AN4460 datasheet(12/54 Pages) STMICROELECTRONICS

12 / 54 page

Design concept

AN4460

12/54

DocID026129 Rev 1

Equation 4

Where Tx is the time to search current and it is equal to 20% of TON. The extra time has two

effects: the former, during the dimming operation, allows a quicker response from OFF to

ON transition; the latter prevents a frequency lower than 15 kHz (in the audible spectrum).

On the board the circuit for the transition mode principle is also present but it is not

described in this AN.

3.1

LED channel schematic

To implement the FOT principle and to use the STLUX385A microcontroller, the following

schematic (single LED chain) is used. It represents a standard reverse buck structure. The

STLUX385A applies the PWM frequency via the PWM0 signal and monitors the voltage of

the VCOM0 signal. It also monitors the LED current during the “on-time” by sensing the

voltage present at CPP3 line. The VLED signal is calculated by the subtraction from VLED_PW

and VCOM0. The VCOM0 signal is acquired 100 µs after the PWM dimming starts.

The schematic for a single LED channel is reported in Figure 4:

Figure 4. Schematic - LED channel 0

Explanation:



The LED chain is connected to the J6 connector.



The MOSFET Q2 drives the current from LED_PW to the LED channel and the

inductor L3, charging it. The Inductor L3 is defined for all selectable current ranges.

Should the user change the inductor values, some parameters, described in this AN,

such as the K constant, have to be recalculated.



The resistors R28 and R29 act as a shunt resistor to sense the current through the

LEDs. The current signal is then fed to CPP3 pin through an RC filter represented by

the resistor R23 and the capacitor C14. The filter removes the spike caused by the

TON MAX



TON

TON 0.2





x2

2.4xTON

CPP3

470uH

1,2A sat

PWM0

R29

1R8

1W 1%

from 240 mA to 1 A

C11

100nF

100V

PM8834

EN1

PWM1

GND

PWM2

OUT2

VCC

OUT1

EN2

C16

10nF

GND

STPS0560Z

RS-250-347

CPP3

CH0

GND

GATE_PWM0

GND

Cut Freq = 723 KHz

VCOM0

LED_PW

82 mA step

VD_10

R39

5K1

TP19

GND

STPS2H100A

RS-249-612

PWM0

GND

R28

1R8

1W 1%

GND

LED_PW

GATE_PWM0

C14

1nF

2N7002

R25

26K1

D8 LL4148

GND

GATE_PWM0

+ C12

1uF

100V

R19

26K1

STN3NF06L

GND

VCOM0

R37

1K2

R23

160R

R21

10R

C18

10nF

GND

AM16704v1

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