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LT1054CN8 Datasheet(PDF) 9 Page - Linear Technology |
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LT1054CN8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1054/LT1054L 1054lfe APPLICATIONS INFORMATION RX = VX/(4.4 IOUT) where VX ≈ VIN – [(LT1054 Voltage Loss)(1.3) + |VOUT|] and IOUT = maximum required output current. The factor of 1.3 will allow some operating margin for the LT1054. For example: assume a 12V to – 5V converter at 100mA output current. First calculate the power dissipation with- out an external resistor: P = (12V – |–5V|)(100mA) + (12V)(100mA)(0.2) P = 700mW + 240mW = 940mW At θJA of 130°C/W for a commercial plastic device this would cause a junction temperature rise of 122 °C so that the device would exceed the maximum junction tempera- ture at an ambient temperature of 25 °C. Now calculate the power dissipation with an external resistor (RX). First find how much voltage can be dropped across RX. The maxi- mum voltage loss of the LT1054 in the standard regulator configuration at 100mA output current is 1.6V, so VX = 12V – [(1.6V)(1.3) + |–5V|] = 4.9V and RX = 4.9V/(4.4)(100mA) = 11Ω This resistor will reduce the power dissipated by the LT1054 by (4.9V)(100mA) = 490mW. The total power dissipated by the LT1054 would then be (940mW – 490mW) = 450mW. The junction temperature rise would now be only 58 °C. Although commercial devices are guaranteed to be functional up to a junction temperature of 125 °C, the specifications are only guaranteed up to a junction temperature of 100 °C, so ideally you should limit the junction temperature to 100 °C. For the above example this would mean limiting the ambient temperature to 42 °C. Other steps can be taken to allow higher ambient tempera- tures. The thermal resistance numbers for the LT1054 packages represent worst case numbers with no heat sinking and still air. Small clip-on type heat sinks can be used to lower the thermal resistance of the LT1054 pack- age. In some systems there may be some available airflow which will help to lower the thermal resistance. Wide PC board traces from the LT1054 leads can also help to remove heat from the device. This is especially true for plastic packages. where dV = peak-to-peak ripple and f = oscillator frequency. For output capacitors with significant ESR a second term must be added to account for the voltage step at the switch transitions. This step is approximately equal to: (2IOUT)(ESR of COUT) Power Dissipation The power dissipation of any LT1054 circuit must be limited such that the junction temperature of the device does not exceed the maximum junction temperature rat- ings. The total power dissipation must be calculated from two components, the power loss due to voltage drops in the switches and the power loss due to drive current losses. The total power dissipated by the LT1054 can be calculated from: P ≈ (VIN – |VOUT|)(IOUT) + (VIN)(IOUT)(0.2) where both VIN and VOUT are referred to the ground pin (Pin 3) of the LT1054. For LT1054 regulator circuits, the power dissipation will be equivalent to that of a linear regulator. Due to the limited power handling capability of the LT1054 packages, the user will have to limit output current require- ments or take steps to dissipate some power external to the LT1054 for large input/output differentials. This can be accomplished by placing a resistor in series with CIN as shown in Figure 6. A portion of the input voltage will then be dropped across this resistor without affecting the output regulation. Because switch current is approximately 2.2 times the output current and the resistor will cause a voltage drop when CIN is both charging and discharging, the resistor should be chosen as: C1 R2 CIN COUT VOUT LT1054 • F06 VIN R1 RX LT1054 FB/SHDN CAP+ GND CAP – V+ OSC VREF VOUT + Figure 6 |
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