Datasheet LT1074, LT1076 (Analog Devices) - 9

LT1074/LT1076
U U PI DESCRIPTIO S
current is well controlled even when switch duty cycle
SHUTDOWN PIN
must be extremely low. Theoretical switch “on” time for a The shutdown pin is used for undervoltage lockout, micro- buck converter in continuous mode is: power shutdown, soft-start, delayed start, or as a general purpose on/off control of the regulator output. It controls V V t OUT D ON = + switching action by pulling the ILIM pin low, which forces V • f IN the switch to a continuous “off” state. Full micropower shutdown is initiated when the shutdown pin drops below VD = Catch diode forward voltage ( ≈ 0.5V) 0.3V. f = Switching frequency The V/I characteristics of the shutdown pin are shown in At f = 100kHz, tON must drop to 0.2µs when VIN = 25V Figure 4. For voltages between 2.5V and ≈V and the output is shorted (V IN, a current of OUT = 0V). In current limit, 10µA flows out of the shutdown pin. This current in- the LT1074 can reduce tON to a minimum value of ≈ creases to ≈25µA as the shutdown pin moves through the 0.6µs, much too long to control current correctly for 2.35V threshold. The current increases further to ≈30µA at VOUT = 0. To correct this problem, switching frequency the 0.3V threshold, then drops to ≈15µA as the shutdown is lowered from 100kHz to 20kHz as the FB pin drops voltage fall below 0.3V. The 10µA current source is in- from 1.3V to 0.5V. This is accomplished by the circuitry cluded to pull the shutdown pin to its high or default state when left open. It also provides a convenient pull-up for TO OSCILLATOR delayed start applications with a capacitor on the shut- V down pin. OUT +2V Q1 When activated, the typical collector current of Q1 in R1 + Figure 5, is 2.21V R3 ≈2mA. A soft-start capacitor on the ILIM pin will ERROR 3k delay regulator shutdown in response to C1, by AMPLIFIER EXTERNAL VC – DIVIDER ≈(5V)(CLIM)/2mA. Soft-start after full micropower shut- FB R2 down is ensured by coupling C2 to Q1. 2.21k 0 LT1074•PD03 T = 25 j °C –5
Figure 3. Frequency Shifting
CURRENT FLOWS OUT OF SHUTDOWN PIN –10 shown in Figure 3. µA) –15 SHUTDOWN THRESHOLD Q1 is off when the output is regulating (V –20 FB = 2.21V). As the output is pulled down by an overload, V –25 FB will eventu- CURRENT ( ally reach 1.3V, turning on Q1. As the output continues to –30 drop, Q1 current increases proportionately and lowers the –35 frequency of the oscillator. Frequency shifting starts when –40 the output is ≈ 60% of normal value, and is down to its 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 minimum value of ≅ 20kHz when the output is ≅ 20% of VOLTAGE (V) LT1074•PC05 normal value. The rate at which frequency is shifted is
Figure 4. Shutdown Pin Characteristics
determined by both the internal 3k resistor R3 and the external divider resistors. For this reason, R2 should not be increased to more than 4kΩ, if the LT1074 will be subjected to the simultaneous conditions of high input voltage and output short-circuit. sn1074 1074fds 9