Datasheet

LTC4446
9
4446f
APPLICATIONS INFORMATION
Unlike a pure capacitive load, a power MOSFET’s gate
capacitance seen by the driver output varies with its V
GS
voltage level during switching. A MOSFET’s capacitive load
power dissipation can be calculated using its gate charge,
Q
G
. The Q
G
value corresponding to the MOSFET’s V
GS
value (V
CC
in this case) can be readily obtained from the
manufacturer’s Q
G
vs V
GS
curves. For identical MOSFETs
on TG and BG:
P
QG
= 2(V
CC
)(Q
G
)(f
IN
)
To avoid damage due to power dissipation, the LTC4446
includes a temperature monitor that will pull BG and TG
low if the junction temperature rises above 160°C. Normal
operation will resume when the junction temperature cools
to less than 135°C.
Bypassing and Grounding
The LTC4446 requires proper bypassing on the V
CC
and V
BOOST-TS
supplies due to its high speed switching
(nanoseconds) and large AC currents (Amperes). Careless
component placement and PCB trace routing may cause
excessive ringing.
To obtain the optimum performance from the LTC4446:
A. Mount the bypass capacitors as close as possible
between the V
CC
and GND pins and the BOOST and
TS pins. The leads should be shortened as much as
possible to reduce lead inductance.
B. Use a low inductance, low impedance ground plane
to reduce any ground drop and stray capacitance.
Remember that the LTC4446 switches greater than
3A peak currents and any signifi cant ground drop will
degrade signal integrity.
C. Plan the power/ground routing carefully. Know where
the large load switching current is coming from and
going to. Maintain separate ground return paths for
the input pin and the output power stage.
D. Keep the copper trace between the driver output pin
and the load short and wide.
E. Be sure to solder the Exposed Pad on the back side of
the LTC4446 package to the board. Correctly soldered
to a 2500mm
2
doublesided 1oz copper board, the
LTC4446 has a thermal resistance of approximately
40°C/W for the MS8E package. Failure to make good
thermal contact between the exposed back side and
the copper board will result in thermal resistances far
greater than 40°C/W.