feature

Very low ground current (ignd=1mA) Good line regulation Good load regulation Very low transient overshoot? ? Low ESR output capacitor stable (ESR=0MΩ)? ? Thermal Shutdown Current Limit Output Options: 3.3V and 1.8V

application

Disk Drive Circuit Desktop Computer Laptop Universal Three Terminal Voltage Regulator

describe

The fan1539/fan1540 series of high current LDOs (1.0A and 1.3A) have been developed for portable applications where low quiescent current is an important requirement. The device has excellent line and load characteristics with a transient response not exceeding 10% of the nominal output value over the full operating temperature range even during power cycling and short circuit removal. An internally trimmed, temperature-compensated bandgap reference guarantees 2.5% accuracy on input voltage, output current, and temperature. Including on-chip precise current limit and thermal shutdown protection. There are only two external low ESR ceramic capacitors. The FAN1539 /FAN1540 are available in thermally enhanced 3x3mm 6-lead MLP, 5x6mm 8-lead MLP and 3-lead to-252 packs. The 5x6mm MLP package version has a separate Kelvin sense pin for high precision applications.

application information

General Circuit Description The FAN1539/FAN1540 is an advanced low dropout regulator designed for portable computer applications where quiescent current is required for high performance and low power consumption. The device features an internally trimmed bandgap reference and an internal output voltage sense divider. These two signals form the input of the error amplifier, which is used to regulate the output voltage. The FAN1539/FAN1540 have a complete set of internal protection circuits including thermal shutdown, short circuit current limiting and electrostatic discharge protection. Low esr ceramic capacitors for input and output pins to keep the circuit stable. Short-Circuit Current Limiting The device has internal over-current limiting and short-circuit protection. Under overcurrent conditions the current is determined by the current limit threshold. Once the device is released from the short circuit, the normal level of the current limit gradually returns to the normal level of the device output voltage. Special circuitry has been added to ensure that the output voltage does not overshoot excessively from short circuit current conditions - a phenomenon often encountered in traditional regulators. The thermal protection FAN1539/FAN1540 is designed to provide at least 1A/1.3A output current. High output loads and excessive input and output voltage differentials will cause the device to heat up and consume energy beyond the maximum ratings. Under an output overload condition, when the die temperature exceeds the shutdown limit of 160 °C, the onboard thermal protection will disable the output until the temperature drops below approximately 15 °C, at which point the output is re-enabled.

Thermal characteristics

The FAN1539/FAN1540 are designed to provide at least 1A/1.3A and operate at die (connection) temperatures up to 125°C at the specified output voltage. Once the power dissipation and thermal resistance are known, the maximum junction temperature of the device can be calculated. While power dissipation is based on known electrical parameters, the actual thermal resistance depends on the thermal characteristics of the chosen package and the surrounding PC board copper mounted on it. Power dissipation is equal to the input-output voltage difference and the output current plus the ground current multiplied by the input voltage, or: ground pin current, ignd can be found in the diagram provided in the "Electrical Characteristics" section. The relational package describing the thermal behavior is: where Tj(max) is the maximum allowable joining temperature of the mold, which is 150°C, and Ta is the ambient operating temperature. θja depends on the surrounding pc board layout and can be obtained empirically. The 6-lead θjc (junction box) MLP package is specified at 8°C/W, and the θja of the smallest PWB package will be greatly improved. This can be improved by providing a heat sink with surrounding copper ground on the PWB. Depending on the size of the copper region and the thickness of the copper layer, the resulting θja can vary over a wide range. This addition of backside copper with vias, stiffeners, and other reinforcements also helps reduce thermal resistance. Conducting thermal simulations to thermally optimize the board layout showed θja as low as 20°C/W. For example, the location near heat dissipation of other equipment must be included in the design considerations. Overload conditions also need to be considered. It is possible for equipment to enter a thermal loop, where the circuit goes off, cools, turns back on, and then overheats and shuts down again due to persistent fault conditions. Capacitor esr and printed circuit board layout The FAN1539/FAN1540 are optimized to accommodate low ESR bypass capacitors (down to 0 MΩ). For best results, input and output bypass capacitors must be placed as close to the input and output pins as possible. Use of X7R types such as Murata GRM31CR70J106KA01B (10 μF) and GRM43ER71A226KE01B (22 μF) or similar components of TDK. Capacitors should be connected directly to the ground plane. Using a ground plane on the top recommends using the bottom of the PCB. As much as possible vias should be used to reduce ground plane resistance.