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ISL97671
Switching Frequency
PWMI
ILED0
60%
40%
60%
40%
The default switching frequency is 600kHz but it can be selected
to 600kHz or 1.2MHz if the SMBus/I 2 C communications is used.
The switching frequency select bit is accessible in the SMBus/I 2 C
ILED1
t D1
Configuration Register 0x08 bit 2.
ILED2
t D1
5V Low Dropout Regulator
ILED3
t D1
A 5V LDO regulator is present at the VDC pin to develop the
necessary low voltage supply, which is used by the chips internal
ILED4
ILED5
t D1
t D1
t D2
t FPWM
control circuitry. Because VDC is an LDO pin, it requires a bypass
capacitor of 1μF or more for the regulation. Low input voltage also
allows only lower output voltage applications only with the
maximum boost ratio defined in “Components Selections” on
page 24. The VDC pin can be used as a coarse reference with a few
ILED0
t ON
t OFF
mA sourcing capability.
FIGURE 22. 6 EQUAL PHASE CHANNELS PHASE SHIFT
ILLUSTRATION
t PWMin
In-rush Control and Soft-start
The ISL97671 has separately built in independent in-rush control and
soft-start functions. The in-rush control function is built around the
PWMI
60%
40%
t FPWM
(t PWMout )
short circuit protection FET, and is only available in applications, which
include this device. At start-up, the fault protection FET is turned on
ILED1
t ON
60%
t OFF
40%
slowly due to a 30μA pull-down current output from the FAULT pin.
ILED2
ILED3
ILED4
t D1
t D1
t D1
This discharges the fault FET's gate-source capacitance, turning on
the FET in a controlled fashion. As this happens, the output capacitor
is charged slowly through the weakly turned on FET before it becomes
fully enhanced. This results in a low in-rush current. This current can
be further reduced by adding a capacitor (in the 1nF to 5nF range)
across the gate-source terminals of the FET.
Once the chip detects that the fault protection FET is turned on
ILED1
t D2
hard, it is assumed that in-rush has completed. At this point, the
boost regulator will begin to switch and the current in the
t D1 = Fixed Delay with Integer only while the decimal value will be discarded (eg. 63.75=63)
FIGURE 23. 4 EQUAL PHASE CHANNELS PHASE SHIFT
ILLUSTRATION
The ISL97671 allows the user to program the amount of phase
shift degree in 7-bit resolution, as shown in Figure 24. To enable
programmable phase shifting, the user must write to the Phase
Shift Control register with EqualPhase = 0 and the desirable
phase shift value of PhaseShift[6:0]. The delay between CH5 and
the repeated CH0 is the rest of the PWM cycle.
t FPWM
inductor will ramp-up. The current in the boost power switch is
monitored and the switching is terminated in any cycle where the
current exceeds the current limit. The ISL97671 includes a soft-
start feature where this current limit starts at a low value
(275mA). This is stepped up to the final 2.2A current limit in 7
further steps of 275mA. These steps will happen over at least
8ms, and will be extended at low LED PWM frequencies if the
LED duty cycle is low. This allows the output capacitor to be
charged to the required value at a low current limit and prevents
high input current for systems that have only a low to medium
output current requirement.
ILED0
ILED1
ILED2
t PD
t PD
t ON
t OFF
For systems with no master fault protection FET, the in-rush
current will flow towards C OUT when VIN is applied and it is
determined by the ramp rate of VIN and the values of C OUT and L.
Fault Protection and Monitoring
ILED3
ILED4
t PD
t PD
The ISL97671 features extensive protection functions to 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
t PD
ILED5
FIGURE 24. PHASE SHIFT WITH 7-BIT PROGRAMMABLE DELAY
13
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 results in that channel being turned
off does not affect other channels unless a similar fault is
FN7631.3
August 14, 2012
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