ISL97684 Intersil Corporation, ISL97684 Datasheet - Page 12

• Manufacturer Part Number: ISL97684
• Description: (ISL97682 - ISL97684) Compact 2-3-4-Ch LED Drivers
• Manufacturer: Intersil Corporation

w w w . D a t a S h e e t 4 U . c o m
PWMI
ILED1
ILED2
ILED3
ILED4
ILED1
Phase Shift Control
The ISL97682, ISL97683, ISL97684 are capable of delaying the
phase of each current source. Conventional LED drivers exhibit
the worst load transients to the boost circuit by turning on all
channels simultaneously as shown in Figures 20 and 21. In
contrast, the ISL97682, ISL97683, ISL97684 phase shifts each
channel by turning them on once during each PWM dimming
period as shown in Figures 23 and 24. At each dimming duty
cycle except at 100%, the sum of the phase shifted total current
will be less than a conventional LED drivers’ total current.
For ISL97682, the two channels are separated by 180°. For
ISL97683, the three channels are separated by 90° and not
120°. For ISL97684, the four channels are separated by 90°. If
the channels are combined for higher current application, the
phase shift function must be disabled by running the part in
direct PWM mode by floating the RFPWM/DirectPWM and
selecting switching frequency by biasing the FSW pin as
explained in Table 2.
PWM Dimming Frequency Adjustment
The dimming frequency is set by an external resistor at the
RFPWM/DirectPWM pin to GND calculated by Equation 6:
where FPWM is the desirable PWM dimming frequency and
R
direct PWM dimming is used; see Table 1 for clarification.
The PWM dimming frequency can be set or applied up to 30kHz
with duty cycle from 0.4% to 100%. The lower limit of 0.4% is the
result of 8-bit digitizer resolution.
Direct PWM Dimming
The ISL97682, ISL97683, ISL97684 can also operate in direct
PWM dimming mode such that the output follows the input PWM
signal without phase shifting. The FSW pin can still be used to
select between 600kHz and 1MHz in PWM or PFM mode as
explained in “Pin Descriptions” on page 4. To use Direct PWM
mode, users should float the RFPWM/DirectPWM pin. The input
PWM frequency should be limited to 30kHz and the minimum
duty cycle be calculated by Equation 7:
Min Duty Cycle
⎛
⎝
F
FPWM
FIGURE 24. ISL97684 4 CHANNELS PHASE SHIFT ILLUSTRATION
PWM
60%
=
is the setting resistor. Do not bias RFPWM/DirectPWM if
(
-------------------------------- -
t
12.4
PWMIN
RFPWM
=
)
×
450ns Input PWM Frequency
10
t
FPWM
7
60%
t
ON
⎞
⎠
×
(t
PWMOUT
12
t
40%
OFF
)
ISL97682, ISL97683, ISL97684
(EQ. 6)
(EQ. 7)
For example, for a 200Hz input PWM frequency, the minimum
duty cycle is:
Min DC
Table 1 shows the PWM Dimming with Phase Shift and Direct
PWM Dimming configurations.
Switching Frequency and PWM/PFM Mode
When the FSW pin is biased from VDC with a resistor divider
R
mode will change according to the following FSW levels shown in
Table 2 with the recommended R
The ISL97682, ISL97683, ISL97684 goes into PFM mode at
F
The part will only go into PFM mode depending on the LED output
voltage and loading conditions and can be more efficient than
running the part in PWM mode as shown in Figures 5 and 6. The
dimming frequency can be set or applied up to 30kHz with duty
cycle from 0.4% to 100%. The lower limit of 0.4% is the result of
an 8-bit digitizer resolution.
Soft-Start
The in-rush current will flow towards C
and it is determined by the ramp rate of V
C
Once the part is enabled, the boost regulator will begin to switch
and the current in the 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
ISL97682, ISL97683, ISL97684 include a soft-start feature
where this current limit starts at a low value (225mA). This is
stepped up to the final 1.8A current limit in 7 further steps of
225mA. These steps will happen over approximately 8ms and
will be extended at a low LED PWM frequency 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.
Connects with
Resistor
Floating
(0 ~ 0.25)*VDC
(0.25~0.5)*VDC
(0.5~0.75)*VDC
(0.75~1) VDC
SW
OUT
UPPER
DIRECTPWM
= 600kHz/1MHz when the FSW pin is biased at 0/VDC volts.
and L.
RFWM/
=
FSW
and R
450ns 200Hz
LOWER
×
PWM Dimming with
frequency adjust
DirectPWM without
frequency adjust
, the switching frequency and PFM/PWM
600kHz
600kHz
1.0MHz
1.0MHz
F
FUNCTION
SW
=
0.009%
TABLE 1.
TABLE 2.
PHASE
SHIFT
UPPER
Yes
Yes
Yes
Yes
OUT
PHASE
and R
SHIFT
Mode
PWM
PWM
PFM
PFM
Yes
IN
No
when V
and the values of
LOWER
R
156kΩ
100kΩ
Open
UPPER
IN
RESOLUTION
0
is applied
DIMMING
March 11, 2011
.
8-bit
N/A
FN7689.0
R
100kΩ
122kΩ
(EQ. 8)
Open
LOWER
0