PDF LM25011 Data sheet ( Hoja de datos )

Número de pieza	LM25011
Descripción	2A Constant On-Time Switching Regulator
Fabricantes	National Semiconductor Corporation
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No Preview Available ! www.DataSheet4U.com LM25011 June 1, 2009 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit General Description The LM25011 Constant On-time Step-Down Switching Reg- ulator features all the functions needed to implement a low cost, efficient, buck bias regulator capable of supplying up to 2A of load current. This high voltage regulator contains an N- Channel Buck switch, a startup regulator, current limit detec- tion, and internal ripple control. The constant on-time regulation principle requires no loop compensation, results in fast load transient response, and simplifies circuit implemen- tation. The operating frequency remains constant with line and load. The adjustable valley current limit detection results in a smooth transition from constant voltage to constant cur- rent mode when current limit is reached, without the use of current limit foldback. The PGD output indicates the output voltage has increased to within 5% of the expected regulation value. Additional features include: Low output ripple, VIN un- der-voltage lock-out, adjustable soft-start timing, thermal shutdown, gate drive pre-charge, gate drive under-voltage lock-out, and maximum duty cycle limit. Features ■ Input operating voltage range: 6V to 42V ■ Absolute maximum input rating: 45V ■ Integrated 2A N-Channel Buck Switch ■ Adjustable current limit ■ Adjustable output voltage from 2.51V ■ Minimum ripple voltage at VOUT ■ Power Good output ■ Switching frequency adjustable to 2 MHz ■ Switching frequency remains nearly constant with load current and input voltage variations ■ Ultra-fast transient response ■ No loop compensation required ■ Stable operation with ceramic output capacitors ■ Adjustable Soft-Start timing ■ Thermal shutdown ■ Precision 2% feedback reference Package ■ MSOP-10EP Typical Application, Basic Step-Down Regulator © 2009 National Semiconductor Corporation 300946 30094601 www.national.com 1 page wwTwy.DpatiacSaheletP4Ue.crofmormance Characteristics Efficiency (Circuit of Figure 5) Efficiency at 2 MHz On-Time vs VIN and RT 30094603 Voltage at the RT Pin 30094604 30094605 Shutdown Current into VIN 30094606 Operating Current into VIN 30094607 5 30094608 www.national.com 5 Page www.DataSheet4U.com FIGURE 2. Normal and Current Limit Operation 30094624 Ripple Requirements The LM25011 requires a minimum of 10 mVp-p ripple voltage at the CS pin. That ripple voltage is generated by the de- creasing recirculating current (the inductor’s ripple current) through RS during the off-time. See Figure 3. FIGURE 3. CS Pin Waveform 30094625 The ripple voltage is equal to: VRIPPLE = ΔI x RS where ΔI is the inductor current ripple amplitude, and RS is the current sense resistor at the CS pin. N-Channel Buck Switch and Driver The LM25011 integrates an N-Channel buck switch and as- sociated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capac- itor (CBST) and an internal high voltage diode. A 0.1 µF ca- pacitor connected between BST and SW provides the supply voltage for the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and CBST is recharged from the internal 5V regulator for the next on-time. The mini- mum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor. Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turn-on, when VIN reaches its under-voltage lock-out threshold an internal 10 µA current source charges the external capacitor at the SS pin to 2.51V (t1 in Figure 1). The ramping voltage at SS ramps the non- inverting input of the regulation comparator, and the output voltage, in a controlled manner. For proper operation, the soft- start capacitor should be no smaller than 1000 pF. The LM25011 can be employed as a tracking regulator by applying the controlling voltage to the SS pin. The regulator’s output voltage tracks the applied voltage, gained up by the ratio of the feedback resistors. The applied voltage at the SS pin must be within the range of 0.5V to 2.6V. The absolute maximum rating for the SS pin is 3.0V. If the tracking function causes the voltage at the FB pin to go below the thresholds for the PGD pin, the PGD pin will switch low (see the Power Good Output section). An internal switch grounds the SS pin if the input voltage at VIN is below its under-voltage lock-out threshold or if the Thermal Shutdown activates. If the tracking function (described above) is used, the tracking voltage ap- plied to the SS pin must be current limited to a maximum of 1 mA. Shutdown Function The SS pin can be used to shutdown the LM25011 by ground- ing the SS pin as shown in Figure 4. Releasing the pin allows normal operation to resume. 11 www.national.com 11 Page

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