Datasheet LT1316 (Analog Devices) - 6

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