LM2746 datasheet(12/24 Pages) NSC

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Application Information (Continued)

too (both drivers are ground referenced, i.e. no floating

driver). To fully turn the top MOSFET on, the BOOT voltage

must be at least one gate threshold greater than V

IN when

the high-side drive goes high. This bootstrap voltage is

usually supplied from a local charge pump structure. But

looking at the Typical Application schematic, this also means

that the difference voltage V

CC -VD1, which is the voltage the

bootstrap capacitor charges up to, must be always greater

than the maximum tolerance limit of the threshold voltage of

the upper MOSFET. Here V

D1 is the forward voltage drop

across the bootstrap diode D1. This therefore may place

restrictions on the minimum input voltage and/or type of

MOSFET used.

Two things must be kept in mind here. First, the BOOT pin

has an absolute maximum rating of 21V. Since the bootstrap

capacitor is connected to the SW node, the peak voltage

impressed on the BOOT pin is the sum of the input voltage

IN) plus the voltage across the bootstrap capacitor (ignor-

ing any forward drop across the bootstrap diode). The boot-

strap capacitor is charged up by a given rail (called

BOOT_DC here) whenever the upper MOSFET turns off. This

rail can be the same as V

CC or it can be any external

ground-referenced DC rail. But care has to be exercised

when choosing this bootstrap DC rail that the BOOT pin is

not damaged. For example, if the desired maximum V

IN is

16V, and V

BOOT_DC is chosen to be the same as VCC, then

clearly if the V

CC rail is 5.5V, the peak voltage on the BOOT

pin is 16V + 5.5V = 21.5V. This is unacceptable, as it is in

excess of the rating of the BOOT pin. A V

CC of 3V would be

acceptable in this case. Or the V

IN range must be reduced

accordingly. There is also the option of deriving the bootstrap

DC rail from another 3V external rail, independent of V

CC.

The second thing to be kept in mind here is that the output of

the low-side driver swings between the bootstrap DC rail

level of V

BOOT_DC and Ground, whereas the output of the

high-side driver swings between V

IN+VBOOT_DC

and

Ground. To keep the high-side MOSFET fully on when de-

sired, the Gate pin voltage of the MOSFET must be higher

than its instantaneous Source pin voltage by an amount

equal to the ’Miller plateau’. It can be shown that this plateau

is equal to the threshold voltage of the chosen MOSFET plus

a small amount equal to Io/g. Here Io is the maximum load

current of the application, and g is the transconductance of

this MOSFET (typically about 100 for logic-level devices).

That means we must choose V

BOOT_DC to at least exceed

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, 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 V

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 MOSFET. Here V

D1 is the

forward voltage drop across the bootstrap diode D1. This

may place restrictions on the minimum input voltage and/or

type of MOSFET used.

The basic bootstrap pump circuit can be built using one

Schottky diode and a small capacitor, as shown in Figure 7.

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 V

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 7

works well for input voltages ranging from 1V up to 16V and

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

CC.

Note that the LM2746 can be paired with a low cost linear

regulator like the LM78L05 to run from a single input rail

between 6.0 and 16V. 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 8.

20147712

FIGURE 7. Basic Charge Pump (Bootstrap)

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