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MAX8548 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX8548 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 18 page Low-Cost, Wide Input Range, Step-Down Controllers with Foldback Current Limit 8 _______________________________________________________________________________________ Detailed Description The MAX8545/MAX8546/MAX8548 are BiCMOS switch- mode power-supply controllers designed to implement simple, buck-topology regulators in cost-sensitive applications. The main power-switching circuit consists of two n-channel MOSFETs, an inductor, and input/out- put filter capacitors. An all n-channel synchronous-rec- tified design provides high efficiency at reduced cost. These devices have an internal 5V linear regulator that steps down the input voltage to supply the IC and the gate drivers. The low-side-switch gate driver is directly powered from the 5V regulator (VL), while the high- side-switch gate driver is indirectly powered from VL plus an external diode-capacitor boost circuit. Current-Limit and Short-Circuit Protection The MAX8545/MAX8546/MAX8548 employ a valley cur- rent-sensing algorithm that uses the RDS(ON) of the low- side n-channel MOSFET to sense the current. This eliminates the need for an external sense resistor usually placed in series with the output. The voltage measured across the low-side MOSFET’s RDS(ON) is compared to a fixed -320mV reference for the MAX8545/MAX8548 and a fixed -165mV reference for the MAX8546. The cur- rent limit is given by the equations below: Aside from current limiting, these devices feature fold- back short-circuit protection. This feature is designed to reduce the current limit by 80% as the output voltage drops to 0V. MOSFET Gate Drivers The DH and DL drivers are optimized for driving n-chan- nel MOSFETs with low gate charge. An adaptive dead- time circuit monitors the DL output and prevents the high-side MOSFET from turning on until the low-side MOSFET is fully off. There must be a low-resistance, low-inductance connection from the DL driver to the MOSFET gate for the adaptive dead-time circuit to work properly. Otherwise, the sense circuitry in the MAX8545/ MAX8546/MAX8548 may detect the MOSFET gate as off while there is actually charge left on the gate. Use very short, wide traces measuring no less than 50mils to 100 mils wide if the MOSFET is 1in away from the MAX8545/ MAX8546/MAX8548. The same type of adaptive dead- time circuit monitors the DH off edge. The same recom- mendations apply for the gate connection of the high-side MOSFET. The internal pulldown transistor that drives DL low is robust, with a 1.1Ω (typ) on-resistance. This helps pre- vent DL from being pulled up due to capacitive cou- pling from the drain to the gate of the low-side synchronous-rectifier MOSFET during the fast rise time of the LX node. Soft-Start The MAX8545/MAX8546/MAX8548 feature an internally set soft-start function that limits inrush current. It accom- plishes this by ramping the internal reference input to the controller’s transconductance error amplifier from 0 to the 0.8V reference voltage. The ramp time is 1024 oscil- lator cycles for the MAX8548 and 2048 oscillator cycles for the MAX8545/MAX8546. At the nominal 100kHz and 300kHz switching rate, the soft-start ramp is approxi- mately 10.2ms and 6.8ms, respectively. High-Side Gate-Drive Supply (BST) A flying-capacitor boost circuit generates gate-drive volt- age for the high-side n-channel MOSFET. The flying capacitor is connected between the BST and LX nodes. On startup, the synchronous rectifier (low-side MOSFET) forces LX to ground and charges the boost capacitor to VL. On the second half-cycle, the MAX8545/MAX8546/ MAX8548 turn on the high-side MOSFET by closing an internal switch between BST and DH. This provides the necessary gate-to-source voltage to drive the high-side MOSFET gate above its source at the input voltage. Internal 5V Linear Regulator All MAX8545/MAX8546/MAX8548 functions are internally powered from an on-chip, low-dropout 5V regulator (VL). These devices have a maximum input voltage (VIN) of 28V. Connect VCC to VL through a 10Ω resistor and bypass VCC to GND with a 0.1µF ceramic capacitor. The VIN-to-VL dropout voltage is typically 140mV, so when VIN is less than 5.5V, VL is typically VIN - 140mV. The internal linear regulator can source a minimum of 25mA and a maximum of approximately 40mA to supply power to the IC low-side and high-side MOSFET drivers. Duty-Cycle Limitations for Low VOUT/VIN Ratios The MAX8545/MAX8546/MAX8548s’ output voltage is adjustable down to 0.8V. However, the minimum duty cycle can limit the ability to supply low-voltage outputs I mV R MAX LIMIT DS ON = 165 8546 () () I mV R MAX MAX LIMIT DS ON = 320 8545 8548 () (/ ) |
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