MIC4605 Datasheet, PDF (14/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
A low-power, high-speed, level-shifting circuit isolates the
low side (VSS pin) referenced circuitry from the high-side
(HS pin) referenced driver. Power to the high-side driver
and UVLO circuit is supplied by the bootstrap circuit while
the voltage level of the HS pin is shifted high.
The bootstrap circuit consists of an internal diode and
external capacitor, C_B. In a typical application, such as
the synchronous buck converter shown in Figure 6, the
HS pin is at ground potential while the low-side MOSFET
is on. The internal diode allows capacitor C_B to charge up
to VDD-VF during this time (where VF is the forward
voltage drop of the internal diode). After the low-side
MOSFET is turned off and the HO pin turns on, the
voltage across capacitor C_B is applied to the gate of the
upper external MOSFET. As the upper MOSFET turns
on, voltage on the HS pin rises with the source of the
high-side MOSFET until it reaches VIN. As the HS and
HB pin rise, the internal diode is reverse biased
preventing capacitor C_B from discharging.
MIC4605
In portable hand tools and other battery-powered
applications, the MIC4605 offers the ability to drive
motors at a lower voltage compared to the traditional
MOSFET drivers because of the wide VDD range (5.5V
to 16V). Traditional MOSFET drivers typically require a
VDD greater than 9V. The MIC4605 drives a motor using
only two Li-ion batteries (total 7.2V) compared to
traditional MOSFET drivers which will require at least
three cells (total of 10.8V) to exceed the minimal VDD
range. As an additional benefit, the low 5.5V gate drive
capability allows a longer run time. This is because the
Li-ion battery can run down to 5.5V, which is just above
its 4.8V minimum recommended discharge voltage. This
is also a benefit in higher current power tools that use five
or six cells. The driver can be operated up to 16V to
minimize the R_DSON of the MOSFETs and use as much of
the discharge battery pack as possible for a longer run
time. For example, an 18V battery pack can be used to
the lowest operating discharge voltage of 13.5V.
Figure 6. MIC4605 Driving a Synchronous Buck Converter
Programmable Gate Drive
The MIC4605 offers programmable gate drive, which
means the MOSFET gate drive (gate to source voltage)
equals the VDD voltage. This feature offers designers
flexibility in driving the MOSFETs. Different MOSFETs
require different VGS characteristics for optimum R_DSON
performance. Typically, the higher the gate voltage (up to
16V), the lower the R_DSON achieved. For example, a
NTMSF4899NF MOSFET can be driven to the ON state
at 4.5V gate voltage but R_DSON is 7.5mΩ. If driven to 10V
gate voltage, R_DSON is 4.5mΩ. In low-current applications,
the losses due to R_DSON are minimal, but in high-current
applications such as power hand tools, the difference in
R_DSON can cut into the efficiency budget.
November 11, 2013
14
Revision 1.0