ISL97673IRZ-TK Intersil, ISL97673IRZ-TK Datasheet - Page 11

• Manufacturer Part Number: ISL97673IRZ-TK
• Description: IC LED DVR PWM CTRL 6CH 20QFN
• Manufacturer: Intersil

Specifications of ISL97673IRZ-TK

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: RGB, White LED
• Frequency: 600kHz, 1.2MHz
• 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 ISL97673 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 change to an
AC/DC adapter without rendering a noticeable visual
nuisance. The number of LEDs that can be driven by
ISL97673 depend on the type of LED chosen in the
application. The ISL97673 are capable of boosting up to
45V and typically driving 13 LEDs in series for each of the
6 channels, enabling a total of 104 pieces of the
3.2V/20mA type of LEDs.
Enable and PWM
The ISL97673 has EN/PWM pin that serves dual
purposes; it is used as an Enable signal and can be used
as a PWM input signal for dimming. If a PWM signal is
applied to this pin, the first pulse of minimum 4ms will be
used as an Enable signal. If there is no signal for longer
than 28ms, the device will enter shutdown.
OVP and V
The Overvoltage Protection (OVP) pin has a function of
setting the overvoltage trip level as well as limiting the
V
The ISL97673 OVP threshold is set by R
R
V
V
Allowable V
For example, if 10 LEDs are used with the worst case
V
level is set at 40V, then the V
between 25.6V and 40V. If the requirement is changed to
a 6 LEDs 21V V
be reduced and users should follow V
~100%) OVP 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.
V
OUT
LOWER
OUT
OUT
OUT
LOWER
OUT_OVP
_
regulation range.
can only regulate between 64% and 100% of the
of 35V. If R
OVP
UPPER
= 3.3nF.
as shown in Equation 1:
such that:
=
OUT
/C
1.21V
OUT
LOWER
OUT
= 64% to 100% of V
1
×
and R
Requirement
(
application, then the OVP level must
R
= 1/33 with C
UPPER
2
11
are chosen such that the OVP
+
OUT
R
LOWER
is allowed to operate
UPPER
UPPER
OUT
) R
OUT
⁄
UPPER
_
/R
LOWER
= 100pF and
OVP
= (64%
LOWER
and
= 33/1,
(EQ. 1)
ISL97673
The LED peak current is set by translating the R
current to the output with a scaling factor of 401.8/R
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 amps offset, internal layout, reference, and current
source resistors. These parameters are optimized for
current matching and absolute current accuracy.
However, the absolute accuracy is additionally
determined by the external R
is recommended.
Dynamic Headroom Control
The ISL97673 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 digitally. When the lowest channel voltage
is lower than the short circuit threshold, V
voltage will be used as the feedback signal for the boost
regulator. The boost makes the output to the correct
level such that the lowest channel 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.
Operating Modes
The ISL97673 has extensive operating modes such as
SMBus controlled PWM or DC dimmings, PWM dimming
with phase shift control and more. Depending on the pin
5 (SEL1) condition, pins 6 and 7 correspond to different
operating modes as shown in Table 1.
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
RSET
RSET
PWM DIMMING
DC DIMMING
+
-
REF
REF
+
+
-
-
SET
. A 1% tolerance resistor
+
+
-
-
SC
, such
June 24, 2010
SET
FN7633.0
SET
.