LM2733YMFX

LM2733 0.6/1.6 MHz Boost Converters With 40V Internal FET Switch in SOT-23 July 2007 LM2733 0.6/1.6 MHz Boost Converters With 40V Internal FET Switch in SOT-23 General Description The LM2733 switching regulators are current-mode boost converters operating fixed frequency of 1.6 MHz (“X” option) and 600 kHz (“Y” option). The use of SOT-23 package, made possible by the minimal power loss of the internal 1A switch, and use of small inductors and capacitors result in the industry's highest power density. The 40V internal switch makes these solutions perfect for boosting to voltages of 16V or greater. These parts have a logic-level shutdown pin that can be used to reduce quiescent current and extend battery life. Protection is provided through cycle-by-cycle current limiting and thermal shutdown. Internal compensation simplifies design and reduces component count. Switch Frequency X 1.6 MHz Y 0.6 MHz Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 40V DMOS FET switch 1.6 MHz (“X”), 0.6 MHz (“Y”) switching frequency Low RDS(ON) DMOS FET Switch current up to 1A Wide input voltage range (2.7V–14V) Low shutdown current ( 1A at duty cycle below 50%. For higher duty cycles, see Typical performance Characteristics curves. THERMAL CONSIDERATIONS At higher duty cycles, the increased ON time of the FET means the maximum output current will be determined by power dissipation within the LM2733 FET switch. The switch power dissipation from ON-state conduction is calculated by: P(SW) = DC x IIND(AVE)2 x RDSON The equation shown to calculate maximum load current takes into account the losses in the inductor or turn-OFF switching losses of the FET and diode. For actual load current in typical applications, we took bench data for various input and output voltages for both the "X" and "Y" versions of the LM2733 and www.national.com 12 LM2733 There will be some switching losses as well, so some derating needs to be applied when calculating IC power dissipation. MINIMUM INDUCTANCE In some applications where the maximum load current is relatively small, it may be advantageous to use the smallest possible inductance value for cost and size savings. The converter will operate in discontinuous mode in such a case. The minimum inductance should be selected such that the inductor (switch) current peak on each cycle does not reach the 1A current limit maximum. To understand how to do this, an example will be presented. In the example, the LM2733X will be used (nominal switching frequency 1.6 MHz, minimum switching frequency 1.15 MHz). This means the maximum cycle period is the reciprocal of the minimum frequency: TON(max) = 1/1.15M = 0.870 µs We will assume the input voltage is 5V, VOUT = 12V, VSW = 0.2V, VDIODE = 0.3V. The duty cycle is: Duty Cycle = 60.3% Therefore, the maximum switch ON time is 0.524 µs. An inductor should be selected with enough inductance to prevent the switch current from reaching 1A in the 0.524 µs ON time interval (see below): The voltage across the inductor during ON time is 4.8V. Minimum inductance value is found by: V = L X dl/dt, L = V X (dt/dl) = 4.8 (0.524µ/1) = 2.5 µH In this case, a 2.7 µH inductor could be used assuming it provided at least that much inductance up to the 1A current value. This same analysis can be used to find the minimum inductance for any boost application. Using the slower switching “Y” version requires a higher amount of minimum inductance because of the longer switching period. INDUCTOR SUPPLIERS Some of the recommended suppliers of inductors for this product include, but not limited to are Sumida, Coilcraft, Panasonic, TDK and Murata. When selecting an inductor, make certain that the continuous current rating is high enough to avoid saturation at peak currents. A suitable core type must be used to minimize core (switching) losses, and wire power losses must be considered when selecting the current rating. SHUTDOWN PIN OPERATION The device is turned off by pulling the shutdown pin low. If this function is not going to be used, the pin should be tied directly to VIN. If the SHDN function will be needed, a pull-up resistor must be used to VIN (approximately 50k-100kΩ recommended). The SHDN pin must not be left unterminated. 20055413 Discontinuous Design, 5V–12V Boost (LM2733X) 13 www.national.com LM2733 Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead SOT-23 Package Order Number LM2733XMF, LM2733XMFX, LM2733YMF or LM2733YMFX NS Package Number MF05A www.national.com 14 LM2733 Notes 15 www.national.com LM2733 0.6/1.6 MHz Boost Converters With 40V Internal FET Switch in SOT-23 Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. 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