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LT1573CS8-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LT1573CS8-3.3 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LT1573 capabilities of the PC board and its copper traces. Table 3 lists some typical values for the thermal resistance of the LT1573. Measured values of thermal resistance for a specific board size with different copper areas are listed. All measurements were taken in still air on 3/32 " FR-4 board with 2oz copper. It is possible to achieve significantly lower values with thinner multilayer boards. Compensation In order to improve the transient response to regulator output load variation, a capacitor in series with a resistor can be inserted between the VOUT and COMP pins. For the microprocessor power supply regulator system based on the LT1573 and the PNP transistor D45H11 with 24 1 µF surface mount ceramic capacitors in parallel with one 220 µF surface mount tantalum capacitor at the output as shown in Figure 1, a 100pF capacitor in series with a 1k resistor is recommended. In theory, the output capacitor forms the dominant pole of the regulator system. An internal compensation capacitor forms another pole. The external compensation capacitor and resistor form a zero which adds phase margin to the regulator system to prevent high frequency oscillation. The LT1573 has an internal pole at approximately 5kHz. An external compen- sation zero between 10kHz and 100kHz is usually required to stabilize the regulator. The zero frequency is primarily determined by the compensation capacitor and can be roughly calculated by the following equation: f kHz pF CpF C ZERO COMP COMP = () ()() ≤≤ 40 30 10 100 , A compensation resistor between 1k and 10k is sug- gested. A compensation resistor of 5k works for most cases. In some cases, a greater compensation resistor is needed to stop oscillation above 1MHz. In some cases, the output capacitor may have enough equivalent series resis- tance (ESR) to generate the required zero and the external compensation zero may not be needed. Output Capacitor The LT1573 is designed to be used with an external PNP transistor with a high gain-bandwidth product fT to make a regulator with a very fast transient response, which can minimize the size of the output capacitor. For a regulator made of an LT1573 and a D45H11, only one 10 µF surface mount ceramic capacitor at the output is enough for the regulator to handle the output load varying up to 5A in a few hundred nanoseconds interval and to remain stable with a 30pF capacitor in series with a 7.5k resistor between the VOUT and COMP pins. If tighter voltage regulation is Table 3. LT1573 Thermal Resistance COPPER AREA THERMAL RESISTANCE TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm2 2500mm2 2500mm2 80 °C/W 1000mm2 2500mm2 2500mm2 80 °C/W 225mm2 2500mm2 2500mm2 85 °C/W *Device is mounted on topside. We can find out the maximum junction temperature of the LT1573 during normal load operation after we calculate the maximum power dissipation of the LT1573 from Eq (2). From the previous design example, the maximum power dissipation of the LT1573 is 0.2W. From Table 3, we know the thermal resistance from junction-to-ambient is around 85 °C/W. The tempera- ture difference between junction and ambient is: (0.25W)(85 °C/W) = 21.25°C If the maximum ambient temperature is specified at 50 °C, the maximum junction temperature will be: TJMAX = 50°C + 21.25°C = 71.25°C The maximum junction temperature must not exceed the specified 125 °C for safe continuous regulator op- eration. Thermal Limiting The thermal shutdown temperature of the LT1573 is approximately 150 °C. The thermal limit of the LT1573 can be used to protect both the LT1573 and the external PNP pass transistor. This is accomplished by thermally cou- pling the LT1573 to the PNP power transistor by locating the LT1573 as close to the PNP transistor as possible. In this case, the power dissipation of the power transistor must be considered in the LT1573 maximum junction temperature calculation. APPLICATIO S I FOR ATIO |
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