Datasheet

LTC3736-2
15
37362fb
The MOSFET power dissipations at maximum output
current are:
P
V
V
IRV
I
TOP
OUT
IN
OUT MAX T DS ON IN
=+••• •
•
() ()
22
2ρ
OOUT MAX RSS OSC
BOT
IN OUT
IN
OUT MA
Cf
P
VV
V
I
()
(
••
–
•=
XX T DS ON
R
)()
••
2
ρ
Both MOSFETs have I
2
R losses and the P
TOP
equation
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short-circuit
when the bottom duty cycle is nearly 100%.
The LTC3736-2 utilizes a nonoverlapping, antishoot-
through gate drive control scheme to ensure that the
P- and N-channel MOSFETs are not turned on at the same
time. To function properly, the control scheme requires
that the MOSFETs used are intended for DC/DC switching
applications. Many power MOSFETs, particularly P-channel
MOSFETs, are intended to be used as static switches and
therefore are slow to turn on or off.
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (Q
G
)
Figure 4. R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
37362 F04
0.5
0
0
50
100
2.0
less than 25nC to 30nC (at 4.5V
GS
) and a turn-off delay
(t
D(OFF)
) of less than approximately 140ns. However, due
to differences in test and specifi cation methods of various
MOSFET manufacturers, and in the variations in Q
G
and
t
D(OFF)
with gate drive (V
IN
) voltage, the P-channel MOSFET
ultimately should be evaluated in the actual LTC3736-2
application circuit to ensure proper operation.
Shoot-through between the P-channel and N-channel
MOSFETs can most easily be spotted by monitoring the
input supply current. As the input supply voltage increases,
if the input supply current increases dramatically, then the
likely cause is shoot-through. Note that some MOSFETs
that do not work well at high input voltages (e.g., V
IN
>
5V) may work fi ne at lower voltages (e.g., 3.3V). Table 1
shows a selection of P-channel MOSFETs from different
manufacturers that are known to work well in LTC3736-2
applications.
Selecting the N-channel MOSFET is typically easier, since for
a given R
DS(ON)
, the gate charge and turn-on and turn-off
delays are much smaller than for a P-channel MOSFET.
Table 1. Selected P-Channel MOSFETs Suitable for LTC3736-2
Applications
PART
NUMBER MANUFACTURER TYPE PACKAGE
Si7540DP Siliconix Complementary
P/N
PowerPak
SO-8
Si9801DY Siliconix Complementary
P/N
SO-8
FDW2520C Fairchild Complementary
P/N
TSSOP-8
FDW2521C Fairchild Complementary
P/N
TSSOP-8
Si3447BDV Siliconix Single P TSOP-6
Si9433BDY Siliconix Single P SO-8
FDC602P Fairchild Single P TSOP-6
FDC606P Fairchild Single P TSOP-6
FDC638P Fairchild Single P TSOP-6
FDW2502P Fairchild Dual P TSSOP-8
FDS6875 Fairchild Dual P SO-8
HAT1054R Hitachi Dual P SO-8
NTMD6P02R2-D On Semiconductor Dual P SO-8
APPLICATIONS INFORMATION