Datasheet LT1185 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Low Dropout Regulator |
| Pages / Page | 16 / 10 — APPLICATIO S I FOR ATIO. Thermal Regulation. Output Voltage Reversal |
| File Format / Size | PDF / 188 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Thermal Regulation. Output Voltage Reversal
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LT1185
U U W U APPLICATIO S I FOR ATIO Thermal Regulation
This shift in output voltage could be in either direction because K1 and K2 can be either positive or negative. IC regulators have a regulation term not found in discrete designs because the power transistor is thermally coupled Thermal regulation is already included in the worst case to the reference. This creates a shift in the output voltage reference specification. which is proportional to power dissipation in the regulator.
Output Voltage Reversal
∆VOUT = P(K1 + K2 θJA) Some IC regulators suffer from a latch-up state when their = (IOUT)(VIN – VOUT)(K1 + K2 θJA) output is forced to a reverse voltage of as little as one diode drop. The latch-up state can be triggered without a fault K1 and K2 are constants. K1 is a fast time constant effect condition when the load is connected to an opposite caused by die temperature gradients which are estab- polarity supply instead of to ground. If the second supply lished within 50ms of a power change. K1 is specified on is turned on first, it will pull the output of the first supply the data sheet as thermal regulation, in percent per watt. to a reverse voltage through the load. The first supply may K2 is a long time constant term caused by the temperature then latch off when turned on. This problem is particularly drift of the regulator reference voltage. It is also specified, annoying because the diode clamps which should always but in percent per degree centigrade. It must be multiplied be used to protect against polarity reversal do not usually by overall thermal resistance, junction-to-ambient, θJA. stop the latch-up problem. As an example, assume a 5V regulator with an input The LT1185 is designed to allow output reverse polarity of voltage of 8V, load current of 2A, and a total thermal several volts without damage or latch-up, so that a simple resistance of 4°C/W, including junction-to-case, (use diode clamp can be used. control area specification), interface, and heat sink resis- tance. K1 and K2, respectively, from the data sheet are 0.014%/W and 0.01%/°C. ∆VOUT = (2A)(8V – 5V)(0.014 + 0.01 • 4) = 0.32% 1185ff 10
U U W U APPLICATIO S I FOR ATIO Thermal Regulation
This shift in output voltage could be in either direction because K1 and K2 can be either positive or negative. IC regulators have a regulation term not found in discrete designs because the power transistor is thermally coupled Thermal regulation is already included in the worst case to the reference. This creates a shift in the output voltage reference specification. which is proportional to power dissipation in the regulator.
Output Voltage Reversal
∆VOUT = P(K1 + K2 θJA) Some IC regulators suffer from a latch-up state when their = (IOUT)(VIN – VOUT)(K1 + K2 θJA) output is forced to a reverse voltage of as little as one diode drop. The latch-up state can be triggered without a fault K1 and K2 are constants. K1 is a fast time constant effect condition when the load is connected to an opposite caused by die temperature gradients which are estab- polarity supply instead of to ground. If the second supply lished within 50ms of a power change. K1 is specified on is turned on first, it will pull the output of the first supply the data sheet as thermal regulation, in percent per watt. to a reverse voltage through the load. The first supply may K2 is a long time constant term caused by the temperature then latch off when turned on. This problem is particularly drift of the regulator reference voltage. It is also specified, annoying because the diode clamps which should always but in percent per degree centigrade. It must be multiplied be used to protect against polarity reversal do not usually by overall thermal resistance, junction-to-ambient, θJA. stop the latch-up problem. As an example, assume a 5V regulator with an input The LT1185 is designed to allow output reverse polarity of voltage of 8V, load current of 2A, and a total thermal several volts without damage or latch-up, so that a simple resistance of 4°C/W, including junction-to-case, (use diode clamp can be used. control area specification), interface, and heat sink resis- tance. K1 and K2, respectively, from the data sheet are 0.014%/W and 0.01%/°C. ∆VOUT = (2A)(8V – 5V)(0.014 + 0.01 • 4) = 0.32% 1185ff 10
