Datasheet MAX1951, MAX1952 (Analog Devices) - 8

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