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TC115501EMTTR Datasheet(PDF) 5 Page - Microchip Technology |
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TC115501EMTTR Datasheet(HTML) 5 Page - Microchip Technology |
5 / 14 page 2002-2012 Microchip Technology Inc. DS21361D-page 5 TC115 4.0 DETAILED DESCRIPTION The TC115 is a combination PFM/PWM step-up (boost) regulator. It is particularly useful in battery-pow- ered 1, 2 and 3 cell applications where the required out- put current is 140 mA or less, and size/cost issues are a concern. The device operates in PWM mode when the output load is sufficient to demand a 10% (or greater) duty cycle. While in PWM mode, the TC115 behaves as any other PWM switching regulator to a maximum duty cycle of 92%. At low output loads (i.e., output loads requiring < 10% duty cycle to support), the TC115 automatically switches to pulse frequency mod- ulation (PFM) operating mode with a fixed duty cycle of 25% (max) (17%, typical). While in PFM mode, the inductor is modulated with individual fixed width pulses only as needed to maintain output voltage. This action reduces supply current, thereby improving power efficiency at low output loads. 4.1 Input Power and Sensing The TC115 is powered from the PS input, which must be connected to the regulated output, as shown in Figure 5-1. 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. 4.2 Output Diode For best results, use a Schottky diode, such as the MA735, 1N5817, EC10 or equivalent. Connect the diode between the PS and LX pins as close to the IC as possible. While ultra fast diodes can be used, lower effi- ciency will result due to their higher forward voltage drop. Ordinary rectifiers should be avoided because of their slow recovery characteristics. 4.3 Low Power Shutdown Mode The TC115 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. SHDN may be tied to the input supply if not used. 4.4 Behavior When VIN is Greater Than the Factory-Programmed VOUT Setting The TC115 is designed to operate as a step-up regulator only. As such, VIN is assumed to always be less than the factory-programmed VOUT setting (VR). Operating the TC115 with VIN > VR causes regulating action to be suspended (and corresponding supply current reduction to 9 µA, typical) until VIN is again less than VR. While regulating action is suspended, VIN is connected to VOUT through the series combination of the inductor and Schottky diode. Care must be taken to add the appropriate isolation (MOSFET output switch or post LDO with shutdown) during system design if this VIN/VOUT leakage path is problematic. Note: Because the TC115 uses an external diode, a leakage path between the input voltage and the output node (through the inductor and diode) exists while the regu- lator is in shutdown. Care must be taken in system design to assure the input supply is isolated from the load during shutdown. |
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