ISL78171 Datasheet, PDF (14/29 Pages) Intersil Corporation – 6-Channel, 50mA Automotive LED Driver with Ultra-high Dimming Ratio and Phase Shift Control

ISL78171
PWM Dimming Frequency Adjustment
For dimming methods 1-3, the PWM dimming frequency is set by
an external resistor at the FPWM pin and can be calculated by
equation 11:
F_PWM = _R-⁶---^.-_F-⁶---^6P----^-_W--1---0-_M--⁷-
(EQ. 11)
Where F_PWM is the PWM dimming frequency in Hz and R_FPWM is
the setting resistor in .
The PWM dimming frequency range is between 200Hz to 30kHz
and the duty cycle range is from 0.4% to 100%.
Phase Shift Control
For dimming methods 1-3, the ISL78171 is capable of delaying
the phase of each current source to minimize load transients. By
default, phase shifting is disabled as shown in Figure 25 where
the channels PWM currents are switching at the same time.
PWMI
60%
40%
ILED0
ILED1
ILED2
ILED3
ILED4
ILED5
ILED0
60%
40%
t_D1
t_D1
t_D1
t_D1
t_D1
t_D2
t_FPWM
t_ON
t_OFF
FIGURE 26. SIX EQUAL PHASE CHANNELS PHASE SHIFT ILLUSTRATION
ILED0
ILED1
ILED2
ILED3
ILED4
t_FPWM
t_ON
t_OFF
PWMI
ILED1
ILED2
ILED3
ILED4
t_PWMin
60 %
40%
t_D1
t_FPWM
(t_PWMout)
t_ON
60%
t_OFF
40%
t_{D 1}
t_D1
ILED5
FIGURE 25. NO DELAY (DEFAULT PHASE SHIFT DISABLED)
When EqualPhase = 1 (register 0x0A, bit 7), the phase shift evenly
spreads the channels switching across the PWM cycle, depending on
how many channels are enabled, as shown in Figures 26 and 27.
Equal phase means there are fixed delays between channels and
such delay can be calculated as Equation 12 in Figures 26 and 27.
t_D1
=
-^t-^F---₂-^P--₅--^W--₅---^M---
x


2_---N-5_---5_{--}
(EQ. 12)
Equation 13 shows the phase delay between the last channel of
the current duty cycle and the first channel of the next duty cycle
in Figures 26 and 27.
t_D2
=
-^t-^F---₂-^P--₅--^W--₅---^M---
x
²⁵⁵
–

N
–
1



2_---N-5_---5_{--}


(EQ. 13)
Where (255/N) is rounded down to the nearest integer. For
example, if N = 6, (255/N) = 42, that leads to:
t_D1 = t_FPWM x 42/255
t_D2 = t_FPWM x 45/255
Where t_FPWM is the sum of t_ON and t_OFF. N is the number of LED
channels. The ISL78171 will detect the number of operating
channels automatically.
t_D2
ILED1
t_D1 = tFD1i=xeFidxeddDeelalayywwithitIhnteignetreognley wr hoilentlhye wdehcimileal vthaleue dweillcbeimdisaclarvdaedlu(eeg.w63i.l7l5b=6e3)discarded (e.g., 3.75 = 63)
FIGURE 27. FOUR EQUAL PHASE CHANNELS PHASE SHIFT ILLUSTRATION
ILED0
ILED1
ILED2
ILED3
ILED4
ILED5
t_FPWM
t_ON
t_PD
t_PD
t_PD
t_PD
t_PD
t_OFF
FIGURE 28. PHASE SHIFT WITH 7-BIT PROGRAMMABLE DELAY
The ISL78171 allows the user to program the amount of phase
shift degree with 7-bit resolution, as shown in Figure 28. 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.
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FN8602.0
June 15, 2015