HV9903 datasheet(6/12 Pages) SUTEX

6 / 12 page

lower standard value is 3.3

µH (10% tolerance) and

the corrected rating is then:

359

µH

320

CORRECTED

Inductor data sheets may rate the inductor in terms of

DC current, RMS current, or saturation current. The

DC or saturation ratings should be used. Confirm that

the inductor is not saturating by observing the SW pin.

When an inductor saturates, current begins to climb

rapidly. This condition is evidenced by a breakpoint in

the SW voltage waveform as indicated in the diagram

below. Normally, the voltage at the SW pin should be

a fairly linear ramp, as the linear rise in inductor

current through the SW resistance produces a linear

voltage ramp. When the inductor saturates, the rapid

rise in current produces a likewise rapid rise in SW

voltage. Test using an HV9903 with a low switching

frequency.

SW Waveform Showing Inductor Saturation

Normal

Saturated

Gnd

VIN

Note: ringing or noise may be present.

Also, confirm that the driver is operating in

discontinuous mode by observing the voltage at the

SW pin while at minimum supply and maximum LED

current.

For worst-case test purposes, select

components within their tolerance range as follows:

Inductor: high value

LED: high voltage drop

HV9903: high switching frequency

Some ringing in the SW waveform will be evident, but

is not a concern as the energy is very low. About 10-

15% idle time should be allowed to assure

discontinuous mode operation.

Idle time is the

interval when there is no inductor current flowing, as

indicated when the SW voltage is at the supply

voltage level (with some ringing).

The following graphs show the SW waveform with

various inductor values and can assist in selecting an

inductor. The top graph shows an inductor value that

is acceptable, however, greater efficiency can be

achieved by increasing inductance. The bottom graph

shows continuous mode operation, which must be

avoided.

SW Waveform with Various Inductor Values

VIN

Inductance too low

Inductance ideal

Inductance too high

(continuous mode)

Gnd

idle

time

Capacitor Selection

Proper selection of CDD and COUT is essential to the

efficient operation of the LED driver. Both CDD and

COUT should be around 1

µF with good high frequency

characteristics (low ESR and ESL).

Ceramic

capacitors are a practical choice for their high

volumetric efficiency and good high frequency

characteristics, but pay attention to the capacitor’s

voltage coefficient. Some small, high value ceramic

capacitors can lose 75% of their capacitance at their

rated voltage! X5R, Y5V, and Z5U formulations are

more susceptible to this effect, as well as possessing

higher temperature coefficients. X7R formulations are

a better choice.

The voltage rating of CDD should be greater than the

maximum supply voltage. To be compatible with the

HV9903’s open LED protection, COUT’s voltage rating

should be 35V or more.

HV9903

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