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MAX1633AEAI Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1633AEAI Datasheet(HTML) 10 Page - Maxim Integrated Products |
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10 / 29 page  Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers 10 ______________________________________________________________________________________ _______________Detailed Description The MAX1630A is a dual, BiCMOS, switch-mode power- supply controller designed primarily for buck-topology regulators in battery-powered applications where high effi- ciency and low quiescent supply current are critical. Light- load efficiency is enhanced by automatic Idle Mode operation, a variable-frequency pulse-skipping mode that reduces transition and gate-charge losses. Each step- down, power-switching circuit consists of two n-channel MOSFETs, a rectifier, and an LC output filter. The output voltage is the average AC voltage at the switching node, which is regulated by changing the duty cycle of the MOSFET switches. The gate-drive signal to the n-channel high-side MOSFET must exceed the battery voltage, and is provided by a flying-capacitor boost circuit that uses a 100nF capacitor connected to BST_. Devices in the MAX1630A family contain 10 major circuit blocks (Figure 2). The two PWM controllers each consist of a Dual Mode feedback network and multiplexer, a multi-input PWM comparator, high-side and low-side gate drivers, and logic. The MAX1630A/MAX1631A/MAX1632A contain fault-protection circuits that monitor the main PWM out- puts for undervoltage and overvoltage. A power-on sequence block controls the power-up timing of the main PWMs and determines whether one or both of the outputs are monitored for undervoltage faults. The MAX1630A/MAX1632A/MAX1633A/MAX1635A include a secondary feedback network and 12V linear regulator to generate a 12V output from a coupled-inductor fly- back winding. The MAX1631A/MAX1634A have an SECFB instead, which allows a quasi-regulated, adjustable-output, coupled-inductor flyback winding to be attached to either the 3.3V or the 5V main inductor. Bias generator blocks include the 5V IC internal rail (VL) linear regulator, 2.5V precision reference, and automatic bootstrap switchover circuit. The PWMs share a com- mon 200kHz/300kHz synchronizable oscillator. These internal IC blocks are not powered directly from the battery. Instead, the 5V VL linear regulator steps down the battery voltage to supply both VL and the gate drivers. The synchronous-switch gate drivers are directly powered from VL, while the high-side switch gate drivers are indirectly powered from VL through an external diode-capacitor boost circuit. An automatic bootstrap circuit turns off the +5V linear regulator and powers the IC from the 5V PWM output voltage if the output is above 4.5V. PWM Controller Block The two PWM controllers are nearly identical. The only differences are fixed output settings (3.3V vs. 5V), the VL/CSL5 bootstrap switch connected to the +5V PWM, and SECFB. The heart of each current-mode PWM con- troller is a multi-input, open-loop comparator that sums three signals: the output voltage error signal with respect to the reference voltage, the current-sense sig- nal, and the slope compensation ramp (Figure 3). The PWM controller is a direct-summing type, lacking a tra- ditional error amplifier and the phase shift associated with it. This direct-summing configuration approaches ideal cycle-by-cycle control over the output voltage. When SKIP = low, Idle Mode circuitry automatically optimizes efficiency throughout the load current range. Idle Mode dramatically improves light-load efficiency by reducing the effective frequency, which reduces switching losses. It keeps the peak inductor current above 25% of the full current limit in an active cycle, allowing subsequent cycles to be skipped. Idle Mode transitions seamlessly to fixed-frequency PWM opera- tion as load current increases. With SKIP = high, the controller always operates in fixed-frequency PWM mode for lowest noise. Each pulse from the oscillator sets the main PWM latch that turns on the high-side switch for a period determined by the duty factor (approximately VOUT/VIN). As the high-side switch turns off, the synchronous rectifier latch sets; 60ns later, the low-side switch turns on. The low-side switch stays on until the beginning of the next clock cycle. In PWM mode, the controller operates as a fixed- frequency current-mode controller where the duty ratio is set by the input/output voltage ratio. The current- mode feedback system regulates the peak inductor Table 3. SKIP PWM Table Low Light LOAD CURRENT Pulse-skipping, supply cur- rent = 250µA at VIN = 12V, discontinuous inductor current DESCRIPTION Low Heavy Constant-frequency PWM, continuous inductor current SKIP Idle MODE PWM High Light PWM Constant-frequency PWM, continuous inductor current PWM High Heavy Constant-frequency PWM, continuous inductor current |
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