ISL97672IRZ Intersil, ISL97672IRZ Datasheet - Page 9

• Manufacturer Part Number: ISL97672IRZ
• Description: IC LED DVR PWM CTRL 6CH 20QFN
• Manufacturer: Intersil

Specifications of ISL97672IRZ

• Constant Current: *
• Constant Voltage: *
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Frequency: 200kHz ~ 1.5MHz
• Voltage - Supply: 4.5 V ~ 26.5 V
• Voltage - Output: *
• Mounting Type: Surface Mount
• Package / Case: 20-VFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 40mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Theory of Operation
PWM Boost Converter
The current mode PWM boost converter produces the
minimal voltage needed to enable the LED stack with the
highest forward voltage drop to run at the programmed
current. The ISL97672 employs current mode control
boost architecture that has a fast current sense loop and
a slow voltage feedback loop. Such architecture achieves
a fast transient response that is essential for the
notebook backlight application where the power can be a
series of drained batteries or instantly changed to an
AC/DC adapter without rendering a noticeable visual
nuisance. The number of LEDs that can be driven by
ISL97672 depends on the type of LED chosen in the
application. The ISL97672 is capable of boosting up to
45V and typically driving 13 LEDs in series for each of the
8 channels, enabling a total of 104 pieces of the
3.2V/20mA type of LEDs.
Enable
The Enable pin is used to enable the device. If there is
no signal for longer than 28ms, the device will enter
shutdown. Do not let Enable pin floating thus a 10k or
higher pull-down resistor should be added.
OVP and V
The Over Voltage Protection (OVP) pin has a function of
setting the overvoltage trip level as well as limiting the
V
The ISL97672 OVP threshold is set by R
R
V
and V
the V
Allowable V
For example, if 10 LEDs are used with the worst case
V
level is set at 40V, then the V
between 24.4V and 40V. If the requirement is changed to
a 6 LEDs of 21V V
must be reduced and users should follow V
~100%) OVP level requirement. Otherwise, the
headroom control will be disturbed such that the channel
voltage can be much higher than expected and
sometimes it can prevent the driver from operating
properly.
The ratio of the OVP capacitors should be the inverse of
the OVP resistors. For example, if R
then C
C
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the
current source circuit, as shown in Figure 19.
The LED peak current is set by translating the R
current to the output with a scaling factor of 401.8/R
OUT
LOWER
OUT
OUT
LOWER
_ovp = 1.21V*(R
OUT
regulation range.
of 35V. If R1 and R2 are chosen such that the OVP
OUT
UPPER
such that:
= 3.3nF.
_ovp such that:
can only regulate between 61% and 100% of
OUT
/C
OUT
LOWER
= 61% to 100% of V
OUT
= 1/33 with C
UPPER
application, then the OVP level
9
+R
OUT
LOWER
is allowed to operate
UPPER
UPPER
)/R
OUT
UPPER
LOWER
_ovp
/R
= 100pF and
LOWER
OUT
and
= (61%
SET
= 33/1,
ISL97672
SET
.
The source terminals of the current source MOSFETs are
designed to run at 500mV to optimize power loss versus
accuracy requirements. The sources of errors of the
channel-to-channel current matching come from the
op amp’s offset, internal layout and reference and these
parameters are optimized for current matching and
absolute current accuracy. The absolute accuracy is also
determined by the external R
should be used.
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:
I
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
LEDmax
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
=
RSET
RSET
-------------- -
R
401.8
PWM DIMMING
SET
+
-
REF
REF
SET
. This should be chosen to fix
+
+
-
-
SET
. A 1% tolerance resistor
SC
, such voltage will be
+
+
-
-
November 1, 2010
FN7632.1
(EQ. 1)