Datasheet

+
LG
HG
BOOT
+
V
IN
D1
C
BOOT
LM2745/8
LM78L05
5V
V
O
V
CC
+
BOOT
HG
LG
V
IN
V
O
V
CC
+
LM2745/8
D1
C
BOOT
LM2745, LM2748
www.ti.com SNOSAL2E –APRIL 2005–REVISED APRIL 2013
Figure 25. Basic Charge Pump (Bootstrap)
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 26.
Figure 26. LM78L05 Feeding Basic Charge Pump
Figure 27 shows a second possibility for bootstrapping the MOSFET drives using a doubler. This circuit provides
an equal voltage drive of V
CC
- 3V
D
+ V
IN
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 V
IN
. Even with V
IN
= V
CC
= 3.0V (10% lower tolerance on
3.3V) and V
D
= 0.5V both high-side and low-side gates will have at least 4.5V of drive. The power dissipation of
the gate drive circuitry 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 V
IN
voltages over 5V.
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