LED7708 STMicroelectronics, LED7708 Datasheet - Page 24

• Manufacturer Part Number: LED7708
• Description: LED Lighting Drivers 16 Ch 85mA LED 3.6 to 36V 250kHz
• Manufacturer: STMicroelectronics

Specifications of LED7708

• Rohs: yes

Recommended operating conditions
8.3.2
24/53
in order to guarantee enough steering room for the optimization. Because of the significant
dependence of the LED forward voltage with temperature, this must be taken into account
when calculating the minimum expected output voltage.
The best way to proceed when designing the external components is the following:
1.
2.
3.
4.
5.
Equation 4
Equation 5
Boost converter loop
The voltage drop of each current generator is sent to the minimum voltage selector: the
lowest one is used as feedback and sent to the window comparator, whose outputs are used
to increase or decrease the loop-counter. The digital outputs of the counter are connected to
the DAC and the target reference is generated. As previously mentioned, this reference
voltage is compared to a partition of the output voltage: the difference is translated into an
error current at the output of the trans-conductance amplifier (see
At this point the compensation network, externally connected to the COMP pin, provides the
programmed trip level for the inductor current. An additional slope compensation ramp is
mixed, as usual, to avoid possible sub-harmonic instability when the boost converter
operates in continuous conduction mode (CCM). The SLOPE pin allows the proper setting
of the amount of slope compensation connecting a simple resistor RSLOPE between the
SLOPE pin and ground. The compensation ramp of a master device starts at 35% (typ.) of
each switching period.
The boost converter switching frequency can be set in the 200 kHz-1 MHz range by
connecting the FSW pin to ground through a resistor, calculated according to
Equation 6
Determine the output voltage range required by the LED strings, considering V
and temperature variation:
Calculate the middle-point of the output voltage range V
Calculate the output divider through
Calculate the resulting four output voltage ranges (VMIN setting options).
Select the output voltage range closest to the one calculated at point 1.
R
Doc ID 022698 Rev 1
2
=
V
(
---------------------------------------------------------------
V
V
--------------------------------- - 100μA
LED
BOOST middle
BOOST MAX
R
1
R
=
Equation 4
+
FSW
max
N
R
RO WS
,
V
,
= i
2
1
VFB
=
(
m
f
K
≤
SW
LE DS
= j
Σ
F
–
1
V
V
and 5.
VFB
F,
j
)
)
⋅
R
BOOST,middle
1
Figure
.
4, COMP pin).
Equation
LED7708
F
spread
6: