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LT124XCS8 Datasheet(PDF) 7 Page - Linear Integrated Systems |
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LT124XCS8 Datasheet(HTML) 7 Page - Linear Integrated Systems |
7 / 16 page 7 LT1241 Series PI FU C S TI COMP (Pin 1): Compensation Pin. This pin is the output of the Error Amplifier and is made available for loop compen- sation. It can also be used to adjust the maximum value of the current sense clamp voltage to less than 1V. This pin can source a minimum of 0.5mA (0.8mA typ) and sink a minimum of 2mA (4mA typ) FB (Pin 2) Voltage Feedback Pin. This pin is the inverting input of the error amplifier. The output voltage is normally fed back to this pin through a resistive divider. The non- inverting input of the error amplifier is internally commit- ted to a 2.5V reference point. ISENSE (Pin 3): Current Sense Pin. This is the input to the current sense comparator. The trip point of the compara- tor is set by, and is proportional to, the output voltage of the Error Amplifier. RT/CT (Pin 4): The oscillator frequency and the deadtime are set by connecting a resistor (RT) from VREF to RT/CT and a capacitor (CT) from RT/CT to GND. The rise time of the oscillator waveform is set by the RC time constant of RT and CT. The fall time, which is equal to the output deadtime, is set by a combination of the RC time constant and the oscillator sink current (8.2mA typ). GND (Pin 5): Ground. OUTPUT (Pin 6): This pin is the output of a high current totem pole output stage. It is capable of driving up to ±1A of current into a capacitive load such as the gate of a MOSFET. VCC (Pin 7): This pin is the positive supply of the control IC. VREF (Pin 8): Reference. This is the reference output of the IC. The reference output is used to supply charging current to the external timing resistor RT. The reference provides biasing to a large portion of the internal circuitry, and is used to generate several internal reference levels includ- ing the VFB level and the current sense clamp voltage. S APPLICATI I FOR ATIO MINIMUM START-UP OPERATING MAXIMUM DEVICE THRESHOLD VOLTAGE DUTY CYCLE REPLACES LT1241 9.6V 7.6V 50% NONE LT1242 16V 10V 100% UC1842 LT1243 8.4V 7.6V 100% UC1843 LT1244 16V 10V 50% UC1844 LT1245 8.4V 7.6V 50% UC1845 Oscillator The LT1241 series devices are fixed frequency current mode pulse width modulators. The oscillator frequency and the oscillator discharge current are both trimmed and tightly specified to minimize the variations in frequency and deadtime. The oscillator frequency is set by choosing a resistor and capacitor combination, RT and CT. This RC combination will determine both the frequency and the maximum duty cycle. The resistor RT is connected from VREF (Pin 8) to the RT/CT pin (Pin 4). The capacitor CT is connected from the RT/CT pin to ground. The charging current for CT is determined by the value of RT. The discharge current for CT is set by the difference between the current supplied by RT and the discharge current of the LT124X. The discharge current of the device is trimmed to 8.2mA. For large values of RT discharge time will be determined by the discharge current of the device and the value of CT. As the value of RT is reduced it will have more effect on the discharge time of CT. During an oscillator cycle capacitor CT is charged to approximately 2.8V and discharged to approximately 1.1V. The output is enabled during the charge time of CT and disabled, in an off state, during the discharge time of CT. The deadtime of the circuit is equal to the discharge time of CT. The maximum duty cycle is limited by controlling the deadtime of the oscilla- tor. There are many combinations of RT and CT that will yield a given oscillator frequency, however there is only one combination that will yield a specific deadtime at that frequency. Curves of oscillator frequency and deadtime |
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