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SC2621A 数据表(PDF) 6 Page - Semtech Corporation |
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SC2621A 数据表(HTML) 6 Page - Semtech Corporation |
6 / 14 page 6 2007 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2621A THEOR THEOR THEOR THEOR THEORY OF OPERA Y OF OPERA Y OF OPERA Y OF OPERA Y OF OPERATION TION TION TION TION The SC2621A integrates a high-speed, voltage mode PWM controller with a linear controller into a single pack- age. It is designed to control two independent output voltages for high performance graphic card applications. As shown in the block diagram of the SC2621A, the volt- age-mode PWM controller consists of an error amplifier, a 450kHz ramp generator, a PWM comparator, a RS latch circuit, and two MOSFET drivers. The buck converter out- put voltage is fed back to the error amplifier negative input and is regulated to a reference voltage level. The error amplifier output is compared with the ramp to gen- erate a PWM wave, which is amplified and used to drive the MOSFETs in the buck converter. The PWM wave at the phase node with the amplitude of Vin is filtered out to get a DC output. The linear controller is an error ampli- fier. It provides the gate drive and output voltage control for a linear regulator. Both PWM controller and linear con- troller work with soft-start and fault monitoring circuitry to meet application requirement. UVLO, Start-up and Shutdown UVLO, Start-up and Shutdown UVLO, Start-up and Shutdown UVLO, Start-up and Shutdown UVLO, Start-up and Shutdown To initiate the SC2621A, a supply voltage is applied to Vcc pin. The top gate (DH) and bottom gate (DL) are held low until Vcc voltage exceed UVLO (Under Voltage Lock Out) threshold, typically 4.0V. Then the internal Soft-Start (SS) capacitor begins to charge, the top gate remains low, and the bottom gate is pulled high to turn on the bottom MOSFET. When the SS voltage at the capacitor reaches 0.4V, the linear controller is enabled and the LDO output is turned on. Meanwhile, the top and bottom gates of PWM controller begin to switch. The switching regulator output is slowly ramping up for a soft turn-on. If the supply voltages at Vcc pin falls below UVLO thresh- old during a normal operation, the SS capacitor begins to discharge. When the SS voltage reaches 0.4V, the PWM controller controls the switching regulator output to ramp down slowly for a soft turn-off. Meanwhile, the linear controller is disabled and LDO output is turned off. Hiccup Mode Short Circuit Protection Hiccup Mode Short Circuit Protection Hiccup Mode Short Circuit Protection Hiccup Mode Short Circuit Protection Hiccup Mode Short Circuit Protection The SC2621A uses low-side MOSFET Rdson sensing for over current protection. In every switching cycle, after the bottom MOSFET is on for 150ns, the SC2621A de- tects the phase node voltage and compares it with an internal setting voltage. If the phase node is lower than the setting voltage, an overcurrent condition occurs. The SC2621A will discharge the internal SS capacitor and shutdown both outputs. After waiting for around 5 milli- seconds, the SC2621A begins to charge SS capacitor again and initiates a fresh startup. The startup and shut- down cycle will repeat until the short circuit is removed. This is called a hiccup mode short circuit protection. To program a load trip point for short circuit protection, it is recommended to connect a 3.3k resistor from the OCS pin to the ground, and a resistor Rset from the OCS pin to the DRV pin, as shown in Fig. 1. SC2621A GND OCS DRV VCC 10 2 12 3.3k Rset 12V 8 SC2621A GND OCS DRV VCC 10 2 12 3.3k Rset 12V 8 Fig. 1. Programming load trip point 150 175 200 225 250 275 300 325 350 0 100 200 300 400 500 600 Rset (k-ohm) 150 175 200 225 250 275 300 325 350 0 100 200 300 400 500 600 Rset (k-ohm) Fig. 2. Pull up resistor (Rset) vs. trip voltage Vpn The resistor Rset can be found in Fig. 2 for a given phase node voltage Vpn at the load trip point. This voltage is the product of the inductor peak current at the load trip point and the Rdson of the low-side MOSFET: on _ ds peak pn R I V ´ = The soft start time of the SC2621A is fixed at around Applications Information |
类似零件编号 - SC2621A |
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类似说明 - SC2621A |
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