SNVS288K September 2004 – September 2018 LM2734
Capacitor CBOOST and diode D2 in Figure 13 are used to generate a voltage VBOOST. VBOOST - VSW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, VBOOST needs to be at least 1.6 V greater than VSW. 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 VBOOST be greater than 2.5 V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5 V.
5.5 V > VBOOST – VSW > 2.5 V for best performance.
When the LM2734 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to CBOOST. This current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin continues to source current to CBOOST until the voltage at the feedback pin is greater than 0.76 V.
There are various methods to derive VBOOST:
In the simplified block diagram of Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive current for the NMOS switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the internal NMOS control switch is off (TOFF) (refer to Figure 12), VBOOST equals VIN minus the forward voltage of D2 (VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore, the voltage stored across CBOOST is:
When the NMOS switch turns on (TON), the switch pin rises to:
forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then:
which is approximately:
for many applications. Thus the gate-drive voltage of the NMOS switch is approximately:
An alternate method for charging CBOOST 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, CBOOST provides a gate drive voltage that is slightly less than VOUT.
In applications where both VIN and VOUT are greater than 5.5 V, or less than 3 V, CBOOST cannot be charged directly from these voltages. If VIN to VOUT are greater than 5.5 V, CBOOST 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 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 VBOOST voltage.
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 VBOOST 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 (IZENER) is 1 mA. The current IBOOST 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:
IBOOST can be calculated for the Y version using the following:
where D is the duty cycle, VZENER and VD2 are in volts, and IBOOST is in milliamps. VZENER is the voltage applied to the anode of the boost diode (D2), and VD2 is the average forward voltage across D2. Note that this formula for IBOOST gives typical current. For the worst case IBOOST, 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 VIN = 10 V, VZENER = 5 V, VD2 = 0.7 V, IZENER = 1 mA, and duty cycle D = 50%. Then: