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SC4612STRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4612STRT Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 24 page  9 2007 Semtech Corp. www.semtech.com SC4612 POWER MANAGEMENT Applications Information INTRODUCTION The SC4612 is a versatile voltage mode synchronous rectified buck PWM convertor, with an input supply (VIN) ranging from 4.5V to 28V designed to control and drive N-channel MOSFETs. The power dissipation is controlled using a novel low volt- age supply technique, allowing high speed and integra- tion with the high drive currents to ensure low MOSFET switching loss. The synchronous buck configuration also allows converter sinking current from load without losing output regulation. The internal reference is trimmed to 500mV with ± 1% accuracy, and the output voltage can be adjusted by two external resistors. A fixed oscillator frequency (up to 1.2MHz) can be programmed by an external capacitor for an optimized design. During the Asynchronous start up, the SC4612 provides a top MOSFET shut down over current protection, while under normal operating conditions a low side MOSFET R DS-ON current sensing with hiccup mode over current protection, minimizes power dissipation and provides further protection. Other features of the SC4612 include: Wide input power voltage range (from 4.5V to 28V), low output voltages down to 500mV, externally programmable soft-start, hiccup over current protection, wide duty cycle range, thermal shutdown, asynchronous start-up protection, and a -40 to 125°C junction operating temperature range. THEORY OF OPERATION SUPPLIES Two pins (VDD and DRV) are used to power up the SC4612. If input supply (VDD) is less than 10V (MAX), tie DRV and VDD together. This supply should be bypassed with a low ESR 2.2uF (or greater) ceramic capacitor directly at the DRV to GND pins of the SC4612. The DRV supply also provides the bias for the low and the high side MOSFET gate drive. The maximum rating for DRV supply is 10V and for applications where input supply is below 10V, it may be connected directly to VDD. START UP SEQUENCE Start up is inhibited until VDD input reaches its UVLO threshold. The UVLO limit is 4.5V (TYP). Meanwhile, the high side and low side gate drivers DH, and DL, are kept low. Once VDD exceeds the UVLO threshold, the external soft-start capacitor starts to be charged by a 25µA current source. If an over current condition occurs, the SS/EN pin will discharge to 500mV by an internal switch. During this time, both DH and DL will be turned off. When the SS pin reaches 0.8V, the converter will start switching. The reference input of the error amplifier is ramped up with the soft-start signal. Initially only the high side driver is enabled. Keeping the low side MOSFET off during start up is useful where multiple convertors are operating in parallel. It prevents forward conduction in the freewheeling MOSFET which might otherwise cause a dip in the common output bus. In case of over current condition which is longer than 10 cycles during the asynchronous start up, SC4612 will turn off the high side MOSFET gate drive, and the soft-start sequence will repeat. When the SS pin reaches 1.3V, the low side MOSFET will begin to switch and the convertor is fully operational in the synchronous mode. The soft-start duration is controlled by the value of the SS cap. If the SS pin is pulled below 0.5V, the SC4612 is disabled and draws a typical quiescent current of 5mA. Bias Generation A 4.5V to 10V (MAX) supply voltage is required to power up the SC4612. This voltage could be provided by an ex- ternal power supply or derived from VDD (VDD >10V) through an internal pass transistor. The internal pass transistor will regulate the DRV from an external supply >10V connected to VDD to produce 7.8V (TYP) at the DRV pin. Soft start / Shut down An external capacitor at the SS/EN pin is used to set up the soft-start duration. The capacitor value in conjunction with the internal current source, controls the duration of soft-start time. If the SS/EN pin is pulled down to GND, the SC4612 is disabled. The soft-start pin is charged by a 25µA current source and discharged by an internal switch. When SS/EN is released it charges up to 0.5V as the con- trol circuit starts up. |
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