LTC4442/LTC4442-1
10
4442fb
APPLICATIONS INFORMATION
Power Dissipation
To ensure proper operation and long-term reliability, the
LTC4442 must not operate beyond its maximum tem-
perature 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 LTC4442 consumes very little quiescent current. The
DC power loss at V
LOGIC
= 5V and V
CC
= V
BOOST
− TS =
7V is only (730A)(5V) + (625A)(7V) = 8mW.
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 across the 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 LTC4442 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.
Bypassing and Grounding
The LTC4442 requires proper bypassing on the V
LOGIC
, 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 and undershoot/overshoot.
