HV9903 데이터 시트보기 (PDF) - Supertex Inc

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HV9903

White LED Driver

Supertex Inc 

HHVV99990033

fairly low frequency so that the 1ms soft-start interval

does not introduce much error. The RSET input is

better for PWM dimming, as it does not include soft-

start. (See below for PWM dimming techniques.)

Soft Start

SHDN

IIN

50mA/div

ILED

10mA/div

inductor rating. Choosing an inductor with lower

resistance results in more efficient operation.

Inductor Value for 2.7V Operation

100 H

10 H

1H

5V

Select the next lower standard value,

taking inductor tolerance into account.

ILED

5mA

10V

15V

20V

25V

VLED-STRING + (ILED · 5? ) + VD

VLED-STRING + (ILED -5Ω) +VD

10mA

15mA

20mA

25mA

30mA

35mA

40mA

Open LED Protection

Open LED protection is integrated into the HV9903.

Without open LED protection, output voltage would

climb to destructive levels as the driver attempts to

correct for the open LED condition.

Should the voltage at the SW pin exceed 33V, the

driver latches off and the RSET pin is pulled to VDD,

indicating a fault condition. To reset the latch, assert

SHDN low for at least 200ns. When SHDN is again

brought high, the driver will be re-enabled, including

soft-start. If the open LED condition persists, the

HV9903 will again latch off.

Inductor Selection

The HV9903 is designed for discontinuous mode

operation. Control loop stability may be compromised

if the converter is allowed to operate in continuous

mode. To assure discontinuous mode operation, the

inductor must not exceed a certain value depending

on supply voltage, output current, and output voltage.

The following graphs show the maximum permissible

inductor value and inductor current rating for a lithium-

ion battery application (2.7V minimum battery

voltage). When calculating LED string voltage drop,

use maximum LED voltage. If using paralleled LED

strings with current balancing resistors, include the

resistor voltage drop in VLED-STRING. VD is the diode’s

forward voltage drop. Always select the next lower

standard value inductor and be conservative on

Inductor Rating for 2.7V Operation

1A

Exceeds max recommended SW current

100mA

40mA

35mA

30mA

25mA

20mA

15mA

10mA

5mA

ILED

Apply correction factor and select an

inductor with an equal or higher rating.

10mA

5V

10V

15V

20V

25V

VLED-STRING + (ILED · 5? ) + VD

VLED-STRING + (ILED -5Ω) +VD

When selecting the next lower standard value

inductor, the current rating must be adjusted

according to the following equation.

ICORRECTED

IGRAPH

LGRAPH

LSELECTED

As an example, 4 LEDs with 4V max drop are to be

driven at 20mA. LED string drop is 16V plus 0.6V for

the diode plus 0.1V for LED pin voltage, for a total of

16.7 volts. From the graphs, inductor value at 16.7V

and 20mA is 3.7µH and rating is 320mA. The next

5

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