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DS3991VT Datasheet(PDF) 9 Page - Maxim Integrated Products |
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DS3991VT Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 16 page Low-Cost CCFL Controller _______________________________________________________________________________________ 9 If the PWM_EN pin is tied high, the digital control method is enabled and an external PWM signal between 80Hz and 300Hz is applied at the POSC/PWM pin to set the brightness of the lamp. In the digital con- trol method, the SLOPE and BRIGHT pins are not used. Lamp Strike On lamp strike, the DS3991 boosts the normal operating lamp frequency by 33%. This is done to increase the voltage created and help ensure that the lamp strikes. Once the controller detects that the lamp has struck, the frequency is returned to the normal lamp frequency. Setting the Lamp and DPWM Frequencies Using External Resistors Both the lamp and DPWM frequencies are set using external resistors. The resistance required for either fre- quency can be determined using the following formula: where K = 4000k Ω x kHz for lamp frequency calcula- tions, K = 4k Ω x kHz for DPWM frequency calculations. Example: Select the resistor values to configure the DS3991 to have a 50kHz lamp frequency and a 160Hz DPWM frequency. For the DPWM resistor calculation, K = 4 k Ω x kHz. For the lamp frequency resistor (RLOSC) calculation, K = 4000 k Ω x kHz. The formula above can now be used to calculate the resistor values for RLOSC and RPOSC as follows: Supply Monitoring The DS3991 has supply-voltage monitors (SVML and SVMH) for the inverter’s DC supply (VINV) and an undervoltage lockout for the VCC supply to ensure that voltage levels are adequate for proper operation. The inverter supply is monitored for overvoltage conditions at the SVMH pin and undervoltage conditions at the SVML pin. External resistor-dividers at each SVM input feed into two comparators, both having 2V thresholds (see Figure 2). Using the equation below to determine the resistor values, the SVMH and SVML trip points (VTRIP) can be customized to shut off the inverter when the inverter supply voltage rises above or drops below specified values. Operating with the inverter supply at too low of a level can prevent the transformer from reaching the strike voltage and could potentially cause numerous other problems. Operating with the inverter voltage at too high of a level can be damaging to the inverter components. Proper use of the SVMs can prevent these problems. If desired, the high and/or low SVMs can be disabled by connecting the SVMH pin to GND and the SVML pin to VCC. The SVMH and SVML are high-impedance inputs and noise on the inverter supply can cause the monitors to inadvertently trigger even though the inputs contain hys- teresis. The user may wish to add a lowpass filter to reduce the noise present at the SVMH and SVML inputs. The VCC monitor is a 5V supply undervoltage lockout (UVLO) that prevents operation when the DS3991 does not have adequate voltage for its analog circuitry to operate or to drive the external MOSFETs. The VCC monitor features hysteresis to prevent VCC noise from VTRIP = 2.0 R1 + R2 R1 RPOSC = 4k kHz 0.160kHz = 25k RLOSC = 4000k kHz 50kHz = 80k ROSC = K fOSC 80Hz TO 300Hz LAMP CURRENT SOFT-START BURST-DIMMING PWM SIGNAL (EITHER CREATED INSIDE THE DS3991 OR SOURCED AT THE POSC/PWM PIN) Figure 1. Digital PWM Dimming and Soft-Start |
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