TENTATIVE
LC41059
Current Regulated White LED Supply with Brightness Control
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General Description
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The LC41059 is a fractional charge pump DC-DC converter with a regulator that provides four regulated
current sources. It accepts an input voltage range from 2.7V to 5.5V and maintains a constant current
determined by an external sense resistor.
The LC41059 delivers up to 105mA of load current to drive one, two, three, or four White LEDs. When it
drives four LEDs, maximum 26mA of each LED is available. When it drives three LEDs, maximum 35mA
of each LED is available. Current of each LEDs are highly matched regardless of the input voltage range
from 2.7V to 5.5V, or the LED forward voltage range from 3V to 4V. It operates with 500kHz
fixed-frequency switching without inductors, therefore the EMI noise is very limited, the pictures or tones
generated in the device will not disturbed.
LED brightness can be controlled by both analog voltage input and pulse input. A voltage between 0V to
1.25V may be applied to the BRGT pin to vary the LED current linearly. Alternatively, a pulse signal can
be applied to the SD pin to vary the perceived LED brightness. An active-low input to the SD pin will
shutdown all the circuit, and the LED will turned off.
The LC41059 is available on WL-CSP, QFN or other packages.
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Features
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Low noise, high efficiency CMOS charge pump
Built-in current mirror with excellent accuracy
Built-in constant current circuit
2.7V to 5.5V input voltage
Drives one, two, three or four white LEDs with maximum total current 105mA
Self-changing of charge pump mode (Pass mode, 3/2 boost mode, 2 boost mode)
+-0.5% current matching of any two LED outputs
Soft start limits inrush current
Charge pump frequency 500kHz
Brightness control with Analog voltage input or Pulse voltage input
Maximum 1uA shutdown current
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Applications
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White LED display backlights
White LED keypad backlights
1-Cell Li-Ion battery-operated equipment including PDAs, hand-held PCs, cellular phones
Flat panel displays
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Package
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WL-CSP, QFN or other packages are available
1
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Block Diagrams
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TENTATIVE
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Pin Assignment
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Pin name
VIN
C1A
C1B
C2A
C2B
SD
Function
Battery Voltage In
Flying Capacitor (1) +
Flying Capacitor (1) -
Flying Capacitor (2) +
Flying Capacitor (2) -
Shutdown Signal Input
PWM Input
BRGT
RSET
GND
VREF
Analog Brightness Control
External Resistor
Ground
VREF Output
White LED Input (1)
White LED Input (2)
White LED Input (3)
White LED Input (4)
Charge Pump Output
SD:
A low level input will disable the device, and turn off LEDs. A high level
input (=V
IN
) will enable the device, and turn on LEDs. Pulse input on
the SD pin makes LEDs flickering in synchronization with the pulse, so
the brightness can be controlled by the duty cycle.
LED_IN1
LED_IN2
LED_IN3
LED_IN4
VOUT
BRGT: Analog brightness control input. It accepts an input voltage range from
0V to 1.25V. Rset resistance and BRGT input voltage determine the
LED current.
RSET: Connect a resistor. Rset resistance and BRGT input voltage determine the LED current.
VREF: 1.2V of internally generated VREF is available. VREF can be used for a BRGT input voltage.
(See “Operation” chapter for more details)
2
TENTATIVE
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Electrical Characteristics
Symbol
VIN
IOUT
VIN = 2.7 to 5.5V
Conditions
Min
2.7
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Typ
3.6
60~80
(20mA x4LEDs)
(20mA x3LEDs)
Max
5.5
105
(26mA x4LEDs)
(35mA x3LEDs)
Hz
V
V
V
Unit
V
mA
(Unless otherwise specified, VIN=3.6V, IOUT=80mA, Ta=27C)
Parameter
Input Voltage
Total Output Current
Charge Pump Frequency
Input
Charge
Pump
FCLK
VCPH
VCPL
Pass mode to 3/2 boost mode
3/2 boost to 2 boost mode
500k
4.6
3.5
0.1
Mode Threshold
Input
Charge
Pump
VCPY
Mode Hysterysis
LED Current Accuracy
(LED Current variation
caused
degradation
variation)
by
or
VIN
VF
VIN = 2.7 to 5.5V
VF = 3.0 to 4.0V
Total Output Current = 80mA
VIN = 3.0 to 5.5V
VF = 3.0 to 4.0V
Total Output Current = 105mA
VIN = 2.7 to 3.0V
VF = 3.0 to 4.0V
Total Output Current = 105mA
LED Current Matching
between
outputs
any
two
ID-MATCH
VIN = 2.7 to 5.5V
VF = 3.0 to 4.0V
Total Output Current = 80mA
VIN = 3.0 to 5.5V
VF = 3.0 to 4.0V
Total Output Current = 105mA
VIN = 2.7 to 3.0V
VF = 3.0 to 4.0V
Total Output Current = 105mA
Shutdown
Current
LED Current Response
against the SD signal
input
TSDH
TSDL
Supply
ISD
VIN = 2.7 to 5.5V
SD = 0V
Shutdown Response
VIN = 2.7 to 5.5V
SD = Low input
Start-up Response
VIN = 2.7 to 5.5V
SD = High input
Recommended SD input
Frequency (PWM)
BRGT input
BRGT
FPWM
PWM Input to SD Pin to
Control Brightness
Analog Input to BRGT Pin to
Control Brightness
0
1.0
1.25
V
100
2000
Hz
4.0
usec
0.1
usec
1
uA
+-3
%
+0.5
%
+-0.5
%
+-5
%
+-3
%
+-3
%
3
TENTATIVE
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Operation
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1. Circuit Description
The LC41059 is a white LED supply which accepts the input voltage from the Li-Ion battery. While the
Li-Ion battery has the output voltage range from 2.7 to 5.5V, the forward voltage (V
F
) of the white LED is
about 3.6V typically. So when the battery degrades the output voltage to about 3.6V, the higher voltage
must be generated internally. The LC41059 employs a charge pump to step up the output voltage to 1.5
times or 2 times the input voltage. This charge pump selects the optimum boost mode, and changes the
boost mode automatically.
LED current (I
LED
) is determined by the BRGT input voltage and the resistance of the external resistor.
Same voltage of BRGT (V
BRGT
) is forced on the external resistor with high accuracy by an internal Op-Amp,
so the current passing through the external resistor (I
RSET
) will be I
RSET
= V
BRGT
/ R
SET
. LC41059 includes
the current regulator to deliver the regulated current to the LEDs, which are composed of current mirrors
with a 40 to 1 ratio. For example, when V
BRGT
= 1.2V, R
SET
= 2000ohms, then I
RSET
=0.6mA, and I
LED
=
I
RSET
x 40= 24mA . If four LEDs are attached, the device drive up a total of 24mA x 4= 96mA through the
LEDs.
The LED brightness can be controlled by both analog and/or digital methods. The digital technique uses a PWM
(Pulse Width Modulation) signal applied to the SD pin (see
4.Brightness Control Using PWM).
The analog
technique applies an analog voltage to the BRGT pin (see
5.BRGT Pin).
2. Soft Start
LC41059 includes a soft start function to reduce the inrush current. Inrush current will be generated when
the charge pump operates with flying capacitors discharged. When the capacitors are being charged with
low impedance, excess current may surge into the capacitors from the battery. Soft start is done to reduce
stress on the battery and external components. During soft start, the switch resistances limit the inrush
current used to charge the flying and hold capacitors.
3. Shutdown Mode
A shutdown pin is available to disable the LC41059 and reduce the quiescent current to 1µA maximum.
During normal operation mode, applying a high level input to SD pin will enable the device. Pulling a low
level input to SD pin will disable the device.
4. Brightness Control Using PWM
Brightness control can be implemented by pulsing a signal at the SD pin. The recommended frequency is
between 100Hz to 2kHz. If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs.
Likewise, if frequency is much higher, brightness in the LEDs will not be linear. Increasing and decreasing
the duty cycle of the PWM signal control the brightness. Zero duty cycle will turn off the LED, and a 50%
duty cycle waveform produces an average current of 10mA if R
SET
is set to produce a maximum LED
current of 20mA. So the LED current varies linearly with the duty cycle.
5. BRGT Pin
The BRGT pin can be used to smoothly vary the brightness of the White LEDs. In the LC41059, this
voltage is fed to the Op-Amp that controls the current through the mirror resistor R
SET
. The nominal range
on BRGT is 0V to 1.25V. Care must be taken to prevent voltages on BRGT that cause LED current to
exceed a total of 100 mA. Although this will not cause damage to the IC, it will not meet the guaranteed
specifications listed in the “Electrical Characteristics” chapter.
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