TX4145

TX4145 is a step-down switching mode converter and a current mode buck regulator, where the EA output voltage is proportional to the peak inductor current. It achieves different peak output currents over a wide input supply range of 6–60V, with excellent line and load regulation. The PWM current mode operation provides easy loop stabilization and fast transient response. An external FS pin allows switching frequency to be set via an external resistor. Integrated protection features include cycle-by-cycle current limiting and thermal shutdown. The device comes in an SOP8 package with minimal external components. At the beginning of each cycle, power switch M1 is off. When the EA output voltage is greater than the current sense amplifier output, the current comparator output is low. The rising edge of CLK triggers the RS flip-flop to set high, turning on M1 and connecting the inductor to the input supply via SW. The rising inductor current is sampled and amplified by the current sense amplifier. Slope compensation is superimposed onto the current sense amplifier output and fed together with the EA output to the PWM comparator. When the slope-compensated current sense amplifier output exceeds the EA output, the RS flip-flop is reset and M1 turns off. The inductor current freewheels through the external Schottky diode D1. The feedback voltage FB is compared with a 0.82V reference voltage through the EA. When the FB pin voltage falls below 0.82V, the EA output increases. The EA output voltage is proportional to the peak inductor current; an increase in EA output voltage results in an increase in output current. TX4145 features a built-in 0.6ms soft-start. Soft-start prevents output voltage overshoot during startup. When the chip starts up, the internal circuit generates a soft-start voltage SS that rises at a fixed slope. When SS is lower than the internal reference voltage, SS is used as the EA reference voltage and the internal reference is masked. When SS exceeds the internal reference voltage, the internal reference takes control of the EA. The output voltage is set by voltage divider resistors R1 and R2 connected to the FB pin. The feedback resistor R2 also sets the feedback loop bandwidth through the internal compensation network. The formula for R1 is: R1 = R2 / (Vout / 0.82V - 1) For most applications, the DC current rating of the inductor should be at least 25% greater than the maximum load current. To achieve higher efficiency, the inductor DC resistance should be less than 200mΩ. The inductance is calculated by: L = (Vout × (Vin - Vout)) / (Vin × ΔIL × fSW), where ΔIL is the inductor ripple current. The inductor ripple current is typically 30% of the maximum load current, and the maximum peak inductor current is calculated as: IL(MAX) = ILOAD + ΔIL / 2. In light load operation (below 100mA), a larger inductance value can be used to improve efficiency. The input capacitor reduces input supply inrush current and suppresses switching noise. The impedance of the input capacitor at the switching frequency should be lower than the input source impedance to prevent high-frequency switching current from flowing into the input. Low-ESR, low-temperature-coefficient electrolytic capacitors are recommended; 47μF is sufficient for most applications. For high input voltage applications, the input electrolytic capacitor also suppresses input voltage spikes during power-on and power-off transitions. The output capacitor maintains low output ripple voltage and ensures feedback loop stability. The impedance of the output capacitor at the switching frequency must be sufficiently low. Low-ESR electrolytic capacitors are recommended; 220μF is sufficient for most applications. A low-ESR ceramic capacitor in parallel with the output capacitor reduces output ripple and improves output stability. The enable pin EN controls the chip enable function and can be driven by an MCU. In applications where enable control is not required, EN should be pulled up directly to the VIN pin and must not be left floating. The switching frequency setting pin FS controls the operating frequency of the chip. Different external resistors to ground set different switching frequencies fSW. The external resistor RFS is calculated by: RFS = 140 / (fSW - 140), where fSW is the switching frequency in KHz. The recommended maximum fSW is 500KHz; the typical operating frequency is 140KHz (FS pin floating). RFS is in units of MΩ.

Applications

• Distributed power systems
• Battery chargers
• Industrial power systems
• Dash cameras
• On-board chargers
• Robotic vacuum cleaners

Key features

• Up to 93% efficiency
• Adjustable frequency
• Thermal shutdown
• Cycle-by-cycle overcurrent protection
• Wide input voltage range: 6~60V
• SOP8 package