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TC126303ECT Datasheet(PDF) 4 Page - Microchip Technology |
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TC126303ECT Datasheet(HTML) 4 Page - Microchip Technology |
4 / 12 page TC125/TC126 DS21372B-page 4 © 2002 Microchip Technology Inc. 3.0 DETAILED DESCRIPTION The TC125/126 are PFM step-up DC/DC regulators for use in systems operating from two or more cells or in low voltage, line powered applications. Because Pulse Frequency Modulation (PFM) is used, the TC125/126 switching frequency (and therefore supply current) is minimized at low output loads. This is especially important in battery operated applications (such as pagers) that operate in standby mode most of the time. For example, a TC125/126 with a 3V output and no load will consume a maximum supply current of only 10 µA versus a supply current of 40µA maximum when IOUT = 30mA. Both devices require only an external inductor, diode and capacitor to implement a complete DC/DC converter. The TC125 is recommended for applications requiring shutdown mode as a means of reducing system supply current. The TC125 is powered from the PS input, which must be connected to the regulated output as shown in Figure 3-1. PS also senses output voltage for closed-loop regulation. Start-up current is furnished through the inductor when input voltage is initially applied. This action starts the oscillator, causing the voltage at the PS input to rise, bootstrapping the regulator into full operation. The TC126 (Figure 3-2) is recommended for all appli- cations not requiring shutdown mode. It has separate VDD and SENSE inputs, allowing it to be powered from any source of 2.2V to 10V in the system. The VDD input of the TC126 may be connected to the VIN,VOUT,oran external DC voltage. Lower values of VDD result in lower supply current, but lower efficiency due to higher switch ON resistance. Higher VDD values increase supply current, but drive the internal switching transistor harder (lowering RDSON), thereby increasing efficiency. 3.1 Low Power Shutdown Mode The TC125 enters a low power shutdown mode when SHDN is brought low. While in shutdown, the oscillator is disabled and the internal switch is shut off. Normal regulator operation resumes when SHDN is brought high. Because the TC125 uses an external diode, a leakage path between the input voltage and the output node (through the inductor and diode) exists while the regulator is in shutdown. Care must be taken in system design to assure the input supply is isolated from the load during shutdown. 3.2 Behavior When VIN is Greater Than the Factory-Programmed OUT Setting The TC125 and TC126 are designed to operate as step-up regulators only. As such, VIN is assumed to always be less than the factory-programmed output voltage setting (VR). Operating the TC125/126 with VIN >VR causes regulating action to be suspended (and corresponding supply current reduction) until VIN is again less than VR. While regulating action is suspended, VIN is connected to the output voltage node through the series combination of the inductor and Schottky diode. Again, care must be taken to add the appropriate isolation (MOSFET series switch or post LDO with shutdown) during system design if this VIN/ VOUT leakage path is problematic. FIGURE 3-1: TYPICAL TC125 CIRCUIT FIGURE 3-2: TYPICAL TC126 CIRCUIT L1 100 µH Sumida CD54 D1 MA735 Shutdown Control 1 2 3 5 4 TC125 LX SHDN NC PS GND 1.5V ON OFF VOUT 3.3V @40mA C1 47 µF/16V Tantalum + 1 2 3 45 TC126 LX SENSE NC VDD GND 3.3V Line Supply L1 100 µH Sumida CD54 D1 MA735 VOUT 5V @80mA C1 47 µF/16V Tantalum + |
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