MAX1760HEUB+T中文资料(7)

0.8A, Low-Noise, 1MHz, Step-Up DC-DC ConverterMAX1760/MAX1760H

Table 2. Selecting the Operating Mode

output filter capacitor and the load. As the energystored in the inductor is depleted, the current rampsdown and the synchronous rectifier turns off. At lightloads, the device operates at fixed frequency or only asneeded to maintain regulation, depending on theCLK/SEL setting (Table 2).

Synchronous Rectifier

The MAX1760 features an internal, P-channel synchro-nous rectifier to enhance efficiency. Synchronous recti-fication provides 5% improved efficiency over similarnonsynchronous boost regulators. In PWM mode, thesynchronous rectifier is turned on during the secondhalf of each switching cycle. In low-power mode, aninternal comparator turns on the synchronous rectifierwhen the voltage at LX exceeds the boost regulatoroutput, and turns it off when the inductor current dropsbelow 60mA. When setting output voltages greater than4V, an external 0.5A Schottky diode must be connectedin parallel with the on-chip synchronous rectifier.

Normal Operation

Pulling CLK/SEL low selects the MAX1760’s normaloperating mode. In this mode, the device operates inPWM when driving medium-to-heavy loads, and auto-matically switches to PFM if the load requires lesspower. PFM operation allows higher efficiency thanPWM under light-load conditions.

Forced-PWM Operation

When CLK/SEL is high, the MAX1760 operates in a low-noise PWM-only mode. During forced-PWM operation,the MAX1760 switches at a constant frequency (1MHz)and modulates the MOSFET switch pulse width to con-trol the power transferred per cycle to regulate the out-put voltage. Switching harmonics generated byfixed-frequency operation are consistent and easily fil-tered. See the Noise Spectrum plot in the TypicalOperating Characteristics.

Low-Voltage Startup Oscillator

The MAX1760 uses a CMOS, low-voltage startup oscil-lator for a 1.1V guaranteed minimum startup input volt-age. At startup, the low-voltage oscillator switches theN-channel MOSFET until the output voltage reaches2.15V. Above this level, the normal boost-converterfeedback and control circuitry take over. Once thedevice is in regulation, it can operate down to 0.7Vinput since internal power for the IC is bootstrappedfrom the output through OUT. Do not apply full loaduntil the output exceeds 2.3V.

Synchronized-PWM Operation

The MAX1760 can be synchronized in PWM mode to anexternal frequency of 500kHz to 1.2MHz by applying anexternal clock signal to CLK/SEL. This allows interfer-ence to be minimized in wireless applications. The syn-chronous rectifier is active during synchronized-PWMoperation.

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Shutdown

The MAX1760 has a shutdown mode that reduces qui-escent current to 0.1µA. During shutdown (ON= highon MAX1760, ON = low on MAX1760H), the referenceand all feedback and control circuitry are off. Duringshutdown, the output voltage is one diode drop belowthe input voltage.

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