LT1768IGN datasheet(12/20 Pages) LINER

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LT1768

APPLICATIONS INFORMATION

and is dependant on the operating mode. For dual lamp

displays, the transfer function for minimum current mode

(IDIO/IRMIN) is equal to 10A/A, and for maximum current

mode (IDIO/IRMAX) is equal to 100A/A.

The transfer functions discussed above are between RMAX

and RMIN current and average lamp current not RMS lamp

current. Due to the differences between the average and

RMS functions, the actual overall transfer function be-

tween actual lamp current and RMIN/RMAX current must be

empirically determined, and is dependant on the particular

lamp/display housing combination used. For example, in

the circuit of Figure 1 setting RRMIN to 10kΩ and RRMAX to

16.8

Ω, sets the minimum and maximum RMS lamp

currents for the example display to 1mA and 9mA per lamp

respectively. Figure 4 shows the lamp current vs program-

ming voltage for the circuit in Figure 1.

RRMIN adjusted to produce the specified current. If a wide

dimming range is desired, VPROG should be set to 0.75V

and RRMIN adjusted to produce the required dimming

ratio. Care must be taken when adjusting RRMIN to pro-

duce extreme dimming ratios. The minimum lamp current

set by RRMIN must be able to fully illuminate the lamp or

thermometering (uneven illumination) will occur. If the

desired dimming ratio can’t be achieved by adjusting

RRMIN, the minimum lamp current can be set to zero by

connecting the RMIN pin to the VREF pin. If the minimum

current is set to less than the open lamp threshold current

(approximately 125

µA), the FAULT pin will be activated for

PROG voltages between 0.5V and 1V.

The values chosen for RRMAX and RRMIN are extremely

critical in determining the lifetime of the display. It is

imperative that proper measurement techniques, such as

those cited in the references, be used when determining

RRMAX and RRMIN values.

Lamp Fault Modes and Single Lamp Operation

The DIO pin diodes that conduct on the positive cycle are

used to detect open lamp fault conditions. If the current

in either of the DIO pins on the positive half cycle is less

than 125

µA due to either an open lamp or lamp lowside

short to ground, for a minimum of 1 PWM cycle, then the

FAULT pin will be activated and the lamp programming

current into the VC pin in high level PWM mode, linear

mode, and maximum current mode, will be reduced by

approximately 50%. Halving the VC source current will cut

the total lamp current to approximately one half of its

programmed value. This function insures that the maxi-

mum lamp current level set by RRMAX will not be exceeded

even under fault conditions. If the current in both of the

DIO pins on the positive cycle is less than 125

µA, and the

VC pin hits its clamp value (indicating an open lamp or

lamp lowside short to ground fault condition) for a mini-

mum of 1 PWM cycle, the gate drive will be latched off. The

latch can be cleared by setting the PROG voltage to zero or

placing the LT1768 in shutdown mode.

Since open lamp fault conditions produce high voltage AC

waveforms, it is imperative that proper layout spacings

between the high voltage and DIO lines be observed.

Coupling capacitance as low as 0.5pF between the high

Figure 4. Lamp Current vs PROG Voltage for

the Circuit in Figure 1

Choosing RRMAX and RRMIN and VPWM

The value for RRMAX should be determined by setting

VPROG to 4.5V then adjusting RRMAX to produce the

maximum allowable current specified by the lamp manu-

facturer.

The voltage for the PWM pin should then be set so that the

LT1768 normally operates in linear mode. A typical value

for VPWM is approximately 2.5V, which limits the PWM

region to 50% of the VPROG input voltage range.

The value for RRMIN should be chosen to either produce

the minimum manufacturer specified lamp current or

enable a wide dimming range. If a minimum specified

current is desired, the VPROG should be set to 0.75V and

VPROG(V)

1.0

0.5

5.0

4.0

3V (VPWM)

MIN

CURRENT

PWM

(FREQ = 220Hz)

MAX

CURRENT

LINEAR

100%

OFF

9mA

6mA

0mA

ICCFL (mA)

1768 F04

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