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参数资料
| 型号： | NCV7680PWR2GEVB |
| 厂商： | ON Semiconductor |
| 文件页数： | 14/20页 |
| 文件大小： | 0K |
| 描述： | BOARD DEMO REAR COMBO LAMP |
| 标准包装： | 1 |
| 系列： | * |
NCV7680
V reg
Using Equation 4
Choose a value for R7.
Oscillator
and PWM
I rstop
R7 = 1k
R6 +
1k 10.5 V * 1.08 V
1.08 V
+ 8.72k
4.4V
0.4V
R TAIL
The closest standard resistor value is 8.87k.
Reduced Channel Operation
The previously shown applications (Figures 4 and 5)
show system operation using all 8 available channels of the
NCV7680. When less than 8 channels are used, the unused
V STRING + V FB
) 1
R TAIL
Figure 22. Duty Cycle Generator Circuitry
Alternative Setup of Duty Cycle
Alternatively, the duty cycle can be controlled by
providing a voltage to the R TAIL pin as per Figure 13 (Duty
Cycle vs. V(R TAIL ). Note the pull-up current source (I rstop )
is still present on the R TAIL pin due to current setting resistor
connected to R STOP . For proper operation the system
designer needs to insure there is sufficient loading on the
R TAIL pin such that I rstop does not pull the referenced voltage
higher than its regulated state.
Setting V STRING
V STRING should be set to a level to allow proper operation
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 STRING is set using resistors R6 and R7 (reference
Figure 4).
R6
(eq. 3)
R7
V FB = Ballast Drive Reference Voltage
This simplifies to an equation for R6.
R7 V STRING * V FB
R6 + (eq. 4)
V FB
Calculate system design V STRING .
Let V LED be the voltage drop across your LEDs (3 included
in Figure 4). 9.5 V
Choose a value for OUTx, 1 V
V STRING + V OUTx ) V LED (eq. 5)
Using Equation 3
V STRING + 1 V ) 9.5 V + 10.5 V
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 P ) while maintaining open circuit detection.
External pullup resistors may be used as an alternative.
Adding LED’s to the String
The NCV7680 can function as a standalone device or in
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)
One of the key contributors to electromagnetic
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.
The NCV7680 employs current slew rate control. Each
output is rated at 6.0 mA/ m s (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.
EMC susceptibility improvements to the NCV7680
device can be made by adding a ferrite bead directly on the
VP (pin 2) of the device. The recommended component for
this setup is the TDK PN/ MMZ2012S601A, FERRITE
CHIP 600 OHM 500MA 0805 device. Care should be taken
to add this component no less than 10mils from the pin.
An R-C network can also be used as an alternative to the
ferrite bead. A 100 W resistor in series with VP (pin 2) with
a 1nF – 10 nF very low ESR ceramic capacitor provides a
similar roll-off. A very-low ESR ceramic capacitor is a
requirement here. A normal ceramic capacitor will not
suffice in this setup. The design should consider that the
dropout voltage of the LED strings must be higher than the
minimum operating voltage of the IC plus the voltage drop
across the 100 W resistor.
http://onsemi.com
14
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