ISL97673_10 Datasheet, PDF (11/27 Pages) Intersil Corporation – 6-Channel SMBus or PWM Dimming LED Driver with Phase Shift Control

ISL97673
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_OUT Requirement
The Overvoltage Protection (OVP) pin has a function of
setting the overvoltage trip level as well as limiting the
V_OUT regulation range.
The ISL97673 OVP threshold is set by R_UPPER and
R_LOWER as shown in Equation 1:
V_{OUT_OVP} = 1.21V × (R_UPPER + R_LOWER) ⁄ R_LOWER (EQ. 1)
V_OUT can only regulate between 64% and 100% of the
V_OUT__OVP such that:
Allowable V_OUT = 64% to 100% of V_OUT__OVP
For example, if 10 LEDs are used with the worst case
V_OUT of 35V. If R₁ and R₂ are chosen such that the OVP
level is set at 40V, then the V_OUT is allowed to operate
between 25.6V and 40V. If the requirement is changed to
a 6 LEDs 21V V_OUT application, then the OVP level must
be reduced and users should follow V_OUT = (64%
~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_UPPER/R_LOWER = 33/1,
then C_UPPER/C_LOWER = 1/33 with C_UPPER = 100pF and
C_LOWER = 3.3nF.
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_SET
current to the output with a scaling factor of 401.8/R_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 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_SET. A 1% tolerance resistor
is recommended.
+
-
+- REF
RSET
+
-
PWM DIMMING
DC DIMMING
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
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_SC, such
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.
11
FN7633.0
June 24, 2010