8.1.1 Boost Function

Capacitor C_BOOST and diode D2 in Figure 13 are used to generate a voltage V_BOOST. V_BOOST - V_SW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, V_BOOST needs to be at least 1.6 V greater than V_SW. Although the LM2734 device will operate with this minimum voltage, it may not have sufficient gate drive to supply large values of output current. Therefore, it is recommended that V_BOOST be greater than 2.5 V above V_SW for best efficiency. V_BOOST – V_SW should not exceed the maximum operating limit of 5.5 V.

5.5 V > V_BOOST – V_SW > 2.5 V for best performance.

Figure 13. V_OUT Charges C_BOOST

When the LM2734 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to C_BOOST. This current charges C_BOOST to a voltage sufficient to turn the switch on. The BOOST pin continues to source current to C_BOOST until the voltage at the feedback pin is greater than 0.76 V.

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 simplified block diagram of Functional Block Diagram, 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 12), V_BOOST equals V_IN minus the forward voltage of D2 (V_FD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (V_FD1). Therefore, the voltage stored across C_BOOST is:

When the NMOS switch turns on (T_ON), the switch pin rises to:

forcing V_BOOST to rise thus reverse biasing D2. The voltage at V_BOOST is then:

which is approximately:

for many applications. Thus the gate-drive voltage of the NMOS switch is approximately:

An alternate method for charging C_BOOST is to connect D2 to the output as shown in Figure 13. The output voltage should be from 2.5 V and 5.5 V, 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 V_OUT are greater than 5.5 V, or less than 3 V, C_BOOST cannot be charged directly from these voltages. If V_IN to V_OUT are greater than 5.5 V, C_BOOST can be charged from V_IN or V_OUT minus a Zener voltage by placing a Zener diode D3 in series with D2, as shown in Figure 14. When using a series Zener diode from the input, ensure that the regulation of the input supply does not create a voltage that falls outside the recommended V_BOOST voltage.

Figure 14. Zener Reduces Boost Voltage from V_IN

An alternative method is to place the Zener diode D3 in a shunt configuration as shown in Figure 15. A small 350 mW to 500 mW 5.1-V Zener diode in a SOT or SOD package can be used for this purpose. A small ceramic capacitor such as a 6.3 V, 0.1-µF 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 current to the Zener diode (D3) and to the BOOST pin. A recommended choice for the Zener current (I_ZENER) is 1 mA. 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 for the X version:

I_BOOST can be calculated for the Y version using the following:

where D is the duty cycle, V_ZENER and V_D2 are in volts, and I_BOOST is in milliamps. V_ZENER 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:

R3 will then be given by:

For example, using the X-version let V_IN = 10 V, V_ZENER = 5 V, V_D2 = 0.7 V, I_ZENER = 1 mA, and duty cycle D = 50%. Then:

Figure 15. Boost Voltage Supplied from the Shunt Zener on V_IN