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MAX1952ESA+ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1952ESA+ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 15 page 1MHz, All-Ceramic, 2.6V to 5.5V Input, 2A PWM Step-Down DC-to-DC Regulators 8 _______________________________________________________________________________________ through the high-side MOSFET body diode, back to the input, until the beginning of the next cycle or until the inductor current drops to zero. The MAX1951/MAX1952 utilize a pulse-skip mode to prevent overheating during short-circuit output conditions. The device enters pulse- skip mode when the FB voltage drops below 300mV, lim- iting the current to 3A (typ) and reducing power dissipation. Normal operation resumes upon removal of the short-circuit condition. VCC Decoupling Due to the high switching frequency and tight output tolerance (1%), decouple VCC with a 0.1µF capacitor connected from VCC to GND, and a 10 Ω resistor con- nected from VCC to IN. Place the capacitor as close to VCC as possible. Soft-Start The MAX1951/MAX1952 employ digital soft-start circuitry to reduce supply inrush current during startup conditions. When the device exits undervoltage lockout (UVLO), shut- down mode, or restarts following a thermal-overload event, or the external pulldown on COMP is released, the digital soft-start circuitry slowly ramps up the voltages at REF and FB (see the Soft-Start Waveforms in the Typical Operating Characteristics). Undervoltage Lockout If VCC drops below 2.25V, the UVLO circuit inhibits switching. Once VCC rises above 2.35V, the UVLO clears, and the soft-start sequence activates. Compensation and Shutdown Mode The output of the internal transconductance voltage error amplifier connects to COMP. The normal operation voltage for COMP is 1V to 2.2V. To shut down the MAX1951/MAX1952, use an NPN bipolar junction transistor or a very low output capacitance open-drain MOSFET to pull COMP to GND. Shutdown mode causes the internal MOSFETs to stop switching, forces LX to a high-impedance state, and shorts REF to GND. Release COMP to exit shutdown and initiate the soft- start sequence. Thermal-Overload Protection Thermal-overload protection limits total power dissipation in the device. When the junction temperature exceeds TJ = +160°C, a thermal sensor forces the device into shut- down, allowing the die to cool. The thermal sensor turns the device on again after the junction temperature cools by 15°C, resulting in a pulsed output during continuous overload conditions. Following a thermal-shutdown condi- tion, the soft-start sequence begins. MAX1951 VCC COMP CURRENT SENSE SLOPE COMP ERROR SIGNAL CLOCK POSITIVE AND NEGATIVE CURRENT LIMITS GND REF FB PGND LX IN DAC gm SOFT-START/ UVLO REF 2V PWM CONTROL THERMAL SHUTDOWN BANDGAP REF 1.25V CLAMP OSC RAMP GEN Figure 1. Functional Diagram |
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