6
February 22, 2006
For a required LED current I
LED
and chosen values of R
1
and R
2
, the dimming DC voltage V
Dim
can be expressed as:
It is clear that as the required LED current I
LED
is closed to
the rate current V
FB
/R
SET
, V
Dim
is closed to V
FB
. As the
required LED current is lower than the rate current, the
dimming DC voltage V
Dim
is increased in R
2
/R
1
factor.
Open-Voltage Protection
In some applications, it is possible that the output is
opened, e.g. when the LEDs are disconnected from the
circuit or the LEDs fail. In this case the feedback voltage
will be zero. The EL7630 will then switch to a high duty
cycle resulting in a high output voltage, which may cause
the LX pin voltage to exceed its maximum 27V rating. To
implement overvoltage protection, a zener diode Dz and a
resistor R
1
can be used at the output and FB pin to limit the
voltage on the LX pin as shown in Figure 10. It is clear that
as the zener is turned on, due to the overvoltage, the zener
diode’s current will set up a voltage on R
1
and R
SET
and this
voltage is applied on FB pin as the feedback node. This
feedback will prevent the output from reaching the
overvoltage condition. In the overvoltage protection circuit
design, the zener voltage should be larger than the
maximum forward voltage of the LED string.
Components Selection
The input capacitance is normally 0.22µF~4.7µF and the
output capacitor is 0.22µF~1µF. X5R or X7R type of ceramic
capacitor with the correct voltage rating is recommended.
The output capacitor value will affect PWM dimming
performance. For lower output capacitor values, the range of
PWM dimming is wider than for higher values of output
capacitor.
When choosing an inductor, make sure the inductor can
handle the average and peak currents given by the following
formulas (80% efficiency assumed):
Where:
• I
L
is the peak-to-peak inductor current ripple in Ampere
• L inductance in H.
•f
OSC
switching frequency, typically 1.2MHz
The boost inductor can be chosen in a wide range of
inductance (10µH~82µH). For 10µH inductor value, the
boost inductor current will be in discontinuous mode. As the
inductor value decreases further, the ripple of the boost
inductor current is increased and can even trigger
overcurrent protection. For high boost inductor value, the
boost inductor current will be in continuous mode. For
general boost converter, as the converter operates in
continuous mode, there is right half plane zero (RHPZ). If
RHPZ frequency is less than or close to the control loop
crossover frequency, there is a stability issue. In EL7630, the
compensation network is well designed and there is no
RHPZ stability issue even if the inductor value is over 82µH.
For the same series of inductors, a lower inductance has
lower DC resistance (DCR), which causes less conducting
loss, but higher peak to peak current variation, which
generates more RMS current loss. Figure 11 shows the
efficiency of the demo board with different LED load for a
specific series of inductor.
The diode used should be a schottky type with minimum
reverse voltage of 28V. The diode’s peak current is the same
as the inductor’s peak current. The schottky RMS current is:
EL7630
C1
VDD
LX
ENAB FB
GND
L1
22µH
D1
C2
0.22µF
R
SET
4.75
OFF/ON
LEDs
1µF
LX
V
IN
2.7V~5.5V
R1
R2
DIMMING SIGNAL
FIGURE 9. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
V
Dim
V
FB
V
FB
I
LED
– R
SET
+
R
2
R
1
-------
=
(EQ. 5)
EL7630
C1
L1
22µH
D1
C2
0.22µF
R
SET
4.75
2.7V~5.5V
OFF/ON
LEDs
1µF
VDD
LX
ENAB FB
GND
V
IN
R1
Dz
FIGURE 10. LED DRIVER WITH OVERVOLTAGE
PROTECTION CIRCUIT
I
LAVG
I
LED
V
OUT
0.8 V
IN
---------------------------------
=
(EQ. 6)
I
LPK
I
LAVG
1
2
---
I
L
+=
(EQ. 7)
I
L
V
IN
V
OUT
V
IN
–
LV
OUT
f
OSC
---------------------------------------------------
=
(EQ. 8)
I
RMS
D2I
LAVG
2
1
6
---
I
L
2
+
=
(EQ. 9)
EL7630
