ISL97672 Intersil, ISL97672 Datasheet - Page 10

• Manufacturer Part Number: ISL97672
• Description: 6-Channel LED Driver
• Manufacturer: Intersil

Dynamic Headroom Control
The ISL97672 features a proprietary Dynamic Headroom
Control circuit that detects the highest forward voltage
string or effectively the lowest voltage from any of the
CH0-CH5 pins. When this lowest channel voltage is lower
than the short circuit threshold, V
used as the feedback signal for the boost regulator. The
boost makes the output to the correct level such that the
lowest channel pin is at the target headroom voltage.
Since all LED stacks are connected to the same output
voltage, the other channel pins will have a higher
voltage, but the regulated current source circuit on each
channel will ensure that each channel has the same
current. The output voltage will regulate cycle by cycle
and it is always referenced to the highest forward voltage
string in the architecture.
Dimming Controls
The ISL97672 allows two ways of controlling the LED
current, and therefore, the brightness. They are:
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an
appropriate value for R
the maximum possible LED current:
For example, if the maximum required LED current
(I
Equation 2:
PWM CURRENT CONTROL
The ISL97672 employs direct PWM dimming such that
the output PWM dimming follows directly with the input
I
R
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
LEDmax
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
SET
LED(max)
=
401.8 0.02
=
) is 20mA, rearranging Equation 1 yields
RSET
RSET
401.8
-------------- -
R
SET
PWM DIMMING
⁄
+
-
=
REF
REF
20.1kΩ
SET
10
. This should be chosen to fix
+
+
-
-
SC
, such voltage will be
+
+
-
-
(EQ. 1)
(EQ. 2)
ISL97672
PWM signal without modifying the input frequency. The
average LED current of each channel can be calculated as
Equation 3:
Switching Frequency
The boost switching frequency can be adjusted by a
resistor as Equation 4:
where f
R
5V Low Dropout Regulator
A 5V LDO regulator is present at the VDC pin to develop
the necessary low voltage supply which is used by the
chips internal control circuitry. Because VDC is an LDO
pin, it requires a bypass capacitor of 1µF or more for the
regulation. The VDC pin can be used as a coarse
reference with few mA sourcing capability.
Inrush Control and Soft-Start
The ISL97672 has separately built-in independent inrush
control and soft-start functions. The inrush control
function is built around the short circuit protection FET,
and is only available in applications which include this
device. At start-up, the fault protection FET is turned on
slowly due to a 30µA pull-down current output from the
FAULT pin. This discharges the fault FET's gate-source
capacitance, turning on the FET in a controlled fashion.
As this happens, the output capacitor is charged slowly
through the weakly turned on FET before it becomes fully
enhanced. This results in a low inrush current. This
current can be further reduced by adding a capacitor (in
the 1nF to 5nF range) across the gate source terminals of
the FET.
Once the chip detects that the fault protection FET is
turned on hard, it is assumed that inrush is complete. At
this point, the boost regulator will begin to switch and the
current in the inductor will ramp-up. The current in the
boost power switch is monitored and the switching
terminated in any cycle where the current exceeds the
current limit. The ISL97672 includes a soft-start feature
where this current limit starts at a low value (275mA).
This is stepped up to the final 2.2A current limit in seven
further steps of 275mA. This is stepped up to the final
2.2A current limit in 7 further steps of 275mA. These
steps will happen over at least 8ms, and will be extended
at low LED PWM frequencies if the LED duty cycle is low.
This allows the output capacitor to be charged to the
required value at a low current limit and prevents high
input current for systems that have only a low to medium
output current requirement.
For systems with no master fault protection FET, the
inrush current will flow towards C
and it is determined by the ramp rate of VIN and the
values of C
I
f
LED ave
SW
FSW
(
=
is the setting resistor.
(
-----------------------
5
R
SW
)
×10
FSW
=
is the desirable boost switching frequency and
OUT
10
I
LED
)
and L.
×
PWM
OUT
when VIN is applied
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June 24, 2010
FN7632.0
(EQ. 3)
(EQ. 4)