LT1768IGN datasheet(13/20 Pages) LINER

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LT1768

APPLICATIONS INFORMATION

voltage and DIO lines can cause enough current flow to

fool the open lamp detection. In situations where coupling

can’t be avoided, resistors can be added from the DIO pins

to ground to increase the open lamp threshold. When

resistors from the DIO pins to ground are added, the

values for RRMAX and RRMIN may need to be increased

from their nominal values to compensate for the additional

current.

For single lamp operation, the lowside of the lamp should

be connected to both DIO pins, and the values of RRMAX

and RRMIN increased to two times the values that would be

used in a dual lamp configuration. In single lamp mode all

fault detection will operate as in the dual lamp configura-

tion, but the open lamp threshold will double. If the

increase in the open lamp threshold is not acceptable, a

positive offset current can be added to reduce the open

lamp threshold by placing a resistor between the REF and

DIO pins (a 33k resistor will reduce the open lamp thresh-

old by approximately 100

µA ((VREF– VDIO+)/33k). When

an offset current is added, the values for RRMAX and RRMIN

may need to be increased from their nominal values to

compensate for the offset current.

VC Compensation

As previously mentioned a single capacitor on the VC pin

combines the error signal conversion, lamp current aver-

aging and frequency compensation. Careful consideration

should be given to the value of capacitance used. A large

value (1

µF) will give excellent stability at high lamp cur-

rents but will result in degraded line regulation in PWM

mode. On the other hand , a small value (10nF) will give

excellent PWM response but might result in overshoot and

poor load regulation. The value chosen will depend on the

maximum load current and dimming range. After these

parameters are decided upon, the value of the VC capacitor

should be increased until the line regulation becomes

unacceptable. A typical value for the VC capacitor is

0.033

µF. For further information on compensation please

refer to the references or consult the factory.

Current Sense Comparator

The LT1768 is a current mode PWM controller. Under

normal operating conditions the GATE is driven high at the

start of every oscillator cycle. The GATE is driven back low

when the current reaches a threshold level proportional to

the voltage on the VC pin. The GATE then remains low until

the start of the next oscillator cycle. The peak current is

thus proportional to the VC voltage and controlled on a

cycle by cycle basis. The peak switch current is normally

sensed by placing a sense resistor in the source lead of the

output MOSFET. This resistor converts the switch current

to a voltage that can be compared to a fraction of the VC

voltage [(VVC – VDIODE)/30] . For normal conditions and a

GATE duty cycle below 50%, the switch current limit will

correspond to IPK = 0.1/RSENSE. For GATE duty cycles

above 50% the switch current limit will be reduced

to approximately 90mV at 80% duty cycle to avoid

subharmonic oscillations associated with current mode

controllers.

When the lamp current is programmed to PWM mode, the

VC pin will slew between voltages that represent the

minimum and maximum PWM lamp currents. The slew

time affects the line regulation at low duty cycle, and

should be kept low by making the sense resistor as small

as possible. The lowest value of sense resistor is deter-

mined by switching transients and other noise due to

layout configurations. A good rule of thumb is to set the

sense resistor so that the voltage on the VC pin equals

2.5V when the PWM current is in maximum mode (VPROG

= VPWM). Typical values of the sense resistor run in the

25m

Ω to 50mΩ range for large displays, and can be

implemented with a copper trace on the PCB.

Since the maximum threshold at the SENSE pin is only

100mV, switching transients and other noise can prema-

turely trip the comparator. The LT1768 has a blanking

period of 100ns which prohibits premature switch turn

off, but further filtering the sense resistor voltage is

recommended. A simple RC filter is adequate for most

applications. (Figure 5.)

Figure 5. Sense Pin Filter

SENSE

GATE

LT1768

100

Ω

2.2nF

0.025m

Ω

1768 •F05

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