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AMS2027 Datasheet(PDF) 4 Page - Advanced Monolithic Systems |
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AMS2027 Datasheet(HTML) 4 Page - Advanced Monolithic Systems |
4 / 5 page Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140 AMS2027 APPLICATION HINTS Power Supply Considerations A 0.047 µF ceramic bypass capacitor close to the device, between input and ground is recommended. When the output load is heavy or has large paralleled capacitors, a high value electrolytic capacitor should be used. To improve the immunity of the device to ESD, use a 0.1 µF ceramic capacitor to bypass the output. Current Limit A sense FET monitors overcurrent conditions. When an overcurrent condition is detected the device maintains a constant output current and decreases the voltage accordingly. If the condition is present long enough to activate the thermal limiting the result is the shutdown of the device. There are three situation in which overload can occur. In the first case, the output has been shorted before the device is enabled or before VI has been applied. The device senses the short and switches into a constant-current output. In the second case, the short occurs while the device is enabled. When this happens, very high currents flow for a short time before the current-limit circuit can react. After the current-limit circuit has tripped, the device limits normally. In the third case, the load has been gradually increased beyond the recommended operating current. The current will rise until the current-limit threshold is reached. The AMS2027 is able to deliver current up to the current-limit threshold without damage. When the threshold has been reached the device switches into the constant current mode. When over current condition is detected the error flag logic output remains low until the condition is removed. Power Dissipation and Junction Temperature The thermal resistance of the surface-mount packages such as SOIC is high compared to that of power packages. The use of the N-cannel MOSFET which has low on-resistance, makes it possible for small surface-mount packages to pass large currents. To determine the power dissipation and junction temperature the first step is to find rON at the input voltage and operating temperature. As an initial estimate use the highest operating ambient temperature of interest and read rON from Figure1. Power dissipation is equal to: PD = rON X I 2 Calculate the junction temperature: TJ = PD X RθJA + TA Where RθJA is the thermal resistance and is 172°C/W for the SOIC package. Compare the calculated junction temperature with the initial estimate and if they don’t mach within a few degrees, repeat the calculations using the calculated value as the new estimate. A few repetitions will be sufficient to give a reasonable answer. Thermal Protection Thermal protection prevents damage to the device when over load or short circuits conditions are present for extended periods of time. These conditions force the AMS2027 into the constant current mode. As a result the voltage across the high-side switch will increase. Under short-circuit conditions the voltage across the switch is equal to the input voltage. Continuous heavy over loads and short circuits increase the power dissipation in the switch and causes the junction temperature to rise to dangerously high levels. The protection circuit shuts off the switch when it senses the high junction temperature. The switch remains off until the device has cooled about 20 °C. The switch continues to cycle off and on until the fault is removed. Undervoltage Lock-out An undervoltage lock-out is provided to insure that the switch is in the off state at start-up. When the input voltage falls below 3.2V the switch will be turned off immediately. This will make it easy for designers of hot plug-in systems that may not be able to turn the switch off before removing the input power. In such cases when the device is reinserted, the turn on will have a controlled rise time to reduce EMI and voltage overshoots. Figure 1 TYPICAL APPLICATIONS POWER SUPPLY 4.0V - 5.0V 0.1 µF 1 µF 1 2 3 10k EXTERNAL LOAD 0.1 µF ENABLE ERROR FLAG 7 8 6 IN OUT OUT EN GND ERROR |
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