Datasheet

15
LTC3736-1
37361f
The MOSFET power dissipations at maximum output
current are:
P
V
V
IRV
ICf
P
VV
V
IR
TOP
OUT
IN
OUT MAX T DS ON IN
OUT MAX RSS OSC
BOT
IN OUT
IN
OUT MAX T DS ON
=+
=
••• •
•••
–
•••
() ()
()
() ()
22
2
2ρ
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 LTC3736-1 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-chan-
nel 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
)
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 specification 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 LTC3736-1
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 in-
creases, 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 fine 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
LTC3736-1 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 LTC3736-1
Applications
PART
NUMBER MANUFACTURER TYPE PACKAGE
Si7540DP Siliconix Complementary PowerPak
P/N SO-8
Si9801DY Siliconix Complementary SO-8
P/N
FDW2520C Fairchild Complementary TSSOP-8
P/N
FDW2521C Fairchild Complementary TSSOP-8
P/N
Si3447BDV Siliconix Single P TSOP-6
Si9803DY 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 Semi Dual P SO-8
APPLICATIO S I FOR ATIO
WUUU
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
37361 F05
0.5
0
0
50
100
2.0
Figure 5. R
DS(ON)
vs Temperature