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LT1245 Datasheet(PDF) 9 Page - Linear Technology |
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LT1245 Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LT1241 Series S APPLICATI I FOR ATIO change in the error amplifier output voltage. The threshold voltage will be 0.333V for an error amplifier voltage of 2.4V. To reduce the maximum current sense threshold to less than 1V the error amplifier output should be clamped to less than 4.4V. Blanking A unique feature of the LT1241 series devices is the built- in blanking circuit at the output of the current sense comparator. A common problem with current mode PWM circuits is erratic operation due to noise at the current sense input. The primary cause of noise problems is the leading edge current spike due to transformer interwinding capacitance and diode reverse recovery time. This current spike can prematurely trip the current sense comparator causing an instability in the regulator circuit. A filter at the current sense input is normally required to eliminate this instability. This filter will in turn slow down the current sense loop. A slow current sense loop will increase the minimum pulse width which will increase the short-circuit current in an overload condition. The LT1241 series devices blank (lock out) the signal at the output of the current sense compara- tor for a fixed amount of time after the switch is turned on. This effectively prevents the PWM latch from tripping due to the leading edge current spike. The blanking time will be a function of the voltage at the feedback pin (Pin 2). The blanking time will be 100ns for normal operating conditions (VFB = 2.5V). The blanking time goes to zero as the feedback pin is pulled to 0V. This means that the blanking time will be minimized during start-up and also during an output short-circuit fault. This blanking circuit eliminates the need for an input filter at the current sense input except in extreme cases. Eliminating the filter allows the current sense loop to operate with minimum delays, reducing peak currents during fault conditions. Current Sense Comparator and PWM Latch LT1241 series devices are current mode controllers. Under normal operating conditions the output (Pin 6) is turned on at the start of every oscillator cycle, coincident with the rising edge of the oscillator waveform. The output is then turned off when the current reaches a threshold level proportional to the error voltage at the output of the error amplifier. Once the output is turned off it is latched off until the start of the next cycle. The peak current is thus proportional to the error voltage and is 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 fed into the current sense input. For normal operating conditions the peak inductor current, which is equal to the peak switch current, will be equal to: I VV R PK PIN S = − () () 1 14 3 . During fault conditions the maximum threshold voltage at the input of the current sense comparator is limited by the internal 1V clamp at the inverting input. The peak switch current will be equal to: I V R PK MAX S () . = 10 In certain applications, such as high power regulators, it may be desirable to limit the maximum threshold voltage to less than 1V in order to limit the power dissipated in the sense resistor or to limit the short-circuit current of the regulator circuit. This can be accomplished by clamping the output of the error amplifier. A voltage level of approximately 1.4V at the output of the error amplifier will give a threshold voltage of 0V. A voltage level of approxi- mately 4.4V at the output of the error amplifier will give a threshold level of 1V. Between 1.4V and 4.4V the threshold voltage will change by a factor of one-third of the |
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