el7630 Intersil Corporation, el7630 Datasheet - Page 5

• Manufacturer Part Number: el7630
• Description: White Led Boost Regulator
• Manufacturer: Intersil Corporation

Detailed Description
EL7630 uses a constant frequency, current mode control
scheme to provide excellent line and load regulation. It
can drive up to 6 LEDs in series or 15 LEDs in
parallel/series configuration, with efficiencies of up 86%.
EL7630 operates from an input voltage of 2.7V to 5.5V and
can boost up to 27V.
Steady-State Operation
EL7630 operates with constant frequency PWM. The
switching frequency is around 1.2MHz. Depending on the
input voltage, inductance, number of LEDs and the LED
current, the converter operates in either continuous
conduction mode or discontinuous conduction mode. Both
are normal. The forward current of the LED is set using the
R
the equation:
Shut-Down
The ENAB pin, when taken low places EL7630 into power
down mode. When in power down, the supply current
reduced to less than 1µA.
Dimming Control
The ENAB pin also doubles as a brightness control. There
are two different types of dimming control methods. The first
dimming control is controlled through the duty-cycle of the
ENAB input PWM waveform, which can operate at
frequencies of 400Hz to 1kHz. The LEDs operate at either
zero or full current. This is called PWM dimming control
method. The relationship between the average LED current
and the duty-cycle (D) of the ENAB pin’s waveform is as
follows:
The magnitude of the PWM signal should be higher than the
minimum ENAB voltage high. The bench PWM dimming test
results are shown in Figure 8. In the test, two PWM
frequencies 400Hz and 1kHz are chosen to compare the
linear dimming range. It is clear that for lower PWM
frequency, the linear dimming range is wider than one for
higher PWM frequency. In the PWM dimming test, the output
capacitor is 0.22µF.
average I
I
LED
SET
=
resistor. In steady state mode, this current is given by
-------------- -
R
V
SET
LED
FB
=
-------------- - D
R
V
SET
FB
⋅
5
(EQ. 1)
(EQ. 2)
EL7630
The second dimming control is to apply a variable DC
voltage to adjust the LED current. This is called analog
dimming control. The dimming control using a DC voltage is
shown in Figure 9. As the DC dimming signal voltage
increases, the voltages drop on R
the voltage drop on R
decreases. The DC dimming signal voltage can be a variable
DC voltage or a DC voltage generated from a PWM control
signal. For some application areas, the PWM control signal
is a high frequency signal. To make dimming controllable
with these high frequency PWM signals, the high frequency
components of the PWM control signal should be filtered to
get the equivalent DC voltage. The equivalent DC voltage is
then used as the variable DC voltage for dimming LED
current.
where F is the brightness with respect to the undimmed
value.
V
I
LED
Dim
FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND
=
=
-------------- -
R
R
------ - V
R
V
SET
2
1
FB
⋅
25
20
15
10
FB
1kHz PWM FREQUENCIES CONDITION,
C
⋅
5
0
R
-------------------- -
OUT
0 10 20 30 40 50 60 70 80 90 10
1
⋅
R



+
1
2
= 0.22µF)
R
+
2
SET
R
------ - F
R
–
1
2
DUTY-CYCLE (%)
-------------------------- -
R
V
–
1kHz
Dim
SET
decreases. Thus, the LED current



⋅
⋅
R
R
1
400Hz
2
1
and R
2
increases and
February 22, 2006
(EQ. 3)
FN7371.1
(EQ. 4)