STMicroelectronics STLDC08 Demo Board STEVAL-ILL039V1/2 STEVAL-ILL039V1 STEVAL-ILL039V1 Hoja De Datos
Los códigos de productos
STEVAL-ILL039V1
Application information
STLDC08
16/29
Doc ID 18476 Rev 1
8 Application
information
8.1
LED current programming
The LED current is set by an external resistor connected between the FB pin and GND. The
following equation can be used to calculate the value of the R
following equation can be used to calculate the value of the R
FB
resistor which guarantees
the desired output current:
Equation 1
The feedback signal V
FB
is compared with the internal precision 100 mV voltage reference
by the error amplifier. The internal reference has a guaranteed tolerance of 10 %. Tolerance
of the sense resistor adds additional error to the output voltage. 1 % resistors are
recommended.
of the sense resistor adds additional error to the output voltage. 1 % resistors are
recommended.
8.2 Duty
cycle
The controlled off-time architecture is a hysteretic mode control. Hysteretic operation
eliminates the need for small signal control loop compensation. When the converter runs in
continuous conduction mode (CCM) the controller adapts the T
eliminates the need for small signal control loop compensation. When the converter runs in
continuous conduction mode (CCM) the controller adapts the T
ON
time in order to obtain the
duty cycle given by the following relationship:
Equation 2
where V
O
is the output voltage given by:
Equation 3
and VD is the forward voltage of the Schottky diode.
8.3 Inductor
selection
As the hysteretic control scheme is inherently stable, the inductor value does not affect the
stability of the regulator. The switching frequency, peak inductor current, and allowable
ripple of the output current determine the value of the inductor.
stability of the regulator. The switching frequency, peak inductor current, and allowable
ripple of the output current determine the value of the inductor.
LED manufacturers generally recommend a value for LED current ripple ranging from 5 % to
20 % of LED average current.
20 % of LED average current.
LED
FB
I
1
.
0
R
=
D
OUT
IN
V
V
V
1
D
+
−
=
FB
)
LED
(
F
O
V
V
n
V
+
×
=