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ISL97678
6.66 × 10
f PWM = ------------------------
( 5 × 10
)
f SW = -----------------------
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 ISL97678 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 R 1 and R 2 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
prevents 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.
Dimming Controls
The ISL97678 allows two ways of controlling the LED
current, and therefore, the brightness. They are:
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
There are various ways to achieve DC or PWM current
control, which will be described in the following.
In any dimming controls, the EN pin must be high. EN
is a high voltage pin that can be applied with a digital
signal or tied directly to V IN for enable function.
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an
appropriate value for R SET . This should be chosen to fix
the maximum possible LED current:
PWM Dimming Frequency Adjustment
The dimming frequencies are set by an external
resistor at the FPWM pin as shown by Equation 3:
7
(EQ. 3)
RPWM
where f PWM is the desirable PWM dimming frequency
and R FPWM is the setting resistor.
Switching Frequency
The boost switching frequency can be adjusted by a
resistor as shown in Equation 4:
10
(EQ. 4)
R OSC
where f SW is the desirable boost switching frequency
and R OSC is the setting resistor.
5V and 2.3V Low Dropout Regulators
A 5V LDO regulator is present at the VDC pin to develop
the necessary low voltage supply, which is used by the
chips internal control circuitry. Because VDC is an LDO
pin, it requires a bypass capacitor of 1μF or more for the
regulation. The VDC pin can be used for a coarse
regulator or reference but do not pull more than few mA
from it.
Similarly, a 2.3V LDO regulator is present at the
VLOGIC pin to develop the necessary low voltage supply
for the chip’s internal logic control circuitry. A 1μF
bypass capacitor or more is needed for regulation. The
VLOGIC pin can be used as a coarse regulator or
reference but do not pull more than few mA from it.
Soft-Start
The ISL97678 uses a digital soft-start where the boost
current limit is stepped up in 8 steps. The initial current
limit level is set to one ninth of the full current limit, with
subsequent steps increasing this by a ninth every 2ms.
In the event that no LEDs have been conducting during
the interval since the last step (for example if the LEDs
are running at low duty cycle at low PWM frequency)
then the step will be delayed until the LEDs are
conducting. If the LEDs are disabled and re-enabled
again then soft start will be restarted when the LEDs are
enabled.
I LEDmax = ---------------
707.9
R SET
(EQ. 2)
Fault Protection and Monitoring
The ISL97678 features extensive protection functions to
Alternatively, the R SET can be replaced by a digital
potentiometer for adjustable current.
PWM CONTROL
The ISL97678 provides PWM dimming by PWM chopping
of the current in the LEDs for all 8 channels. To achieve
PWM dimming, the users need to apply a PWM signal at
the PWM pin. The PWM output will follow the PWM input
and the dimming frequency will be set by R PWM . During
the On periods, the LED current will be defined by the
value of R SET , as described in Equation 1.
12
cover all the perceivable failure conditions. The failure
mode of a LED can be either open circuit or as a short.
The behavior of an open circuited LED can additionally
take the form of either infinite resistance or, for some
LEDs, a zener diode, which is integrated into the device
in parallel with the now opened LED.
For basic LEDs (which do not have built-in zener diodes),
an open circuit failure of an LED will only result in the loss
of one channel of LEDs without affecting other channels.
Similarly, a short circuit condition on a channel that
FN6998.1
November 5, 2009
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