LTC1697EMS データシートの表示（PDF） - Linear Technology

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LTC1697EMS

High Efficiency Low Power 1W CCFL Switching Regulator

Linear Technology 

LTC1697

APPLICATIO S I FOR ATIO

VDIM pin where DC = 0% at VDIM = 1V and DC = 100% at

VDIM = 2V. If desired, the internal dimming PWM can be

overridden by grounding the CDIM pin and applying the

input PWM signal to the VDIM pin. This external PWM

signal should be in the range of 150Hz to 500Hz.

Lamp Feedback

In a typical application, the LAMP pin is connected to the

low voltage side of the lamp. The lamp pin is internally

connected to ground by a ~50Ω resistor. This resistor will

limit the voltage on the LAMP pin to ±0.35V for a 5mARMS

lamp current. The lamp feedback circuit removes a current

from VC approximating 1/40 of the absolute value of the

current through the 50Ω resistor.

Current Programming Input (RPROG)

The ILAMP current is set with an external resistor con-

nected between this pin and ground. ILAMP = 32V/RPROG.

VC Compensation

The VC node is the point where the lamp feedback current,

the programming current, and the control for the switch-

ing controller meet. A single capacitor must be connected

from the VC pin to ground to compensate the feedback

loop. Careful consideration should be given to the value of

capacitance used. A large value (1µF) will give excellent

stability at high lamp currents but will result in degraded

line regulation. On the other hand, a small value (10nF) will

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 0.1µF. For further information on

compensation please refer to Application Note 65 or

consult the factory.

OVSEN Operation

The OVSEN pin can be used to protect the high voltage

transformer from an overvoltage condition that can occur

when the lamp is open or not present. Connect this pin

through a resistor to the emitters of the drive transistors

of the Royer oscillator. The voltage at the OVSEN pin is

specified by VOVSEN. The duty cycle of the LTC1697 SW pin

will be 0% when the current flowing out of the OVSEN pin

reaches IOVSEN (protect). See the manufacturers trans-

former specifications for transformer voltage ratings.

Thermal Shutdown

This IC includes overtemperature protection that is in-

tended to protect the device during momentary overload

conditions. Junction temperature will exceed 125°C when

overtemperature protection is active. Continuous opera-

tion above the specified maximum operating junction

temperature may result in device degradation or failure.

Shutdown Operation

There are two ways to place the LTC1697 in shutdown. The

SHDN pin can be pulled below VSHDN-1, or the VDIM pin can

be pulled below VDIM(SD-I) for more than approximately

50mS. For normal operation, both pins must be pulled

high. The SHDN pin must be pulled above VSHDN-H, and the

VDIM pin must be pulled above VDIM (SD-H).

1697f

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