isl97635 Intersil Corporation, isl97635 Datasheet - Page 26

• Manufacturer Part Number: isl97635
• Description: Smbus 8-channel Led Driver
• Manufacturer: Intersil Corporation

16-Bit Dimming
The SMBus controlled PWM and DC dimmings can be
combined to effectively provide 16 bits of dimming capability,
which can be valuable for automotive and avionics display
applications. Figure 32 illustrates one programming example
where 256 steps of PWM dimming can be programmed
between each DC dimming steps, or vice versa.
RGB LED Backlight or Scrolling Backlight
Operation
The SMBus control features of PWM dimming, DC dimming,
and random channels selection have offered many driving
possibilities. For example, red, green, and blue LEDs can be
arranged in Ch0 and Ch1, Ch2 and Ch3, Ch4 and Ch5
respectively such that each group can be controlled
independently in sequential order for RGB or RGGB LED
backlighting applications.
Compensation
The ISL97635 has two main elements in the system; the
Current Mode Boost Regulator and the op amp based
multi-channel current sources. The ISL97635 incorporates a
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ISL97635
transconductance amplifier in its feedback path to allow the
user some levels of adjustment on the transient response,
as well as better regulation. The ISL97635 uses current
mode control architecture, which has a fast current sense
loop and a slow voltage feedback loop. The fast current
feedback loop does not require any compensation. The slow
voltage loop must be compensated for stable operation. The
compensation network is a series Rc, Cc1 network from
COMP pin to ground and an optional Cc2 capacitor
connected to the COMP pin. The Rc sets the high frequency
integrator gain for fast transient response and the Cc1 sets
the integrator zero to ensure loop stability. For most
applications, Rc is in the range of 200Ω to 3kΩ and Cc1 is in
the range of 27nF to 37nF. Depending on the PCB layout, a
Cc2, in range of 100nF, may be needed to create a pole to
cancel the output capacitor ESR’s zero effect for stability.
The ISL97635 evaluation board is configured with Rc1 of
500Ω, Cc1 of 33nF, and Cc2 of 0, which achieves stability. In
the actual applications, these values may need to be tuned
empirically but these recommended values are usually a
good starting point.
December 22, 2008
FN6434.2