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MAX1632 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1632 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 28 page _______________Detailed Description The MAX1630 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 MAX1630 family contain ten major circuit blocks (Figure 2). The two pulse-width modulation (PWM) controllers each consist of a Dual Mode™ feedback network and multi- plexer, a multi-input PWM comparator, high-side and low-side gate drivers, and logic. MAX1630/MAX1631/ MAX1632 contain fault-protection circuits that monitor the main PWM outputs for undervoltage and overvolt- age. 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 undervolt- age faults. The MAX1630/MAX1632/MAX1633/ MAX1635 include a secondary feedback network and 12V linear regulator to generate a 12V output from a coupled-inductor flyback winding. The MAX1631/ MAX1634 have a secondary feedback input (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 genera- tor blocks include the 5V IC internal rail (VL) linear regu- lator, 2.5V precision reference, and automatic bootstrap switchover circuit. The PWMs share a common 200kHz/300kHz synchronizable oscillator. These internal IC blocks aren’t 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 via 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. Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers ______________________________________________________________________________________ 11 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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Similar Description - MAX1632 |
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