ZHCSAW4M December 2004 – December 2015 LP38690 , LP38692
PRODUCTION DATA.
7 Detailed Description
7.1 Overview
The LP38690 and LP38692 devices are designed to meet the requirements of portable, battery-powered digital systems providing an accurate output voltage with fast start-up. When disabled via a low logic signal at the enable pin (EN), the power consumption is reduced to virtually zero (LP38692 only). The LP38690 and LP38692 perform well with a single 1-μF input capacitor and a single 1-μF ceramic output capacitor.
7.2 Functional Block Diagrams
Figure 26. LP38690 Functional Diagram (TO-252)
Figure 27. LP38690 Functional Diagram (WSON)
Figure 28. LP38692 Functional Diagram (SOT-223)
Figure 29. LP38692 Functional Diagram (WSON)
7.3 Feature Description
7.3.1 Enable (EN)
The LP38692 has an Enable pin (EN) which allows an external control signal to turn the regulator output On and Off. The Enable On/Off threshold has no hysteresis. The voltage signal must rise and fall cleanly, and promptly, through the ON and OFF voltage thresholds. The EN pin voltage must be higher than the VEN(MIN) threshold to ensure that the device is fully enabled under all operating conditions. The EN pin voltage must be lower than the VEN(MAX) threshold to ensure that the device is fully disabled. The Enable pin has no internal pull-up or pull-down to establish a default condition and, as a result, this pin must be terminated either actively or passively. If the Enable pin is driven from a source that actively pulls high and low, the drive voltage should not be allowed to go below ground potential or higher than VIN. If the application does not require the Enable function, the pin should be connected directly to the IN pin.
7.3.2 Thermal Shutdown Protection (TSD)
Thermal shutdown disables the output when the junction temperature rises to approximately 160°C which allows the device to cool. When the junction temperature cools to approximately 150°C, the output circuitry enables.
Based on power dissipation, thermal resistance, and ambient temperature, the thermal protection circuit may cycle on and off. This thermal cycling limits the dissipation of the regulator and protects it from damage as a result of overheating.
The TSD circuitry of the LP38692 has been designed to protect against temporary thermal overload conditions. The TSD circuitry was not intended to replace proper heat-sinking. Continuously running the LP38692 device into thermal shutdown degrades device reliability.
7.3.3 Foldback Current Limiting
Foldback current limiting is built into the LP38690 and LP38692 which reduces the amount of output current the part can deliver as the output voltage is reduced. The amount of load current is dependent on the differential voltage between VIN and VOUT. Typically, when this differential voltage exceeds 5 V, the load current limits at about 450 mA. When the VIN – VOUT differential is reduced below 4 V, load current is limited to about 1500 mA.
CAUTION
When toggling the LP38692 Enable (EN) after the input voltage (VIN) is applied, the foldback current limit circuitry is functional the first time that the EN pin is taken high. The foldback current limit circuitry is non-functional the second, and subsequent, times that the EN pin is taken high. Depending on the input and output capacitance values the input inrush current may be higher than expected which can cause the input voltage to droop.
If the EN pin is connected to the IN pin, the foldback current limit circuitry is functional when VIN is applied if VIN starts from less than 0.4 V.
7.4 Device Functional Modes
7.4.1 Enable (EN)
The EN pin voltage must be higher than the VEN(MIN) threshold to ensure that the device is fully enabled under all operating conditions.
7.4.2 Minimum Operating Input Voltage (VIN)
The LP38690 and LP38692 devices do not include any dedicated UVLO circuitry. The LP38690 and LP38692 internal circuitry is not fully functional until VIN is at least 2.7 V. The output voltage is not regulated until VIN ≥ (VOUT + VDO), or 2.7 V, whichever is higher.