6

FN7371.0

June 28, 2005

required LED current is lower than the rate current, the

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

voltage on the LX pin as shown in Figure 11. It is clear that

as the zener is turned on, due to the overvoltage, the zener

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:

•

• L inductance in H.

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 12 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

4.75

Ω

OFF/ON

LEDs

1µF

LX

2.7V~5.5V

R1

R2

DIMMING SIGNAL

FIGURE 10. ANALOG DIMMING CONTROL APPLICATION

CIRCUIT

–

⋅

()

+

-------

⋅

=

(EQ. 5)

EL7630

C1

L1

22µH

D1

C2

0.22µF

4.75

Ω

2.7V~5.5V

OFF/ON

LEDs

1µF

VDD

LX

ENAB FB

GND

R1

Dz

FIGURE 11. LED DRIVER WITH OVERVOLTAGE

PROTECTION CIRCUIT

⋅

⋅

---------------------------------

=

(EQ. 6)

1

2

---

∆

⋅

+

=

(EQ. 7)

∆

–

()

⋅

⋅⋅

---------------------------------------------------

=

(EQ. 8)

2

⋅

1

6

---

2

∆

⋅

+





⋅

=

(EQ. 9)

EL7630