Technical Note

7/12

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2010.02 - Rev.A

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BD1204GWL

b) LED current level

The interface records rising edges of the EN pin and decodes them into 16 different indicated in following table.

Data

Output current [mA]

Data

Output current [mA]

1

20.0

9

5.0

2

17.0

10

4.0

3

14.0

11

3.0

4

12.0

12

2.0

5

10.0

13

1.0

6

8.5

14

0.5

7

7.0

15

0.25

8

6.0

16

0.125

(RISET=120 kΩ)

Moreover, LED current can be set up with the resistance RISET connected to ISET terminal,

and the maximum current is decided by the following formula.

(2) Charge pump

a) Description of operations

Pin voltage comparison takes place at Vout control section, and then Vout generation takes place so that the LED

cathode voltage with the highest Vf is set to 0.15V. A boost rate is changed automatically to a proper one at the

Charge Pump Mode Control section so that operation can take place at possible low boost rate. In addition, when the

VOUT output is short-circuited to GND, the leak current is suppressed via the overcurrent protection function.

b) Soft start function

BD1204GWL have a soft start function that prevents the rush current.

Soft Start

Ordinal mode

EN/LED*

VOUT

* EN/LED is an internal enable

Fig.7 Soft Start

c) Automatic boost rate change

The boost rate automatically switches to the best mode.

* (×1 mode  ×1.5 mode) or (×1.5 mode  ×2 mode)

If a battery voltage drop occurs BD1204GWL cannot maintain the LED constant current, and then mode transition

begins.

* (×1.5 mode  ×1 mode) or (×2 mode  ×1.5 mode)

If a battery voltage rise occurs, VOUT and VBAT detection are activated, and then mode transition begins.

(3) UVLO (Under Voltage Lock Out)

If the input voltage falls below 2.2V(Typ.), BD1204GWL is shut down to prevent malfunction due to ultra-low voltage.

(4) OVP (Over Voltage Protection)

This circuit protects this IC against damage when the C/P output voltage (VOUT) rises extremely for some external

factors.

(5) Thermal shutdown (TSD)

To protect this IC against thermal damage or heat-driven uncontrolled operations, this circuit turns off the output if the

chip temperature rises over 175ºC. In addition, it turns on the output if the temperature returns to the normal

temperature. Because the built-in thermal protection circuit is intended to protect the IC itself, the thermal shutdown

detection temperature must be set to below 175ºC in thermal design.