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LT1316CS8 Datasheet(PDF) 6 Page - Linear Technology |
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LT1316CS8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LT1316 APPLICATIONS INFORMATION 10 µs/DIV 1316 F02 Figure 2. Switching Waveforms RSET (kΩ) 10 100 1000 10 100 1316 F03 Figure 3. DC Current Limit vs RSET Resistor Note: DC Current is the Peak Switch Current if the Power Transistor had Zero Turn-Off Delay VOUT AC COUPLED 200mV/DIV VSW 5V/DIV INDUCTOR CURRENT 100mA/DIV During the portion of the switch cycle when Q1 is turned off, current is forced through D1 to C1 causing output voltage to rise. This switching action continues until output voltage rises enough to overcome A1’s hysteresis. Peak switch current is set by a resistor from the RSET pin to ground. Voltage at the RSET pin is forced to 0.5V by A4 and is used to set up a constant current through R5. This current also flows through R3 which sets the voltage at the positive input of comparator A2. When Q1 turns on, the SW pin goes low and current ramps up at the rate VIN/L. Current through Q2 is equal to Q1’s current divided by 200. When current through Q2 causes the voltage drop across R4 and R3 to be equal, A2 changes state and resets the oscillator, causing Q1 to turn off. Shutdown is accom- plished by grounding the SHDN pin. The low-battery detector A3 has its own 1.17V reference and is always on. The open collector output device can sink up to 500 µA. Approximately 35mV of hysteresis is built into A3 to reduce “buzzing” as the battery voltage reaches the trip level. Current Limit During active mode when the part is switching, current in the inductor ramps up each switch cycle until reaching a preprogrammed current limit. This current limit value must be set by placing the appropriate resistor from the RSET pin to ground. This resistance value can be found by using Figure 3 to locate the desired DC current limit and Table 1 simplifies component selection for commonly used input and output voltages. The methods used in determining these values are discussed in more detail later in this data sheet. VOUT can be set using the equation: VOUT = 1.23 R2 + R1 R2 ) ) R1 VOUT R2 1316 EQF01 FB Table 1. RSET Resistor and Inductor Values LOAD RSET PEAK SWITCH VIN VOUT CURRENT RESISTOR INDUCTOR CURRENT 2 5 10mA 36.8k 100 µH 80mA 2 5 25mA 18.2k 68 µH 165mA 2 5 50mA 10k 47 µH 320mA 2 5 75mA 6.81k 33 µH 500mA 5 12 100mA 6.81k 82 µH 490mA 5 28 1mA 75k 100 µH 56mA 5 28 5mA 22.1k 100 µH 140mA 5 28 10mA 10k 100 µH 270mA Operation To understand operation of the LT1316, first examine Figure 1. Comparator A1 monitors FB voltage which is VOUT divided down by resistor divider network R1/R2. When voltage at the FB pin drops below the reference voltage (1.23V), A1’s output goes high and the oscillator is enabled. The oscillator has an off-time fixed at 2 µs and an on-time limited to 6.3 µs. Power transistor Q1 is cycled on and off by the oscillator forcing current through the inductor to alternately ramp up and down (see Figure 2). |
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