LM2744 datasheet(12/22 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.

The most basic charge 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 MOS-

FETs is V

CC -VD. This method works well when VCC 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

D-MAX = 0.5V. Logic level MOSFETs generally specify their

on-resistance at V

GS = 4.5V. When VCC = 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, usu-

ally specified at V

GS = 2.5V, will work, but are more expen-

sive, and tend to have higher on-resistance. The circuit in

Figure 7 works well for input voltages ranging from 1V up to

16V and V

CC =5V ±10%, because the drive voltage de-

pends only on V

CC.

Note that the LM2744 can be paired with a low cost linear

regulator like the LP8340 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.

Figure 9 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 LM2744 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 100nF ceramic capacitors in the bootstrap locations.

20106012

FIGURE 7. Basic Charge Pump (Bootstrap)

20106013

FIGURE 8. LP8340 Feeding Basic Charge Pump

20106019

FIGURE 9. Charge Pump with Added Gate Drive

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