LTC4444 Linear Technology, LTC4444 Datasheet - Page 9

• Manufacturer Part Number: LTC4444
• Description: High Voltage Synchronous N-Channel MOSFET Driver
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
Power Dissipation
To ensure proper operation and long-term reliability, the
LTC4444 must not operate beyond its maximum tem-
perature rating. Package junction temperature can be
calculated by:
where:
Power dissipation consists of standby and switching
power losses:
where:
The LTC4444 consumes very little quiescent current. The
DC power loss at V
(350μA)(12V) = 4.2mW.
At a particular switching frequency, the internal power loss
increases due to both AC currents required to charge and
discharge internal node capacitances and cross-conduc-
tion currents in the internal logic gates. The sum of the
quiescent current and internal switching current with no
load are shown in the Typical Performance Characteristics
plot of Switching Supply Current vs Input Frequency.
The gate charge losses are primarily due to the large AC
currents required to charge and discharge the capacitance
of the external MOSFETs during switching. For identical
pure capacitive loads C
frequency f
T
T
T
P
θ
P
P
P
P
with gate charge QG at frequency f
P
A
J
J
JA
AC
D
D
DC
QG
CLOAD
= T
= Junction temperature
= Ambient temperature
= Power dissipation
= P
= Junction-to-ambient thermal resistance
= Internal switching loss at input frequency, f
= Quiescent power loss
= Loss due turning on and off the external MOSFET
A
DC
+ P
= (C
IN
+ P
, the load losses would be:
D
LOAD
(θ
AC
JA
+ P
CC
)(f)[(V
)
QG
= 12V and V
LOAD
BOOST-TS
on TG and BG at switching
BOOST-TS
)
2
+ (V
IN
CC
)
= 12V is only
2
]
IN
In a typical synchronous buck confi guration, V
is equal to V
drop across the diode between V
drop is small relative to V
approximated as:
Unlike a pure capacitive load, a power MOSFET’s gate
capacitance seen by the driver output varies with its V
voltage level during switching. A MOSFET’s capacitive load
power dissipation can be calculated using its gate charge,
QG. The QG value corresponding to the MOSFET’s V
value (V
manufacturer’s QG vs V
on TG and BG:
To avoid damage due to power dissipation, the LTC4444
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 LTC4444 requires proper bypassing on the V
and V
(nanoseconds) and large AC currents (Amperes). Careless
component placement and PCB trace routing may cause
excessive ringing.
To obtain the optimum performance from the LTC4444:
A. Mount the bypass capacitors as close as possible
B. Use a low inductance, low impedance ground plane
P
P
between the V
TS pins. The leads should be shortened as much as
possible to reduce lead inductance.
to reduce any ground drop and stray capacitance.
Remember that the LTC4444 switches greater than
3A peak currents and any signifi cant ground drop will
degrade signal integrity.
CLOAD
QG
BOOST-TS
= 2(V
CC
= 2(C
in this case) can be readily obtained from the
CC
CC
)(Q
supplies due to its high speed switching
LOAD
– V
CC
G
)(f
D
and GND pins and the BOOST and
)(f
, where V
IN
IN
GS
)
)(V
curves. For identical MOSFETs
CC
CC
)
, the load losses can be
2
D
www.DataSheet4U.com
is the forward voltage
CC
and BOOST. If this
LTC4444
BOOST-TS
9
4444f
GS
GS
CC