AN4291 Datasheet, PDF (12/53 Pages) STMicroelectronics – STEVAL385LED4CH: four independent high brightness LED

Design concept
AN4291
Where T_x is the time to search current and it is equal to 20% of T_ON. 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 manual.
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 P_WM0 signal and monitors the voltage of
the V_COM0 signal. It also monitors the LED current during the "on-time" by sensing the
voltage present at CPP3 line. The V_LED signal is calculated by the subtraction from V_{LED_PW}
and V_COM0. The V_COM0 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
PWM0
U2
PWM0
GND
PM8834
1
EN1
2
PWM1
3
GND
4
PWM2
EN2
OUT1
VCC
OUT2
8
TP19
7 GATE_PWM0
6
5
VD_10
2 ^D4 1
STPS2H100A
RS-249-612
1 ^L3 2
470uH
GATE_PWM0 R21 10R 1
Q2 ^{1,2A sat}
STN3NF06L
D6
R19
26K1
STPS0560Z
CPP3
CPP3
R23 160R
RS-250-347
C14
1nF
R28 R29
1R8 1R8
GND
R25
26K1
1W 1% 1W 1%
J6
CH0
-
+
C11
100nF
100V
LED_PW
+ C12
1uF
100V
GND
GND
Cut Freq = 723 KHz
GND GND
from 240 mA to 1 A
82 mA step
Q4
1
2N7002
D8 LL4148
GATE_PWM0
C18 R39
10nF 5K1
R37 C16
1K2 ^10nF
VCOM0
VCOM0
LED_PW
GND GND ^{GND GND}
AM16704v1
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
manual, such as the K constant, have to be recalculated.
• The resistors R28 and R29 act as 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 turn-on
current generated by MOSFET Q2. The STLUX385A detects the peak current by
comparing CPP3 with an internal, DAC generated, voltage.
• The R19, R25, Q4 and R37 are used to feed the voltage of the LED chain cathode to
the STLUX385A's ADC. The MOSFET Q4 is used to prevent the small biasing current
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