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LM2736XMK Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2736XMK Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 22 page Application Information (Continued) current. Therefore, it is recommended that V BOOST be greater than 2.5V above V SW for best efficiency. VBOOST – V SW should not exceed the maximum operating limit of 5.5V. 5.5V > V BOOST –VSW > 2.5V for best performance. When the LM2736 starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA to C BOOST. This current charges C BOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to source current to C BOOST until the voltage at the feedback pin is greater than 1.18V. There are various methods to derive V BOOST: 1. From the input voltage (V IN) 2. From the output voltage (V OUT) 3. From an external distributed voltage rail (V EXT) 4. From a shunt or series zener diode In the Simplifed Block Diagram of Figure 1, capacitor C BOOST and diode D2 supply the gate-drive current for the NMOS switch. Capacitor C BOOST is charged via diode D2 by V IN. During a normal switching cycle, when the internal NMOS control switch is off (T OFF) (refer to Figure 2), VBOOST equals V IN minus the forward voltage of D2 (VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (V FD1). Therefore the voltage stored across C BOOST is V BOOST -VSW =VIN -VFD2 +VFD1 When the NMOS switch turns on (T ON), the switch pin rises to V SW =VIN –(RDSON xIL), forcing V BOOST to rise thus reverse biasing D2. The voltage at V BOOST is then V BOOST =2VIN –(RDSON xIL)– VFD2 +VFD1 which is approximately 2V IN - 0.4V for many applications. Thus the gate-drive voltage of the NMOS switch is approximately V IN - 0.2V An alternate method for charging C BOOST is to connect D2 to the output as shown in Figure 3. The output voltage should be between 2.5V and 5.5V, so that proper gate voltage will be applied to the internal switch. In this circuit, C BOOST provides a gate drive voltage that is slightly less than V OUT. In applications where both V IN and VOUT are greater than 5.5V, or less than 3V, C BOOST cannot be charged directly from these voltages. If V IN and VOUT are greater than 5.5V, C BOOST can be charged from VIN or VOUT minus a zener voltage by placing a zener diode D3 in series with D2, as shown in Figure 4. When using a series zener diode from the input, ensure that the regulation of the input supply doesn’t create a voltage that falls outside the recommended V BOOST voltage. (V INMAX –VD3) < 5.5V (V INMIN –VD3) > 1.6V An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 5. A small 350mW to 500mW 5.1V zener in a SOT-23 or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3V, 0.1µF capacitor (C4) should be placed in parallel with the zener diode. When the internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The 0.1 µF parallel shunt capacitor ensures that the V BOOST voltage is maintained during this time. Resistor R3 should be chosen to provide enough RMS cur- rent to the zener diode (D3) and to the BOOST pin. A recommended choice for the zener current (I ZENER)is1mA. The current I BOOST into the BOOST pin supplies the gate current of the NMOS control switch and varies typically according to the following formula: I BOOST = 0.56 x (D + 0.54) x (VZENER –VD2)mA where D is the duty cycle, V ZENER and VD2 are in volts, and I BOOST is in milliamps. VZENER is the voltage applied to the anode of the boost diode (D2), and V D2 is the average forward voltage across D2. Note that this formula for I BOOST gives typical current. For the worst case I BOOST, increase the current by 40%. In that case, the worst case boost current will be I BOOST-MAX =1.4xIBOOST R3 will then be given by R3=(V IN -VZENER)/(1.4xIBOOST +IZENER) For example, let V IN = 10V, VZENER =5V, VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then I BOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11k Ω 20124208 FIGURE 3. V OUT Charges CBOOST 20124209 FIGURE 4. Zener Reduces Boost Voltage from V IN www.national.com 8 |
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