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TPS54350 Datasheet(PDF) 7 Page - Texas Instruments |
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TPS54350 Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 34 page  TPS54350 SLVS456C − OCTOBER 2003 − REVISED OCTOBER 2004 www.ti.com 7 320 k Ω 125 k Ω R1 R2 Input Voltage Supply 1 k Ω Figure 1. Circuit Using External UVLO Function The equations for selecting the UVLO resistors are: R1 + VIN(start) 1k W 1.24 V * 1kW VIN(stop) + (R1 ) 1kW) 1.02 V 1k W For applications which require an undervoltage lock out (UVLO) threshold greater than 4.49 V, external resistors may be implemented, see Figure 1, to adjust the start voltage threshold. For example, an application needing an UVLO start voltage of approximately 7.8 V using the equation (1), R1 is calculated to the nearest standard resistor value of 5.36 k Ω. Using equation (2), the input voltage stop threshold is calculated as 6.48 V. Enable (ENA) and Internal Slow Start Once the ENA pin voltage exceeds 0.5 V, the TPS54350 starts operation. The TPS54350 has an internal digital slow start that ramps the reference voltage to its final value in 1150 switching cycles. The internal slow start time (10% − 90%) is approximated by the following expression: T SS_INTERNAL(ms) + 1.15k ƒ s(kHz) Once the TPS54350 device is in normal regulation, the ENA pin is high. If the ENA pin is pulled below the stop threshold of 0.5 V, switching stops and the internal slow start resets. If an application requires the TPS54350 to be disabled, use open drain or open collector output logic to interface to the ENA pin (see Figure 2). The ENA pin has an internal pullup current source. Do not use external pullup resistors. 5 µA Disabled Enabled RSS CSS Figure 2. Interfacing to the ENA Pin Extending Slow Start Time In applications that use large values of output capacitance there may be a need to extend the slow start time to prevent the startup current from tripping the current limit. The current limit circuit is designed to disable the high side MOSFET and reset the internal voltage reference for a short amount of time when the high side MOSFET current exceeds the current limit threshold. If the output capacitance and load current cause the startup current to exceed the current limit threshold, the power supply output will not reach the desied output voltage. To extend the slow start time and to reduce the startup current, an external resistor and capcitor can be added to the ENA pin. The slow start capacitance is calculated using the following equation: CSS(µF) = 5.55e−3 Tss(ms) The RSS resistor must be 2 kΩ and the slow start capacitor must be less than 0.47 µF. Switching Frequency (RT) The TPS54350 has an internal oscillator that operates at twice the PWM switching frequency. The internal oscillator frequency is controlled by the RT pin. Grounding the RT pin sets the PWM switching frequency to a default frequency of 250 kHz. Floating the RT pin sets the PWM switching frequency to 500 kHz. Connecting a resistor from RT to AGND sets the frequency according to the following equation (also see Figure 30). RT(k W) + 46000 ƒ s(kHz)–35.9 The RT pin controls the SYNC pin functions. If the RT pin is floating or grounded, SYNC is an output. If the switching frequency has been programmed using a resistor from RT to AGND, then SYNC functions as an input. The internal voltage ramp charging current increases linearly with the set frequency and keeps the feed forward modulator constant (Km = 8) regardless of the frequency set point. (1) (2) (3) (4) |
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