16

NXP Semiconductors

34845

Figure 11. Typical application circuit for single wire control, fS = 300 kHz

(VIN = 8.0V, ILED = 20 mA/channel, 14 LEDs/channel, OVP = 49V, VPWM = 3.3V)

6.2

Components calculation

The following formulas are intended for the calculation of all external components related with the boost converter and network

compensation. To calculate the duty cycle, the internal losses of the MOSFET and diode should be taken into consideration:

The average input current depends directly on the output current when the internal switch is off.

6.2.1

Inductor

For calculating the Inductor, consider the losses of the internal switch and winding resistance of the inductor:

It is important to look for an inductor rated at least for the maximum input current:

MC34845B/D

CH5

CH1

CH2

CH3

33uH

CH4

20

23

18

11

10

9

8

7

3

4

80V, 1A

LED

LEG 2

VIN

LED

LEG 1

CH6

12

LED

LEG 3

SWA

SWB

LED

LEG 4

LED

LEG 5

LED

LEG 6

OVP

14

FAIL

22

1

VIN

2. 2uF

10V

VDC1

2.2uF

10V

VDC2

EN

6

24

VOUT

PGND

2

5

PGND

WAKE

EP

10uf

25V

15

0.1%

21

13

GND

GND

PWM

16

100 pF

100pF

100pF

100 pF

100pF

100 pF

0.1uf

220pF

COMP

56nF

2kΩ

Control

Unit

2uF

10kΩ

114kΩ

680kΩ

2uF 2uF 2uF

765kΩ

Caps should be located

as close as possible to the

MC34845 Device

7.65k

Ω

D

VOUT VD VIN

–

+

VOUT VD VSW

–

+

-----------------------------------------------

=

IIN AVG

–

IOUT

1D

–

-------------

=

L

VIN VSW

–IIN AVG

–

RINDUCTOR

×

()

–

() D

×

IIN AVG

–

r

×

FSW

×

------------------------------------------------------------------------------------------------------------------

=

IIN MAX

–

IIN AVG

–

VIN VOUT VIN

–

()

×

2L

×

FSW

×

VOUT

×

---------------------------------------------------------

+

=