LM3405AXMY datasheet(10/32 Pages) TI

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LM3405A

SNVS508C – OCTOBER 2007 – REVISED MAY 2013

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The third method can be used in the applications where both VIN and VOUT are greater than 5.5V. In these cases,

C3 cannot be charged directly from these voltages; instead C3 can be charged from VIN or VOUT minus a zener

voltage (VD3) by placing a zener diode D3 in series with D2 as shown in Figure 16. When using a series zener

diode from the input, the gate drive voltage is VIN - VD3 - VD2 + VD1.

Figure 16. VBOOST Derived from VIN through a Series Zener

An alternate method is to place the zener diode D3 in a shunt configuration as shown in Figure 17. 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 (C5) 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

0.1µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time. Resistor R2 should

be chosen to provide enough RMS current to the zener diode and to the BOOST pin. A recommended choice for

the zener current (IZENER) is 1mA. The current IBOOST into the BOOST pin supplies the gate current of the NMOS

power switch. It reaches a maximum of around 3.6mA at the highest gate drive voltage of 5.5V over the

LM3405A operating range.

For the worst case IBOOST, increase the current by 50%. In that case, the maximum boost current will be:

IBOOST-MAX = 1.5 x 3.6mA = 5.4mA

(6)

R2 will then be given by:

R2 = (VIN - VZENER) / (IBOOST_MAX + IZENER)

(7)

For example, let VIN = 12V, VZENER = 5V, IZENER = 1mA, then:

R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09k

Ω

(8)

Figure 17. VBOOST derived from VIN through a Shunt Zener

The fourth method can be used in an application which has an external low voltage rail, VEXT. C3 can be charged

through D2 from VEXT, independent of VIN and VOUT voltage levels. Again for best performance, ensure that the

gate drive voltage, VEXT - VD2 + VD1, falls in the range of 2.5V to 5.5V.

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LM3405AXMY Datasheet(PDF) 10 Page - Texas Instruments