed to support such a large
demand.
High powered LED capable to sustaining up to 800 mA
are under development and drivers for these devices should
be available within a few months.
This application note describes a multi−functional
system, capable of generating and controlling the power
needed to utilized three features available in modern cellular
phones. In addition to larger displays, with full color
capability, flash and torch features have now been added to
support the embedded camera and the night path finder.
These features are made possible by using an ultra bright
LED powered by standard battery cells.
BASIC CIRCUIT DESCRIPTION
Since the LED have a forward drop voltage ranging from
3 V to 4.5 V, depending upon the forward current, a
straightforward connection to a standard battery is not feasible
as depicted Figure 1. A boost structure must be used to make
the power supply voltage compatible with the LED.
Battery Voltage = F(Capacity) @ TA = +20°C
OSRAM − LWY87S @10 mA−3.8 V
100
CITIZEN− CL590S @20 mA−3.9 V
90
NICHIA−NECWB205 @20 mA−4.0 V
Absorbed Capacity (%)
80
OSRAM − LWT67C @20 mA−4.1 V
70
60
Vout−3 Cell Alkaline
Vout−Li−ion
50
Vout−2 cell Alkaline
40
30
20
10
0
2
MB−JUNE 2004
2.5
3
2.0 X
3.5
1.5 X
4
1.33 X
4.5
5
Vout (V)
1.0 X
Figure 1. Typical Lithium−Ion Battery Voltage and White LED
 Semiconductor Components Industries, LLC, 2004
December, 2004 − Rev. 0
1
Publication Order Number:
AND8192/D
AND8192/D
Table 1. White LED Typical Applications
LED
Backlight
OSRAM LWY85S
1 mA – 10 mA
OSRAM – LWT67C
1 mA – 20 mA
Torch
OSRAM
Flash
100 mA
OSRAM – LWW5SG
350 mA
CITIZEN − CL590S
1 mA – 20 mA
NICHIA−NECWB205
1 mA – 20 mA
LUMILED
800 mA
LED, a good thermal contact to a dedicated layer on the
printed board is essential. The LWW5SG specifications give
a maximum 9°C/W junction−to−case thermal resistance,
capable of limiting the temperature of the silicon to the
100°C maximum specified in the OSRAM data sheet. After
dissipating 1.6 W, the maximum thermal to air resistance
acceptable by the chip can be calculated as:
Along with the amount of current the converter provides,
it is worthwhile to note the thermal behavior of both the
silicon and the power LED.
According to the OSRAM’s data sheet, the Dragon LED
(LWW5SG ) should have a maximum 4.5 V forward drop
with 350 mA current. The power absorbed by the load will
be 1.57 W and, assuming a 75% efficiency of the DC/DC
converter, will translate to almost 2 W of input power.
Consequently, some 400 mW will be dissipated as heat into
the silicon and, according to the NCP5603 data sheet, the
chip temperature will increase by RJA x Pin = 85 x 0.4 =
34°C. Such a temperature increase is acceptable since,
even under the worst case +85°C ambient temperature, the
junction will be below the maximum rating defined for this
chip.
However, we must take into account the low battery
situation: in this case, the efficiency of the converter can
decrease and we end up with 60% efficiency, yielding
almost 54°C temperature increases. At this point, the
silicon can rise above 125°C, under extreme high ambient
temperature, and the global long−term reliability of the chip
will be impaired. This can be avoided by either reducing the
thermal resistance (using a heatsink by means of the PCB
layer) or by ensuring the duty cycle is short enough to
properly cool off the chip between pulses.
Generally speaking, the High Intensity LED are power
limited and care must be observed to avoid any thermal run
out during normal operation. This is particularly true for the
flash mode in which, as depicted above, nearly 1.6 W are
dissipated into the LED junctions. Because the junction to
ambient thermal resistance is limited by the packaging of the
RJA RJA Tjmax Tamb
Pchip
100 85
9.37°CW
1.6
Since the RJC is 9°C/W, it is practically impossible to
achieve a 0.38°C/W case to ambient thermal resistance and
the only alternative is to limit the operating ambient
temperature.
Assuming Tamb = 60°C, then RJA = (100−60) / 1.6 =
25°C/W.
In this case, the case−to−ambient thermal resistance is
25 − 9 = 16°C/W, a value more realistic, although not so easy
to achieve with a room limited PCB.
NCP5603 operates without special treatment in terms of
thermal sinking and a simple copper flag is built underneath
the QFN package as depicted Figure 3.
The schematic of the multiple application, Figure 2,
illustrates the three functions:
• Backlight four LED in parallel, dimming capability.
• Torch one LED, no output adjustment.
• Flash one power LED, pulse width adjustable.
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2
AND8192/D
GND
J3
10 F/10 V
10R
D5
R8
Q2
MGSF1N03L
GND
LW67C
D3
82R
R5
LW67C
D2
82R
R4
LW67C
D1
82R
R3
LW67C
82R
R7
200R
Q1
U4B
GND
1 F
74HC08
U4A
EN
Vout
U1
NCP5603
EN/PWM
Fsel
Vsel
GND
1.5 k
C6
R11
VCC
100 k
10 k
TRA
TRB
R
Q
CNT/PWM
FLASH
10 k
S1
GND
Figure 2. Multiple LED Driver Application
3
GND
10 k
VCC
VCC
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S6
S5
NL37WZ04
R1A
U2A
P1
100 kA
100 k
GND
R10
10 k
NL37WZ04
10 k
R13
R12
Adjust PWM
10 k
R1C
10 k
R1B
+
VCC
33 nF
GND
U2B
2 x 1.5 V
−
+
+
PK1
S3
Adjust FLASH
S2
VSEL
GND
FSEL
100 nF/6.3 V
VCC
200 kA
R2
GND
P2
C8
2.2 F
TORCH
GND
S7
PWR
CX
C7
100 nF
R1D
GND
R14
CLR
Q
D7
PWM
A
B
C9
C
4.7 F/10 V
Q
GND
RC
C1
VCC
U5A
MC14538B
EN
FSEL
VSEL
GND
RC
1 F/6.3 V
Q
GND
C2
U5B
MC74HC4538
C1P
C1N
C2N
C1P
C3
74HC08
C4
1 F/6.3 V
GND
MMBF0201N
NL37WZ04
LWG6SC
GOLDEN DRAGON 5.1R
R6
D4
Vout
Vbat
Q3
MGSF1N03L
GND
74HC08
TP1
GND
R9
U2C
D6
U4C
C5
GND
VCC
AND8192/D
TOP Layer
BOTTOM Layer
Figure 3. Printer Circuit Board GERBER Files (scale 1:1)
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AND8192/D
switch S1 is flipped to the Vcc position, the RESET of U5A
is released and the EN pin is clocked High / Low by the clock
generated by U2A / U2B. Simultaneously, diode LED D7
turns ON to identify the PWM mode of operation. The duty
cycle of the U5A / Q output is manually adjusted by
potentiometer P1 to set the brightness of the four associated
LED.
The efficiency of the system has been evaluated at room
temperature (see Table 2), the results being fully within the
NCP5603 data sheet specifications.
The system is powered by two AA cells in series,
assembled in a standard battery holder, the operating mode
being selected by the S1, S5 and S6 switches. Since the total
current is limited by the DC/DC converter, the backlights
LEDs are automatically deactivated when either the Torch
or the Flash are selected. Moreover, the Flash is not available
while the Torch is running.
An extra feature, backlight dimming, is provided by
switch S1 is associated with potentiometer P1. When the
switch is connected to ground, the NCP5603 enabling pin
EN is high and the brightness is maximized. When the
Table 2. Demo Board Efficiency
Vbat
Ibat
Vout
Iout/LED
Iout Total
Yield
Comments
3.50 V
2.3 mA
0V
0 mA
0 mA
−
No Load
3.50 V
132 mA
4.42 V
16.5 mA
66 mA
63.14%
3.50 V
170 mA
4.92 V
21.4 mA
85.6 mA
70.78%
3.10 V
131 mA
4.42 V
16.5 mA
66 mA
71.83%
3.10 V
169 mA
4.92 V
21.4 mA
85.6 mA
80.38%
3.10 V
300 mA
4.92 V
142 mA
142 mA
75.12%
Torch operation
The inrush current is internally limited by the chip, as
depicted Figure 4, and no uncontrolled current takes place
when the system starts up from scratch.
Figure 5. Typical Digital Dimming
Although there is no dedicated pin, the LED brightness can
be dimmed by means of the EN digital control. The
waveform captured in Figure 5 illustrate this behavior, the
PWM being intentionally arranged out of the audio band for
a portable system.
Figure 4. Typical Startup Timing
With a startup time well below 1 ms (from zero to full Vout,
see Figure 4), the NCP5603 is fast enough to accommodate a
flash application as shown in the demo board.
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5
AND8192/D
NCP5603 – MULTIPLE DRIVE CIRCUIT − Bill Of Material
QTY
Designator
Description
Footprint
Manufacturer
Part Number
1
R1
10 k, pack four independent elements
SIP−5
BOURNS
4605X series
3
R2, R10, R13
10 k
0805
Vishay Draloric
4
R3, R4, R5, R6
82 0805
Vishay Draloric
1
R7
200 0805
Vishay Draloric
1
R8
1.2 0805
Vishay Draloric
1
R9
10 0805
Vishay Draloric
2
R11, R12
100 k
0805
Vishay Draloric
1
R14
1.5 k
0805
Vishay Draloric
1
P1
100 k A Potentiometer
VR4
BOURNS
3386F−TW
1
P2
200 k A Potentiometer
VR4
BOURNS
3386F−TW
1
C1
4.7 F/10 V
1206
TDK
3
C2, C3, C4
1 F/6.3 V
1206
TDK
1
C5
10 F/10 V
1206
TDK
1
C6
33 nF
0805
KEMET
1
C7
2.2 F
0805
TDK
2
C8, C9
100 nF
0805
KEMET
4
D1, D2, D3, D4
LW67C
OSRAM_LED
OSRAM
1
D5
GOLDEN DRAGON
OSRAM_DRAGON
OSRAM
1
D6
LWG6SC
OSRAM_LWG
OSRAM
1
D7
LED
OSRAM_LED
OSRAM
1
Q1
MMBF0201N
SOT_23A
ON Semiconductor
2
Q2, Q3
MGSF1N03L
SOT_23A
ON Semiconductor
1
U1
NCP5603
QFN10_COB
ON Semiconductor
1
U2
NL37WZ04
US8
ON Semiconductor
1
U4
74HC08
SO−14
ON Semiconductor
1
U5
MC14538B
SO−16
ON Semiconductor
4
S1, S2, S3, S6
Toggle Switch
APEM_CMS
APEM
TL36WS84000
1
S5
Push Button
PUSH_BUT_B
CANNON ITT
KSA 0M210
1
S7
Toggle Switch
CKSWITCH_V
C&K
ET01MD1 CBE
1
TP1
TEST POINT
TEST_POINT
KEYSTONE
5005 (THM)
1
J3
GND
GND_TEST
HARWIN
D3082−01 (tin)
D3082−05 (gold)
1
PK1
2 x 1.5V
Battery holder, 2 x AA
BPACK2
KEYSTONE
2223
LWW5SG
ON Semiconductor and
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