LM2734

BOOST

SW

GND

L

D1

D2

D3

LM2734Z

SNVS334E – JANUARY 2005 – REVISED APRIL 2013

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(1)

(2)

(3)

which is approximately

(4)

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

(5)

voltage should be between 2.5V and 5.5V, so that proper gate voltage will be applied to the internal switch. In

minus a zener voltage by placing a zener diode D3 in series with D2, as shown in Figure 11. When using a

series zener diode from the input, ensure that the regulation of the input supply doesn’t create a voltage that falls

(6)

(7)

An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 12. A small

350mW to 500mW 5.1V zener in a SOT 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

Resistor R3 should be chosen to provide enough RMS current to the zener diode (D3) and to the BOOST pin. A

gate current of the NMOS control switch and varies typically according to the following formula:

(8)

boost current will be

(9)

8

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