Datasheet LT1175 (Analog Devices) - 9

LT1175
APPLICATIONS INFORMATION Note to Reader:
To avoid confusion when working with The LT1175-5 is a fi xed 5V design with the SENSE pin negative voltages (is –6V more or less than –5V?), I have acting as a Kelvin connection to the output. Normally the decided to treat the LT1175 as if it were a positive regulator SENSE pin and the OUTPUT pin are connected directly and express all voltages as positive values, both in text and together, either close to the regulator or at the remote in formulas. If you do the same and simply add a negative load point. sign to the eventual answer, confusion should be avoided. SHUTDOWN > 2V OR < –2V TO Please don’t give me a hard time about “preciseness” or LOGIC TURN REGULATOR ON “correctness.” I have to fi eld phone calls from around the world and this is my way of dealing with a multitude + + R1 of conventions. Thanks for your patience. CIN 383k SHDN GND 1% COUT VIN SENSE ≥ 0.1μF
Setting Output Voltage
R2 LT1175 I 825k LIM2 1% The LT1175 adjustable version has a feedback sense VOUT ILIM4 OUTPUT –12V voltage of 3.8V with a bias current of approximately 75nA 1175 F01 fl owing out of the SENSE pin. To avoid output voltage
Figure 1. Typical LT1175 Adjustable Connection
errors caused by this current, the output divider string (see Figure 1) should draw about 25μA. Table 1 shows
Setting Current Limit
suggested resistor values for a range of output voltages. The LT1175 uses two ILIM pins to set current limit (typical) The second part of the table shows resistor values which at 200mA, 400mA, 600mA or 800mA. The corresponding draw only 10μA of current. Output voltage error caused minimum guaranteed currents are 130mA, 260mA, 390mA by bias current with the lower valued resistors is about and 520mA. This allows the user to select a current limit 0.4% maximum and with the higher values, about 1% tailored to his specifi c application and prevents the situa- maximum. A formula is also shown for calculating the tion where short-circuit current is many times higher than resistors for any output voltage. full-load current. Problems with input supply overload or
Table 1. Suggested Divider Resistors
excessive power dissipation in a faulted load are prevented.
OUTPUT R1 R2 R1 R2
Power limiting in the form of foldback current limit is built
VOLTAGE IDIV = 25μA NEAREST 1% IDIV = 10μA NEAREST 1%
in and reduces current limit as a function of input-to-output 5V 150k 47.5k 383k 121k voltage differential for differentials exceeding 14V. See the 6V 150k 86.6k 383k 221k graph in Typical Performance Characteristics. The LT1175 8V 150k 165k 383k 422k is guaranteed to be blowout-proof regardless of current 10V 150k 243k 383k 619k limit setting. The power limiting combined with thermal 12V 150k 324k 383k 825k shutdown protects the device from destructive junction 15V 150k 442k 383k 1.13M temperatures under all load conditions. 3 8 . V
Shutdown
R1= I In shutdown, the LT1175 draws only about 10μA. Special DIV R1 V − 3 8 . V ( ) circuitry is used to minimize increases in shutdown cur- OUT R2 = Simple formula rent at high temperatures, but a slight increase is seen . ( ) 3 8V above 125°C. One option not taken was to actively pull R1 V − 3 8 . V ( ) OUT ⎛ ⎞ Taking SENSE pin bias down on the output during shutdown. This means that the R2 = output will fall slowly after shutdown is initiated, at a rate 3 8 . V + R1 I ( ) current into account FB ⎝⎜ ⎠⎟ determined by load current plus the 12μA internal load, and the size of the output capacitor. Active pull-down is I = Desired DIV divider current 1175ff 9