Datasheet

LTC4449
9
4449fa
APPLICATIONS INFORMATION
OPERATION
The LTC4449’s powerful parallel combination of the
N-channel MOSFET (N2) and NPN (Q3) on the BG
pull-down generates a phenomenal 4ns fall time on BG
while driving a 3nF load. Similarly, the 0.8 pull-down
Power Dissipation
To ensure proper operation and long-term reliability,
the LTC4449 must not operate beyond its maximum
temperature rating. Package junction temperature can
be calculated by:
T
J
= T
A
+ (P
D
)(θ
JA
)
where:
T
J
= junction temperature
T
A
= ambient temperature
P
D
= power dissipation
θ
JA
= junction-to-ambient thermal resistance
Power dissipation consists of standby, switching and
capacitive load power losses:
P
D
= P
DC
+ P
AC
+ P
QG
where:
P
DC
= quiescent power loss
P
AC
= internal switching loss at input frequency f
IN
P
QG
= loss due turning on and off the external
MOSFET with gate charge Q
G
at frequency f
IN
The LTC4449 consumes very little quiescent current. The
DC power loss at V
LOGIC
= 5V and V
CC
= 5V is only (730A
+ 600µA)(5V) = 6.65mW.
At a particular switching frequency, the internal power loss
increases due to both AC currents required to charge and
discharge internal nodal 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
LOAD
on TG and BG at switching
frequency fi n, the load losses would be:
P
CLOAD
= (C
LOAD
)(f
IN
)[(V
BOOST – TS
)
2
+ (V
CC
)
2
]
In a typical synchronous buck confi guration, V
BOOST – TS
is equal to V
CC
– V
D
, where V
D
is the forward voltage drop
of the external Schottky diode between V
CC
and BOOST.
If this drop is small relative to V
CC
, the load losses can
be approximated as:
P
CLOAD
≈ 2(C
LOAD
)(f
IN
)(V
CC
)
2
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 damaging junction temperatures due to power
dissipation, the LTC4449 includes a temperature monitor
that will pull BG and TG low if the junction temperature
exceeds 160°C. Normal operation will resume when the
junction temperature cools to less than 135°C.
MOSFET (N1) on TG results in a rapid 7ns fall time with
a 3nF load. These powerful pull-down devices minimize
the power loss associated with MOSFET turn-off time and
cross-conduction current.