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Texas Instruments LM3464 Скачать руководство пользователя страница 14

30127165

FIGURE 10. Additional Voltage Clamping Circuits for V

RAIL(peak)

> 80V/95V (LM3464A)

Applications With High Rail Voltage

Since the LM3464A is rated to 95V supply voltage, applying
a voltage to any pin of the device exceeding the absolute rated
voltage could damage the device permanently. For the appli-
cations that the rail voltage could increase to exceed 95V,
external voltage clamping circuit must be added to the Vin and
DRx pins to avoid system breakdown. Figure 13 shows a typ-
ical application circuit with 150V peak rail voltage.

In figure 13, Z

1

, Z

2

, Z

3

, Z

4

and Z

IN

are zener diodes for limiting

voltages at the DRx and VIN pins of the LM3464A. The re-
verse voltage of the selected zener diodes must not exceed
the rated voltage of the corresponding pin. For the LM3464A

evaluation board, the reverse voltage of the additional zener
diodes must not exceed 95V. The resistors R

DR1

, R

DR2

,

R

DR3

, R

DR4

and R

IN

are resistors for absorbing the voltage

difference across the DRx pins and V

RAIL

.

Calculating the Values of Z

x

and R

DRx

:

Since the current being passed through the zener diodes are
derived by the resistance of R

DRx

, the value of the R

DRx

must

be calculated properly according to the reverse current of the
zener diode and input current of the DRx pins of the LM3464A
avoid unnecessary power dissipations. For instant, a 500mW/
75V zener diode CMHZ5267B (Central Semiconductor) is
used to clamp the DRx pins at 75V. Because the reverse cur-

13

www.national.com

AN-2071

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Содержание LM3464

Страница 1: ...LM3464 Application Note 2071 LM3464A 4 Channel LED Driver Evaluation Board Literature Number SNVA449C ...

Страница 2: ...wing the LM3464A to adapt to difference source voltages easily The integrated thermal foldback control circuit protects the LED Strings from damages due to over temperature This eventually secures the lifetime of the entire lighting system The LM3464A in cludes a fault handling mechanism which latches off output channels upon open or short circuit of the LED strings pre venting substantial damages...

Страница 3: ...Evaluation Board Schematic 30127101 FIGURE 1 LM3464A Evaluation Board Schematic www national com 2 AN 2071 ...

Страница 4: ... RTHM2 Chip Resistor 232ohm 1 0603 0603 CRCW0603232RFKEA Vishay RDMIN1 Chip Resistor 15 4Kohm 1 0603 0603 CRCW060315K4FKEA Vishay RDMIN2 Chip Resistor 1 05Kohm 1 0603 0603 CRCW06031K05FKEA Vishay RDHC Chip Resistor 2 67Kohm 1 0603 0603 CRCW06032K67FKEA Vishay RFB1 Chip Resistor 48 7Kohm 1 0603 0603 CRCW060348K7FKEA Vishay RFB2 Chip Resistor 2 67Kohm 1 0603 0603 CRCW06032K67FKEA Vishay RISNS1 RISNS...

Страница 5: ... LED4 Output Channel 4 Connect to cathode of LED string 4 EN LM3464A enable pin pull down to disable VFB Connect to voltage feedback node of primary power supply for DHC FAULTb Acknowledgement signal for arising of FAULT DIM PWM dimming signal input TTL signal compatible SYNC Synchronization signal for cascade operation THM Connect to NTC thermal sensor for thermal foldback control THM Connect to ...

Страница 6: ...erve voltage headroom for DHC to take place This is achieved by modifying the resistance of the output voltage sensing resistors of the AC DC power supply To adapt the AC DC power supply to the LM3464A evaluation board the nominal output voltage of the AC DC power supply is recommended to be reduced from 48V to 36V Usually the nominal output voltage of the AC DC power supply can be reduced by chan...

Страница 7: ...from the DHC loop to sustain normal operation of the remaining LED strings The LM3464A will maintain the failed channels in disable state until the EN pin is pulled low or the entire system is re powered When the test is completed the LM3464A enables the output channels and provides constant current to the LED strings The level of VDHC_READY is defined by the values of RFB1 and RFB2 on the evaluat...

Страница 8: ...ents of the LM3464A evaluation board can be pro grammed individually by changing the value of the resistors RISNS1 RISNS2 RISNS3 and RISNS4 accordingly The LED driving current is governed by the following equation 8 Adjusting Frequency Response of the LM3464A Circuit The frequency response of the LM3464A evaluation board can be adjusted by changing the value of the capacitor CDHC Higher capacitanc...

Страница 9: ...dback dimming fre quency at 258Hz Thermal foldback control is activated when the voltage at the Thermal pin VThermalis in between 3 25V and 0 4V as shown in figure 5 Thermal foldback control begins when VThermal is below 3 25V The LED current will be reduced to zero as VThermal falls below 0 4V The average LED current varies ac cording to the Thermal pin voltage following the equation 11 30127113 ...

Страница 10: ...age at the Thermal pin and DTHMFB is the dimming duty cycle under thermal fold back control When thermal foldback begins 17 18 19 When the temperature goes up to 125 C 20 By combining the equations 17 and 18 the values of RTHM1 and RTHM2 can be obtained 21 The default values of RTHM1 and RTHM2 on the LM3464A evaluation board are 4 87kΩ and 232Ω respectively Minimum Dimming Duty Cycle for Thermal F...

Страница 11: ... on the MASTER unit propagates the PWM dimming signal on its DIM pin to the slave units one by one through the SYNC pin PWM dimming control is allowed when thermal foldback control is activated When PWM dimming and ther mal foldback controls are required simultaneously the PWM dimming frequency should be set at least ten times below the thermal foldback dimming frequency The thermal foldback dimmi...

Страница 12: ...Cascade Operation 30127162 FIGURE 8 Cascading LM3464A Evaluation Boards for Output Channel Expansion 11 www national com AN 2071 ...

Страница 13: ... nections are required Connect the VIN terminal of master and slave units together and then to the POSITIVE output of the AC DC power supply Connect the PGND terminal of master and slave units to gether and then to the NEGATIVE output of the AC DC power supply Connect the SYNC terminal of the master unit to DIM ter minal of the next slave unit in the chain Connect the VFB terminal of master and sl...

Страница 14: ...ns of the LM3464A The re verse voltage of the selected zener diodes must not exceed the rated voltage of the corresponding pin For the LM3464A evaluation board the reverse voltage of the additional zener diodes must not exceed 95V The resistors RDR1 RDR2 RDR3 RDR4 and RIN are resistors for absorbing the voltage difference across the DRx pins and VRAIL Calculating the Values of Zx and RDRx Since th...

Страница 15: ... 2kΩ resistor with 0 25W power rating 1206 package and 1 tolerance can be used Calculating the Values of ZIN and RIN Assume the VIN pin of the LM3464A is about to be clamped to 75V a 1 5W 75V zener diode CMZ5946B from Central Semiconductor is used to ensure adequate conduction cur rent for ZIN Because the reverse current of the CMZ5946B is 5mA at 75V the allowable current flows through ZIN is in b...

Страница 16: ...lve power LEDs with four output channels active and output current per channel 350mA TA 25 C unless otherwise specified Output Current Variation 30127120 Efficiency 30127121 VCC Variation 30127122 DHC in Cascade Operation 30127123 VSYNC of Master Unit 30127124 VSYNC at System Startup 30127125 15 www national com AN 2071 ...

Страница 17: ...Evaluation Board Layout 30127126 FIGURE 11 Top Layer and Top Overlay 30127127 FIGURE 12 Bottom Layer and Bottom Overlay www national com 16 AN 2071 ...

Страница 18: ...Notes 17 www national com AN 2071 ...

Страница 19: ...ETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS BUYERS SHOULD PROVIDE ADEQUATE DESIGN TESTING AND OPERATING SAFEGUARDS EXCEPT AS PROVIDED IN NATIONAL S T...

Страница 20: ...for use in safety critical applications such as life support where a failure of the TI product would reasonably be expected to cause severe personal injury or death unless officers of the parties have executed an agreement specifically governing such use Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications and acknowledge and agre...

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