HV9980 查看數據表（PDF） - Supertex Inc

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HV9980

3-Channel LED Array Driver IC

Supertex Inc 

HV9980

Application Information

Programming LED current and selecting L and D

The required value of the output inductor L is inversely pro-

portional to the ripple current ΔIO in it. Setting the relative

peak-to-peak ripple to 20~30% is a good practice to ensure

noise immunity of the current sense comparator.

L = (VO • TOFF) / ΔIO = (VO • [1 - D]) / fSΔIO

(1)

Using an ultra-fast rectifier diode for D1 is recommended to

achieve high efficiency and reduce the risk of false triggering

of the current sense comparator. Using diodes with shorter

reverse recovery time trr and lower junction capacitance CJ

achieves better performance. The reverse voltage rating VR

of the diode must be greater than the maximum input volt-

age of the LED lamp.

VO is the forward voltage of the LED string, fS is the switching The total parasitic capacitance present at the DRAIN output

frequency, D = VO/VIN is the switching duty cycle.

of the HV9980 can be calculated as:

The output current in the LED string (IO) is calculated as:

CP = CDRAIN + CPCB + CL + CJ

(3)

IO = (VREF / RSENSE) - 1/2 • ΔIO

(2)

where VREF is the voltage at REF1-3, and RSENSE is the cur-

rent sense resistor at RSENSE1-3. (The ripple current intro-

duces a peak-to-average error in the output current setting

that needs to be accounted for.)

Adding a filter capacitor across the LED string can reduce

the output current ripple yielding a reduced value of L. How-

ever, one must keep in mind that the peak-to-average cur-

rent error is affected by the variation of the input and output

voltage. Therefore, the line and load regulation of the LED

current might be sacrificed at large ripple current in L.

When the switch turns on, the capacitance CP is discharged

into the DRAIN output of the IC. The discharge current is

limited to about 300mA typically. However, it may become

lower at increased junction temperature. The duration of the

leading edge current spike can be estimated as:

TSPIKE = [(VIN • CP) / ISAT ] = trr

(4)

In order to avoid false triggering of the current sense com-

parator, CP must be minimized in accordance with the follow-

ing expression:

CP < [ISAT • (TBLANK(MIN) - trr)] / VIN(MAX)

(5)

Another important aspect of designing an LED driver with

the HV9980 is related to certain parasitic elements of the

circuit, including distributed coil capacitance of L1, junction

capacitance and reverse recovery of the rectifier diode D1,

capacitance of the printed circuit board traces CPCB and out-

put capacitance CDRAIN of the controller itself. These parasitic

elements affect the efficiency of the switching converter and

could potentially cause false triggering of the current sense

comparator if not properly managed. Minimizing these

parasitics is essential for efficient and reliable operation of

the HV9980.

Coil capacitance of inductors is typically provided in the

manufacturer’s data books either directly or in terms of the

self-resonant frequency (SRF).

SRF = 1 / (2π√L • CL)

where L is the inductance value, and CL is the coil capaci-

tance.) Charging and discharging this capacitance every

switching cycle causes high-current spikes in the LED string.

Therefore, connecting a small capacitor CO (~10nF) is rec-

ommended to bypass these spikes.

where TBLANK(MIN) is the minimum blanking time of 120ns, and

VIN(MAX) is the maximum instantaneous input voltage.

Layout Considerations

The HV9980 provides three independent power ground

connections PGND1-3 for each channel. The PGND pins

must be wired together on the printed circuit board (PCB).

To minimize interference between the channels, the PGND

pins should be wired to the negative terminal of the input

filter capacitor CIN using separate tracks. All four power sup-

ply inputs VDD, VDD1-3 must be connected together on the

PCB also.

Although in many layout arrangements wiring the reference

pins REF1-3 together is acceptable, further reduction of

the “cross-talk” between the channels is possible by add-

ing low-pass RC filters with the filter capacitors referenced

to the corresponding PGND pins. These filters composed

from RREF1-3 and CREF1-3 are shown in the Typical Application

Circuit diagram.

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