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EL1848
0.8 × V IN
I LPK = I LAVG + --- × ? I L
? I L = ---------------------------------------------
Detailed Description
The EL1848 is a constant current boost regulator specially
designed for driving white LEDs. It can drive up to 3 LEDs in
series or 9 LEDs in parallel/series configuration and
achieves efficiency up to 91%.
The brightness of the LEDs is adjusted through a voltage
level on the CNTL pin. When the level falls below 0.1V, the
chip goes into shut-down mode and consumes less than
1μA of current for V IN less than 5.5V.
Steady-State Operation
EL1848 is operated in constant frequency PWM. The
switching is around 1MHz. Depending on the input voltage,
the inductance, the type of LEDs driven, and the LED’s
current, the converter operates at either continuous
conduction mode or discontinuous conduction mode (see
waveforms). Both are normal.
Brightness Control
LED’s current is controlled by the voltage level on CNTL pin
(V CNTL ). This voltage can be either a DC or a PWM signal
with frequency less than 200Hz (for C 3 =4700pF). When a
higher frequency PWM is used, an RC filter is recommended
before the CNTL pin (see Figure 17).
hiccough continues until LED is applied or converter is shut
down.
When designing the converter, caution should be taken to
ensure the highest operating LED voltage does not exceed
13V, the minimum shut-down voltage. There is no external
component required for this function.
Component Selection
The input and output capacitors are not very important for
the converter to operate normally. The input capacitance
is normally 0.22μF - 4.7μF and output capacitance
0.22μF - 1μF. Higher capacitance is allowed to reduce the
voltage/current ripple, but at added cost. Use X5R or X7R
type (for its good temperature characteristics) of ceramic
capacitors with correct voltage rating and maximum height.
When choosing an inductor, make sure the inductor can
handle the average and peak currents giving by following
formulas (80% efficiency assumed):
I O × V O
I LAVG = ------------------------
1
2
V IN × ( V O – V IN )
L × V O × F S
where:
PWM
SIGNAL
100K
0.1μF
CNTL
COMP
? ? I L is the peak-to-peak inductor current ripple in Ampere
? L inductance in μH
? FS switching frequency, typical 1MHz
13.33 × R 1
FIGURE 19. PWM BRIGHTNESS CONTROL
The relationship between the LED current and CNTL voltage
level is as follows:
V CNTL
I LED = ----------------------------
When R 1 is 5 ? , 1V of V CNTL conveniently sets I LED to
15mA. The range of V CNTL is 250mV to 5.5V.
Shut-Down
When V CNTL is less than 100mV, the converter is in shut-
down mode. The max current consumed by the chip is less
than 1μA for V IN less than 5.5V.
Over-Voltage Protection
When an LED string is disconnected from the output, V O will
continue to rise because of no current feedback. When V O
reaches 14V (nominal), the chip will shut down. The output
voltage will drop. When V O drops below 11V (nominal), the
chip will boost output voltage again until it reaches 14V. This
9
A wide range of inductance (6.8μH - 68μH) can be used for
the converter to function correctly. For the same series of
inductors, the lower inductance has lower DC resistance
(DCR), which has less conducting loss. But the ripple current
is bigger, which generates more RMS current loss. Figure 9
shows the efficiency of the demo board under different
inductance for a specific series of inductor. For optimal
efficiency in an application, it is a good exercise to check
several adjacent inductance values of your preferred series
of inductors.
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