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LT1074CT Datasheet(PDF) 9 Page - Linear Technology |
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LT1074CT Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LT1074/LT1076 9 Figure 3. Frequency Shifting Figure 4. Shutdown Pin Characteristics current is well controlled even when switch duty cycle must be extremely low. Theoretical switch “on” time for a buck converter in continuous mode is: t VV Vf ON OUT D IN = + • VD = Catch diode forward voltage ( ≈ 0.5V) f = Switching frequency At f = 100kHz, tON must drop to 0.2µs when VIN = 25V and the output is shorted (VOUT = 0V). In current limit, the LT1074 can reduce tON to a minimum value of ≈0.6µs, much too long to control current correctly for VOUT = 0. To correct this problem, switching frequency is lowered from 100kHz to 20kHz as the FB pin drops from 1.3V to 0.5V. This is accomplished by the circuitry shown in Figure 3. Q1 is off when the output is regulating (VFB = 2.21V). As the output is pulled down by an overload, VFB will eventu- ally reach 1.3V, turning on Q1. As the output continues to drop, Q1 current increases proportionately and lowers the frequency of the oscillator. Frequency shifting starts when the output is ≈ 60% of normal value, and is down to its minimum value of ≅20kHz when the output is ≅ 20% of normal value. The rate at which frequency is shifted is determined by both the internal 3k resistor R3 and the external divider resistors. For this reason, R2 should not be increased to more than 4k Ω, if the LT1074 will be subjected to the simultaneous conditions of high input voltage and output short-circuit. DESCRIPTIO S PI SHUTDOWN PIN The shutdown pin is used for undervoltage lockout, micro- power shutdown, soft-start, delayed start, or as a general purpose on/off control of the regulator output. It controls switching action by pulling the ILIM pin low, which forces the switch to a continuous “off” state. Full micropower shutdown is initiated when the shutdown pin drops below 0.3V. The V/I characteristics of the shutdown pin are shown in Figure 4. For voltages between 2.5V and ≈VIN, a current of 10 µA flows out of the shutdown pin. This current in- creases to ≈25µA as the shutdown pin moves through the 2.35V threshold. The current increases further to ≈30µAat the 0.3V threshold, then drops to ≈15µA as the shutdown voltage fall below 0.3V. The 10 µA current source is in- cluded to pull the shutdown pin to its high or default state when left open. It also provides a convenient pull-up for delayed start applications with a capacitor on the shut- down pin. When activated, the typical collector current of Q1 in Figure 5, is ≈2mA. A soft-start capacitor on the ILIM pinwill delay regulator shutdown in response to C1, by ≈(5V)(CLIM)/2mA. Soft-start after full micropower shut- down is ensured by coupling C2 to Q1. + – 2.21V +2V ERROR AMPLIFIER VC FB R2 2.21k R1 VOUT EXTERNAL DIVIDER R3 3k Q1 TO OSCILLATOR LT1074•PD03 VOLTAGE (V) 0 –40 1.0 2.0 LT1074•PC05 1.5 2.5 3.0 3.5 4.0 –35 –30 –25 –20 –15 –10 –5 0 SHUTDOWN THRESHOLD CURRENT FLOWS OUT OF SHUTDOWN PIN = 25 °C Tj 0.5 |
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