NCV7680 ON Semiconductor, NCV7680 Datasheet - Page 14

• Manufacturer Part Number: NCV7680
• Description: Linear Current Regulator and Controller
• Manufacturer: ON Semiconductor

duty cycle as specified in the electrical parameter table plus
an additional error due to the Irstop current which generates
this voltage in the system.
Alternative Setup of Duty Cycle
providing a voltage to the R
Cycle vs. V(R
is still present on the R
connected to R
designer needs to insure there is sufficient loading on the
R
higher than its regulated state.
Setting V
of the IC without detecting an open circuit (0.5 V max on
OUTx) and to keep power to the IC at reduced levels below
the 150°C max die temperature thermal limit (die
temperature will depend on printed circuit board
composition, PCB size, thermal via number and placement,
module component placement, and air flow).
Example:
V
Figure 4).
V
This simplifies to an equation for R6.
TAIL
STRING
FB
Alternatively, the duty cycle can be controlled by
V
STRING
= Ballast Drive Reference Voltage
pin such that I
Figure 22. Duty Cycle Generator Circuitry
STRING
is set using resistors R6 and R7 (reference
should be set to a level to allow proper operation
and PWM
Oscillator
TAIL
R6 +
V
STOP
STRING
). Note the pull-up current source (I
rstop
. For proper operation the system
TAIL
R7 V
does not pull the referenced voltage
+ V
4.4V
0.4V
pin due to current setting resistor
STRING
TAIL
FB
R
V
V
TAIL
reg
FB
pin as per Figure 13 (Duty
R6
R7
R
* V
I
rstop
−
TAIL
+
) 1
FB
(eq. 3)
(eq. 4)
http://onsemi.com
rstop
)
14
Calculate system design V
Let V
in Figure 4). 9.5 V
Choose a value for OUTx, 1 V
Using Equation 3
Using Equation 4
Choose a value for R7.
R7 = 1k
The closest standard resistor value is 8.87k.
Reduced Channel Operation
show system operation using all 8 available channels of the
NCV7680. When less than 8 channels are used, the unused
OUTx pins can be grounded eliminating the unused OUTx
drive current. This is accomplished by voltage threshold
detection on OUTx (100 mV typ). A voltage less than
100 mV on OUTx turns the driver off, reducing quiescent
current to the IC. This also helps reduce system power by
eliminating the need for an external pullup resistor (from
OUTx to V
External pullup resistors may be used as an alternative.
Adding LED’s to the String
conjunction with additional support circuitry for more
complex systems. Figure 23 shows the NCV7680 operating
with a boost controller. This setup allows additional LEDs
in a string to be increased. Eight are shown in the diagram.
Consideration of the 45 V maximum limit on the OUTx Pin
is the limitation of this configuration. The DC on voltage
level on OUTx must also be considered for thermal reasons.
Electromagnetic Interference (EMI)
interference is the rise and fall times of the electrical signals.
This is a concern with both the initial startup of a device, and
the repeated turn on/off cycles of a device.
output is rated at 6.0 mA/ms (typ). Slew rate control reduces
overshoot and allows for a predictable electrical signal. Slew
rate control is used in the stop mode for soft−start and in the
tail mode for low EMI operation.
The previously shown applications (Figures 4 and 5)
The NCV7680 can function as a standalone device or in
One of the key contributors to electromagnetic
The NCV7680 employs current slew rate control. Each
LED
be the voltage drop across your LEDs (3 included
R6 +
P
V
) while maintaining open circuit detection.
STRING
V
STRING
1k 10.5 V * 1.08 V
+ 1 V ) 9.5 V + 10.5 V
+ V
1.08 V
STRING
OUTx
.
) V
LED
+ 8.72k
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(eq. 5)