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TPS63700 Datasheet(PDF) 11 Page - Texas Instruments |
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TPS63700 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 22 page www.ti.com DETAILED DESCRIPTION Power Conversion Control Enable Load Disconnect Soft Start Output Overvoltage Protection TPS63700 SLVS530 – SEPTEMBER 2005 The TPS63700 is a dc-dc converter for negative output voltages using buck-boost topology. It operates with an input voltage range of 2.7 V to 5.5 V and generates a negative output voltage down to –15 V. The output is controlled by a fixed-frequency, pulse-width-modulated (PWM) regulator. In normal operation mode, the converter operates at continuous conduction mode (CCM). At light loads it can enter discontinuous conduction mode (DCM). The converter operates in a fixed-frequency, pulse-width-modulated control scheme. So, the on-time of the switches varies depending on input-to-output voltage ratio and the load. During this on-time, the inductor connected to the converter is charged with current. In the remaining time, the time period set by the fixed operating frequency, the inductor discharges into the output capacitor via the rectifier diode. Usually, at higher loads the inductor current is continuous. During light load, the inductor current of this converter can become discontinuous. In this case, the control circuit of the controller output automatically takes care of these changing conditions to always operate with an optimum control setup. The controller circuit of the converter is based on a fixed-frequency, multiple-feedforward controller topology. Input voltage, output voltage, and voltage drop across the switch are monitored and forwarded to the regulator. Changes in the operating conditions of the converter directly affect the duty cycle. The error amplifier compares the voltage on FB pin with GND to generate an accurate and stable output voltage. The error amplifier is internally compensated. At light loads, the converter operates in discontinuous conduction mode (DCM). If the load will be further decreased, the energy transmitted to the output capacitor can't be absorbed by the load and would lead to an increase of the output voltage. In this case, the converter limits the output voltage increase by skipping switch pulses. Applying GND signal at the EN pin disables the converter, where all internal circuitry is turned off. The device now just consumes low shutdown current flowing into the VIN pin. The output load of the converter is also disconnected from the battery as described in the following paragraph. Pulling the EN pin to VIN enables the converter. Internal circuitry, necessary to operate the converter, is then turned on. The device supports complete load disconnection when the converter is disabled. The converter turns off the internal PMOS switch, thus no DC current path remains between load and input voltage source. The converter has a soft-start function. When the converter is enabled, the implemented switch current limit ramps up slowly to its nominal value. Soft start is implemented to limit the input current during start-up to avoid high peak currents at the battery which could interfere with other systems connected to the same battery. Without soft start, uncontrolled input peak currents flow to charge up the output capacitors and to supply the load during start-up. This would cause significant voltage drops across the series resistance of the battery and its connections. The converter has an implemented output overvoltage protection. The output voltage is limited to –19 V in case the feedback connection from the output to the FB pin is open. 11 |
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