LM2745 datasheet(12/23 Pages) NSC

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Application Information (Continued)

the Miller plateau level. This may therefore affect the choice

of the threshold voltage of the external MOSFETs, and that

in turn may depend on the chosen V

BOOT_DC rail.

So far, in the discussion above, the forward drop across the

bootstrap diode has been ignored. But since that does affect

the output of the driver somewhat, it is a good idea to include

this drop in the following examples. Looking at the Typical

Application schematic, this means that the difference voltage

CC -VD1, which is the voltage the bootstrap capacitor

charges up to, must always be greater than the maximum

tolerance limit of the threshold voltage of the upper MOS-

FET. Here V

D1 is the forward voltage drop across the boot-

strap diode D1. This may place restrictions on the minimum

input voltage and/or type of MOSFET used.

A basic bootstrap circuit can be built using one Schottky

diode and a small capacitor, as shown in Figure 9. The

capacitor C

BOOT serves to maintain enough voltage between

the top MOSFET gate and source to control the device even

when the top MOSFET is on and its source has risen up to

the input voltage level. The charge pump circuitry is fed from

CC, which can operate over a range from 3.0V to 6.0V.

Using this basic method the voltage applied to the gates of

both high-side and low-side MOSFETs is V

CC -VD. This

method works well when V

CC is 5V±10%, because the gate

drives will get at least 4.0V of drive voltage during the worst

case of V

CC-MIN = 4.5V and VD-MAX = 0.5V. Logic level

MOSFETs generally specify their on-resistance at V

GS =

4.5V. When V

CC = 3.3V±10%, the gate drive at worst case

could go as low as 2.5V. Logic level MOSFETs are not

guaranteed to turn on, or may have much higher on-

resistance at 2.5V. Sub-logic level MOSFETs, usually speci-

fied at V

GS = 2.5V, will work, but are more expensive, and

tend to have higher on-resistance. The circuit in Figure 9

works well for input voltages ranging from 1V up to 14V and

CC =5V±10%, because the drive voltage depends only on

CC.

Note that the LM2745/8 can be paired with a low cost linear

regulator like the LM78L05 to run from a single input rail

between 6.0 and 14V. The 5V output of the linear regulator

powers both the V

CC and the bootstrap circuit, providing

efficient drive for logic level MOSFETs. An example of this

circuit is shown in Figure 10.

Figure 11 shows a second possibility for bootstrapping the

MOSFET drives using a doubler. This circuit provides an

equal voltage drive of V

CC -3VD +VIN to both the high-side

and low-side MOSFET drives. This method should only be

used in circuits that use 3.3V for both V

CC and VIN. Even with

IN =VCC = 3.0V (10% lower tolerance on 3.3V) and VD =

0.5V both high-side and low-side gates will have at least

4.5V of drive. The power dissipation of the gate drive cir-

cuitry is directly proportional to gate drive voltage, hence the

thermal limits of the LM2745/8 IC will quickly be reached if

this circuit is used with V

CC or VIN voltages over 5V.

All the gate drive circuits shown in the above figures typically

use 100 nF ceramic capacitors in the bootstrap locations.

20137412

FIGURE 9. Basic Charge Pump (Bootstrap)

20137413

FIGURE 10. LM78L05 Feeding Basic Charge Pump

20137419

FIGURE 11. Charge Pump with Added Gate Drive

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