Selection of external components

AN2827

6/30

Doc ID 15047 Rev 1

resistor to the NTC; this creates a voltage divider supplied by the external reference voltage

connected to the NTC.

If the temperature of the NTC thermistor rises as a result of the heat dissipated by the LED,

the voltage on the NTC pin increases. When this voltage exceeds 0.56 V, the NTC_W bit in

the STATUS register is set to High, and the ATN pin is set to Low to inform the

microcontroller that the LED is becoming hot. The NTC_W bit is cleared by reading the

STATUS register.

If the voltage on the NTC pin rises further and exceeds 1.2 V, the NTC_H bit in the STATUS

register is set to High, and the ATN pin is set to Low to inform the microcontroller that the

LED is too hot, and the device switches automatically to READY mode to avoid damaging

the LED. This status is latched until the microcontroller reads the STATUS register. Reading

the STATUS register clears the NTC_H bit.

by the LED and all the components surrounding the NTC thermistor, and on the cooling

demonstration board presented in this application note. A real-life application may require a

different type of NTC thermistor to achieve optimal thermal protection.

The procedure to activate the NTC feature is described in Section 5.2: SHUTDOWN mode

with NTC feature activated.

Table 1.

List of components

Component

Manufacturer

Part number

Value

Size

TDK

C1608X5R0J106M

10 µF

0603

TDK

C1608X5R1A105M

1 µF

0603

LTDK

1 µH

3.7 x 3.5 x 1.2 mm

1 µH

2.5 x 2 x 1.2 mm

NTC

Murata

NCP21WF104J03RA

100 k

Ω

0805

Rohm

MCR01MZPJ15K

15 k

Ω

0402

LED

Luxeon LED

2x LXCL-PWF1

0805

AUXLED

Rohm

SML-210VT

0805

1.

Used in v2 version to achieve best efficiency.

2.

Used in v3 version, when low application area down to 23.9 mm² is preferred.