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PREMA SEMICONDUCTORS
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1. 0.9v Boost driver for white LEDs PR4401/PR4402 :
Specifications:
Single cell step-up converters for white LEDs operatingfrom a supply voltage of less than 0.9V. Only one external inductor is needed for operation of a whiteLED
Features
• minimum start up voltage 0.9V• 200/250mA peak output current
• only one external component required• battery deep discharge protection
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Applications:
• Small-sized LED torches• LCD backlighting• LED indicator lights
Application Notes
Selection of PR4401 and PR4402The circuit type should be selected according to the required LED current.
PR4401 is best operated with inductors between 10 and 22μH.
PR4402 is best operated with inductors between 4.7 and 10μH.
Using lower inductances may lead to erratic behaviour, especially at low supply voltages and
should be avoided. Operating with higher inductances is possible and will lead to lower
supply and LED current. However the quiescent current which is independent of the
inductance will lead to a lower overall efficiency. Since PR4402 has about twice the
quiescent current of PR4401, it is not recommended for small LED currents.
LED compatibility It must be considered that the peak current through the LED is a factor of up to 7 higher than
the mean current. LED lifetime may be affected if operated outside the range
specified by the LED vendor.
Since the emission spectrum of white LEDs usually depends on the current, the light colour
may shift to bluish white. High peak currents may also saturate the LED and reduce the light
efficiency of the LED.
If the rated LED peak current is exceeded, it is recommended to use a smoothing
capacitor and diode to provide a continuous output current (see below). With most
standard LEDs, this will improve the overall performance with inductors of 10μH and less, or
mean output currents of 23mA and more.
Note that with especially with the higher currents of PR4402 the current rating of most
standard LEDs is exceeded, and more powerful or multiple LEDs must be used.
Inductor compatibility
While the series resistance of the coil has a small impact on the LED current, it is
important that the saturation current is higher than the maximum peak current over the supply
voltage range. Inductors optimized for DC-DC converters are mostly suitable.
Connection from Battery
Due to high peak currents, it is important to connect the PR4401/PR4402 to the battery withshort, low resistance wires, to achieve the best performance. A voltage drop along the wire
affects LED current, efficiency and minimum startup and operating voltage.
This is most critical for applications with low inductivity and high current.
In cases where a longer wire from the battery cannot be avoided, a capacitor should be placed
close to the Vcc and Gnd pin of PR4401. Typically capacitors between 220nF and 1μF are
used.
With PR4402 operated at higher currents, a blocking capacitor is usually necessary even if
wires or board layout are optimized.
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Using Different Battery TypesThe input voltage must be 1.90V at maximum. At higher voltages the circuit may not trigger
and start up correctly. Therefore operation with one Alkaline, NiCd, or NiMH cell (AA or
AAA type) is recommended.
Alkaline button cells can also be used for supply. However, since high peak currents are
drawn from the battery, button cells are recommended only with inductors of 22μH or more,depending on the battery type. A capacitor at the supply pins of PR4401 may also improve
performance with button cells.
Lithium batteries are not suitable due to their higher voltage.
Connecting several LEDs in parallelWhen several LEDs are connected in parallel, it is necessary to match the forward voltage of
these LEDs, to achieve a uniform brightness. The total current of all LEDs together
corresponds approximately to the mean output current for operation with one LED.
Operation of LED with smoothed current (rectifier)With a diode (preferably a Schottky diode) and a smoothing capacitor the voltage at the LED
can be buffered if necessary. The capacitance must be small enough so that the voltage at the
capacitor will exceed a voltage of 2.5V in the first cycle, otherwise the circuit may not start
u p. In most cases, values between 100nF and 1μF are appropriate.
Due to different load characteristics, output current and efficiency are typically higher than
without capacitor, especially in the high-current range.
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Recommended configuration for higher LEDs currents
While for lower LED currents it is possible to operate PR4401 with the minimum number of
components, it is recommended to provide both rectifier circuit a the output and buffer capacitor at the input at high LED currents, to achieve the best performance.
Whether the extra components are necessary or not depends largely on the performance of the
components used, most importantly the peak current of the LED, the internal resistance of the
battery and the resistance of the battery cables. If by adding the extra components the mean
LED current increases significantly, it is usually advisable to add them permanently to
achieve a high efficiency.
As a rule of thumb, with a 22μH inductor the extra components will usually not increase thecurrent significantly, while for inductors below 10μH they will usually improve the
performance noticeably. Buffer capacitor and rectifier circuit are independent measures. For
powerful LEDs, or two or more LEDs in parallel at the output, the rectifier may not be
necessary, but the buffer capacitor will still prevent high voltage drops along the supply wire.
For the buffer capacitor, values between 220nF and 1μF are common.
Connecting two LEDs in series
It is possible to operate PR4401 and PR4402 with two LEDs in series at the output.
However, while the peak output current is nearly independent of the output load, the mean
output current with two LEDs in series is reduced to half the current with one LED. In
addition, at high output voltages the efficiency drops significantly, depending on operating
conditions, and current pulses become shorter and sharper.
Therefore, although possible and useful in some cases, this operation mode is not
recommended for general applications and not specified further.
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Using red, green or yellow LEDsAlthough PR4401/PR4402 is optimized for operation with white or blue LEDs, it will usually
also work with red, green or yellow LEDs, with the following restrictions:
a) The LED must build up a sufficient forward voltage to trigger PR4401/PR4402. Due to the
internal resistance of the LED, this condition is usually met. However, no guarantee can beassumed for proper operation under all conditions, and you need to qualify the system
yourself
b) Due to the different forward voltage level and internal resistance of colored LEDs, the
timing is different, and mean currents are mostly lower than for white LEDs. Also other
parameters may deviate from this data sheet.
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2. Low Voltage Boost Driver PR4404 for 0.5W / 1W Power LEDs. –
PR4404
PR4404 is a step-up converter for white LEDs, operating with single battery cell
supply (1.2/1.5V) at up to 150mA LED current or dual cell supply (2.4/3.0V) at up to
300mA LED current.A minimum part count allows compact and cost-efficient solutions.
The converter can be switched on and off with a logic signal, which is useful e.g. for PWM
control, timer circuits etc.
Features• minimum start up voltage 1.0V
• supply by one or two battery cells
• low number of external components
• battery deep discharge protection
Applications• LED torches
• LCD panel backlighting
• home lighting
• toys
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Typical CharacteristicsThis datasheet presents characteristics for different typical applications:
A. Supply voltage 1...2V - target current 100mA - one LED
B. Supply voltage 1...2V - target current 150mA - one LED
C. Supply voltage 1.5...3V - target current 300mA - one LED
D. Supply voltage 2...5V - target current 200mA - two LEDs in seriesA and B are typical single battery-cell applications, C for two, and D for three cells.
The behaviour at many other operating conditions can be estimated by interpolation or
extrapolation.
Note that in the following diagrams the peak current through the output switch exceeds the
maximum rating.
Also the power dissipation can cause a junction temperature higher than the maximum
rating under some operating conditions.
These data should give an impression of the performance over the whole set of
parameters, but the fact that they are shown here should not be regarded as an approval
for operation under these conditions
Application Notes
Current sense resistor RsRs sets the peak output current, which is defined by I peak =VFB/Rs.
Inductor L1
The best inductance depends mainly on the ratio between input and output voltage.
A high inductance results in a low frequency, limiting the transfered power at low supply
voltages.A low inductance causes a shorter charging time, allowing a higher power transfer, but
resulting in a lower efficiency and eventually in an insufficient current regulation at higher
supply voltages.
So a proper balance between behaviour at the low end and high end of the supply voltage
range in the respective application should be found.
See diagrams and application examples for some recommendations.
The saturation current of the inductor must be at least equal to the peak current.
The peak current is approximately two times the average input current under worst-case
conditions, which mostly occur at low supply voltages.
Schottky diode D1D1 must be capable driving the peak current, as for the inductor.
A low forward voltage at this current and a fast recovery provide a high efficiency.
Recommended diode types are MBRS140 or 1N5817.
LEDAny type of LED can be used, as far as specified for the current.
However, the output voltage Vout must be higher than the input voltage Vcc; otherwise the
current regulation will not work properly. Therefore it is usually possible to drive one white
light LED with VF=3.2V from two alkaline batteries with a nominal voltage of 3.0V, but not
from three NiMH cells with 3.6V.
The voltage at pin SW is internally clamped by a zener diode (for clamping voltage see
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electrical specification). Therefore operation without output load does not damage the
chip. Nevertheless, prolonged operation without load should be avoided.
It is possible to connect two or more LEDs in series at the output. With increasing ratio of
Vout/Vcc the maximum output current decreases.
Mind that the built-in zener diode at SW limits the achievable output voltage and therefore
the number of LEDs that can be connected.
Capacitors C1, C2C1 is a reservoir capacitor to stabilise the input voltage and prevent regulator oscillations.
It usually consists of a ceramic capacitor C1a in parallel to a tantalum capacitor C1b.
It is important to connect a ceramic capacitor of approx. 1μF between Vcc and Gnd very
close to the IC.
The size of the tantalum or electrolytic capacitor depends on the application. In some
cases it may be even dispensable, but mostly a capacitor of up to 100μF will improve the
performance, as it smoothes out the peak current drain from the power source and allows
a battery to be used to a deeper discharge level.
Also C2 consists of a ceramic capacitor C2a and an electrolytic reservoir capacitor C2b atthe output, responsible for smoothing the pulsating output current through diode D1. Its
value depends on the current level and the allowed ripple height. Find some
recommended values in the application example circuits.
PCBWhen designing the PCB layout, keep in mind that currents of up to 2A are involved. All
power lines, especially from the battery, inductor and SW input pin, must be sufficiently
wide to keep the voltage drop as low as possible.
The ceramic capacitor C1b must be placed close between Vcc and Gnd of the IC.
A large ground plane is helpful for good performance and low EMI.
Output voltages up to 16V are possible. As the output power of the circuit is limited, the
maximum LED current is lower by a factor roughly equal to the number of LEDs compared
with single LED applications.
Recommended dimensions: see power flashlight, except R s changed to VFB/ILED.
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PWM controlled LED
The Hold input allows to switch on and off the LED with a digital circuit, such as a
microcontroller or a timer circuit.
This can be used e.g. for a PWM brightness control, for blinker circuits and many other
purposes. Mind that no voltage must be applied at the Hold pin, but it must only be pulled
down to Gnd. Only an open collector or open drain output must be connected to Hold!
Due to the remaining quiescent current this feature is not recommended for soft-switching.