PDF LM2746 Data sheet ( Hoja de datos )

Número de pieza	LM2746
Descripción	Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller
Fabricantes	National Semiconductor
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No Preview Available ! May 2005 www.DataSheet4U.com LM2746 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller General Description The LM2746 is a high-speed synchronous buck regulator controller with a feedback voltage accuracy of ±1%. It can provide simple down conversion to output voltages as low as 0.6V. Though the control sections of the IC are rated for 3 to 5.5V, the driver sections are designed to accept input supply rails as high as 16V. The use of non-overlapping MOSFET gate drivers helps avoid potential shoot-through problems while maintaining high efficiency. The IC is designed for the more cost-effective option of driving only N-channel MOS- FETs in both the high-side and low-side positions. It senses the low-side switch voltage drop for providing a simple, adjustable current limit. The fixed-frequency voltage-mode PWM control architecture is adjustable from 50 kHz to 1 MHz with one external resis- tor. This wide range of switching frequency gives the power supply designer the flexibility to make better tradeoffs be- tween component size, cost and efficiency. Features include soft-start, input undervoltage lockout (UVLO) and Power Good (based on both undervoltage and overvoltage detection). In addition, the shutdown pin of the IC can be used for providing startup delay, and the soft-start pin can be used for implementing precise tracking, for the purpose of sequencing with respect to an external rail. Features n Power stage input voltage from 1V to 16V n Control stage input voltage from 3V to 5.5V n Output voltage adjustable down to 0.6V n Power good flag and shutdown n Output overvoltage and undervoltage detection n ±1% feedback voltage accuracy over temperature n Low-side adjustable current sensing n Adjustable soft-start n Tracking and sequencing with shutdown and soft start pins n Switching frequency from 50 kHz to 1 MHz n Exposed pad TSSOP-14 package Applications n Cable Modem, DSL and ADSL n Laser Jet and Ink Jet Printers n Low Voltage Power Modules n DSP, ASIC, Core and I/O Typical Application © 2005 National Semiconductor Corporation DS201477 20147701 www.national.com 1 page Typical Performance Characteristics Efficiency (VOUT = 1.2V) VCC = 3.3V, fSW = 300kHz www.DataSheet4U.com Efficiency (VOUT = 2.5V) VCC = 3.3V, fSW = 300kHz Efficiency (VOUT = 3.3V) VCC = 5V, fSW = 300kHz 20147740 20147757 VCC Operating Current plus BOOT Current vs Frequency FDS6898A FET (TA = 25˚C) 20147741 BOOT Pin Current vs Temperature for BOOT Voltage = 3.3V fSW = 300kHz, FDS6898A FET, No-Load 20147745 BOOT Pin Current vs Temperature for BOOT Voltage = 5V fSW = 300kHz, FDS6898A FET, No-Load 20147742 5 20147743 www.national.com 5 Page Application Information (Continued) www.DataSheet4U.com 20147714 FIGURE 4. Sequencing Circuit A desired delay time tDELAY between the startup of the master supply output voltage and the LM2746 output voltage can be set based on the SD pin low-to-high threshold VSD-IH and the slew rate of the voltage at the SD pin, SRSD: tDELAY = VSD-IH / SRSD Note again, that in Figure 4, the LM2746’s output voltage has been represented symbolically as VOUT2, i.e. without explicitly showing the power components. VSD-IH is typically 1.08V and SRSD is the slew rate of the SD pin voltage. The values of the sequencing divider resistors RS1 and RS2 set the SRSD based on the master supply output voltage slew rate, SROUT1, using the following equa- tion: FIGURE 5. Delay for Sequencing 20147711 SD PIN IMPEDANCE When connecting a resistor divider to the SD pin of the LM2746 some care has to be taken. Once the SD voltage goes above VSD-IH, a 17 µA pull-up current is activated as shown in Figure 6. This current is used to create the internal hysteresis ()170mV); however, high external impedances will affect the SD pin logic thresholds as well. The external impedance used for the sequencing divider network should preferably be a small fraction of the impedance of the SD pin for good performance (around 1kΩ). For example, if the master supply output voltage slew rate were 1V/ms and the desired delay time between the startup of the master supply and LM2746 output voltage were 5ms, then the desired SD pin slew rate would be (1.08V/5ms) = 0.216V/ms. Due to the internal impedance of the SD pin, the maximum recommended value for RS2 is 1kΩ. To achieve the desired slew rate, RS1 would then be 274Ω. A timing diagram for this example is shown in Figure 5. FIGURE 6. SD Pin Logic 20147706 MOSFET GATE DRIVERS The LM2746 has two gate drivers designed for driving N-channel MOSFETs in a synchronous mode. Note that unlike most other synchronous controllers, the bootstrap capacitor of the LM2746 provides power not only to the driver of the upper MOSFET, but the lower MOSFET driver 11 www.national.com 11 Page

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