AL8821SP-13 Просмотр технического описания (PDF) - Diodes Incorporated.

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AL8821SP-13

40V, 2A BOOST LED DRIVER

Diodes Incorporated. 

AL8821

Application Information (Cont.)

K  I L

I L( peak)

RSET and RHYS can be obtained from above equations:

RSET

=













((VCOMP

44 (2

(VLED 

1.5) 0.6 1.4) (2  K

32 (VLED  0.444)  ILED

 K)

0.444) 

ILVEDIN1（11-0K3）, 0V

)  VIN1

（1-K）

 VCOMP 

103

1.5V

1.5V

 VCOMP



5V

RHYS

=

2

(VLED



0.444) ILED  K 

 VIN1  (2  K)

RSET

104

When the value of K, η and VCOMP are provided, the value of resister RSET and RHYS can be calculated according to these above equations. In

order to get appropriate efficiency and Electronic Transformer (ET) compatibility, generally K is set between 0.4 and 0.8. Due to the range of

VCOMP is from 0V to 5V, in order to get output voltage regulation, generally VCOMP is set as 3V at rated input voltage.

Inductor Selection

Because of the using of the hysteretic control scheme, the switching frequency in a boost configuration can be adjusted in accordance to the value

of the inductor being used. The value of the inductor can be determined on the desired switching frequency by using the following equation:

L  [VIN1  (RSET  RL  RDSON )  I L(ave) ][VLED  0.444 VIN1 VF  (RL  RSET )  IL(ave) ]

IL[VLED  0.444 VF  (2RSET  2RL  RDSON )  IL(ave) ] fSW

Where:

L is the coil inductance.

RL is the coil resistance.

RDSON is the switch SW resistance.

VIN1 is the rated input voltage.

VF is the diode forward voltage.

fSW is the desired switching frequency. Generally 500kHz to 800kHz switching frequency is suggested. Low switching frequency can decrease the

switching loss but need to choose higher inductor values that will result in larger size in order to meet the saturation current. For example the

relationship between switching frequency and inductor value is shown as below Table 1 in the same application system. Considering these factors,

500kHz switching frequency is recommend in typical application.

Inductance Value of L @ Vac = 12Vac, VIN = 22V

VLED = 28V, ILED = 180mA

10µH

15µH

22µH

33µH

Table 1

Operation Frequency of SW at Peak Voltage Of Vac

840kHz

800kHz

680kHz

465kHZ

AL8821

Document number: DS37671 Rev. 1 - 2

9 of 14

www.diodes.com

January 2015

© Diodes Incorporated

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