ISL97672AIRZ Intersil, ISL97672AIRZ Datasheet - Page 9

• Manufacturer Part Number: ISL97672AIRZ
• Description: 6-Ch LED Driver With PWM Dimming. Vsc 7.5V Min
• Manufacturer: Intersil
• Series: -r

Specifications of ISL97672AIRZ

• Constant Current: -
• Constant Voltage: -
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: -
• Frequency: 200kHz ~ 1.5MHz
• Voltage - Supply: 4.5 V ~ 26.5 V
• Voltage - Output: 45V
• Mounting Type: Surface Mount
• Package / Case: 20-VFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Typical Performance Curves
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 ISL97672A
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 notebook backlight applications in which drained batteries
can be instantly changed to an AC/DC adapter without
noticeable visual disturbance. The number of LEDs that can be
driven by ISL97672A depends on the type of LED chosen in the
application. The ISL97672A 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 enters shutdown. The Enable pin
should not float. If it does, a 10k or higher pull-down resistor should
be added.
OVP and V
The Overvoltage Protection (OVP) pin has a function of setting the
overvoltage trip level as well as limiting the V
range.
The ISL97672A OVP threshold is set by R
that:
V
and V
V
Allowable V
if, for example, 10 LEDs are used with the worst-case V
35V.
If R
then V
V
OUT
OUT
OUT
1
_ovp = 1.21V * (R
_ovp such that:
requirement is changed to an application of six LEDs of 21V,
and R
OUT
OUT
FIGURE 19. MINIMUM DIMMING LINEARITY AT 200Hz
30
25
20
15
10
can only regulate between 61% and 100% of the
is allowed to operate between 24.4V and 40V. If the
2
6
OUT
are chosen such that the OVP level is set at 40V,
OUT
= 61% to 100% of V
7
PWM DIMMING DUTY CYCLE (%)
UPPER
8
+ R
9
9
LOWER
10
OUT
)/R
_ovp
11
UPPER
LOWER
I
F
NO CH CAPS
OUT
LED
PWM
12
and R
= 20mA
regulation
= 200Hz
13
LOWER
OUT
(Continued)
ISL97672A
of
14
such
then the OVP level must be reduced. Users should follow the
V
headroom control will be disturbed such that the channel voltage
can be much higher than expected. This can sometimes prevent
the driver from operating properly.
The ratio of the OVP capacitors should be the inverse of the OVP
resistors. For example, if R
C
C
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 21.
The LED peak current is set by translating the R
output, with a scaling factor of 401.8/R
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
OUT
UPPER
LOWER
= (61% ~100%) OVP level requirement; otherwise, the
FIGURE 20. MINIMUM DIMMING LINEARITY AT 20kHz
/C
FIGURE 21. SIMPLIFIED CURRENT SOURCE CIRCUIT
1.2
1.0
0.8
0.6
0.4
0.2
0.0
= 3.3nF.
LOWER
0 0.4
RSET
RSET
PWM DIMMING
SET
= 1/33 with C
. A 1% tolerance resistor should be used.
0.8
+
-
1.2
PWM DIMMING DUTY CYCLE (%)
REF
REF
1.6
UPPER
2.0
UPPER
+
+
-
-
2.4
/R
LOWER
2.8 3.2 3.6
= 100pF and
SET
+
+
= 33/1, then
-
-
. The source terminals
I
F
LED
PWM
4.0
SET
= 20mA
4.4 4.8
= 20kHz
current to the
May 2, 2011
5.2
FN7710.2