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LT1950IGN Datasheet(PDF) 11 Page - Linear Technology |
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LT1950IGN Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT1950 11 sn1950 1950fs APPLICATIO S I FOR ATIO Shutdown and Undervoltage Lockout Figure 4 shows how to program undervoltage lockout (UVLO) for the VIN supply. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, hence source current increases as source voltage drops. This looks like a negative load resistance to the source and can cause the source to current limit or latch low under low source voltage conditions. An internally set undervoltage lockout (UVLO) threshold prevents the regulator from operating at source voltages where these problems might occur. An internal comparator will force the part into shutdown below the minimum VIN of 2.6V. This feature can be used to prevent excessive discharge of battery-operated sys- tems. Alternatively, UVLO threshold is adjustable. The shutdown threshold voltage of the SHDN pin is 1.32V. Forcing the SHDN pin below this 1.32V threshold causes VREF to be disabled and stops switching at the GATE pin. If the SHDN pin is left open circuit, a permanent 3 µA flows out of the pin to ensure that the pin defaults high to allow normal operation. Voltages above the 1.32V threshold cause an extra 7 µA to be sourced out of the pin, providing current hysteresis. This can be used to set voltage hyster- esis of the UVLO threshold using the following equations: R VV A R V VV R A HL H 1 7 2 132 132 1 3 = µ = +µ – . (– . ) VH = Turn on threshold VL = Turn off threshold Example: switching should not start until the input is above 11V and is to stop if the input falls below 9V. VH = 11V VL = 9V R VV A k R V VV k A k 1 11 9 7 286 2 132 11 1 32 286 3 36 = µ = = +µ = – . (– . ) Keep the connections from the resistors to the SHDN pin short and make sure that the interplane or surface capaci- tance to the switching nodes are minimized. If high resis- tance values are used, the SHDN pin should be bypassed with a 1nF capacitor to prevent coupling problems from the switch node. Figure 4. Undervoltage Lockout 3 µA 7 µA 1.32V GND VREF VIN R1 R2 C1 + 1950 F04 LT1950 Generating VIN2 Supply Using BOOST Pin The LT1950’s BOOST pin is used to provide a “boosted” 11V supply at the VIN2 pin for VIN voltages as low as 3V. Since VIN2 supplies the output driver for the GATE pin of the IC, it is advantageous to generate a boosted VIN2. This architecture achieves high GATE drive for an external |
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