ISL97672 Intersil, ISL97672 Datasheet - Page 9

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

Typical Performance Curves
FIGURE 17. LOAD REGULATION WITH I
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
OUT
LOWER
OUT
_ovp = 1.21V*(R
OUT
regulation range.
OUT
such that:
_ovp such that:
can only regulate between 61% and 100% of
FROM 100% TO 0% PWM DIMMING,
V
OUT
IN
= 12V, 6P12S AT 20mA/CHANNEL
UPPER
9
+R
LOWER
)/R
UPPER
LOWER
LED
CHANGE
and
ISL97672
(Continued)
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
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.
OUT
LOWER
FIGURE 18. ISL97671 SHUTS DOWN AND STOPS
of 35V. If R1 and R2 are chosen such that the OVP
UPPER
SWITCHING ~ 30ms AFTER EN GOES LOW
= 3.3nF.
OUT
/C
LOWER
= 61% to 100% of V
OUT
= 1/33 with C
application, then the OVP level
SET
OUT
. A 1% tolerance resistor
is allowed to operate
UPPER
UPPER
OUT
www.DataSheet4U.com
_ovp
/R
= 100pF and
LOWER
OUT
June 24, 2010
= (61%
SET
= 33/1,
FN7632.0
SET
.