LTC3736
W U U
APPLICATIO S I FOR ATIO
U
less than 25nC to 30nC (at 4.5VGS) and a turn-off delay
(tD(OFF)) of less than approximately 140ns. However, due
to differences in test and specification methods of various
MOSFET manufacturers, and in the variations in QG and
tD(OFF)withgatedrive(VIN)voltage,theP-channelMOSFET
ultimately should be evaluated in the actual LTC3736
application circuit to ensure proper operation.
2.0
1.5
1.0
0.5
0
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,iftheinputsupplycurrentincreasesdramatically,
then the likely cause is shoot-through. Note that some
MOSFETsthatdonotworkwellathighinputvoltages(e.g.,
VIN > 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 applications.
50
100
–50
150
0
JUNCTION TEMPERATURE (°C)
3736 F04
Figure 4. RDS(ON) vs Temperature
The MOSFET power dissipations at maximum output
current are:
Selecting the N-channel MOSFET is typically easier, since
for a given RDS(ON), the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
VOUT
V
IN
2
PTOP
=
•IOUT(MAX)2 • ρT •RDS(ON) +k • V
IN
•IOUT(MAX) • ρT •RDS(ON)
Table 1. Selected P-Channel MOSFETs Suitable for LTC3736
Applications
V – VOUT
IN
P
BOT
=
•IOUT(MAX)2 • ρT •RDS(ON)
PART
V
IN
NUMBER
MANUFACTURER
TYPE
PACKAGE
Si7540DP
Siliconix
Complementary
P/N
PowerPak
SO-8
Both MOSFETs have I2R losses and the PTOP equation
includesanadditionaltermfortransitionlosses,whichare
largest at high input voltages. The constant k = 2A–1 can
be used to estimate the amount of transition loss. The
bottom MOSFET losses are greatest at high input voltage
or during a short circuit when the bottom duty cycle is
nearly 100%.
Si9801DY
FDW2520C
FDW2521C
Siliconix
Fairchild
Fairchild
Complementary
P/N
SO-8
Complementary
P/N
TSSOP-8
TSSOP-8
Complementary
P/N
Si3447BDV
Si9803DY
FDC602P
Siliconix
Siliconix
Fairchild
Fairchild
Fairchild
Fairchild
Fairchild
Hitachi
Single P
Single P
Single P
Single P
Single P
Dual P
TSOP-6
SO-8
TheLTC3736utilizesanonoverlapping,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 applica-
tions. Many power MOSFETs, particularly P-channel
MOSFETs, are intended to be used as static switches and
therefore are slow to turn on or off.
TSOP-6
TSOP-6
TSOP-6
TSSOP-8
SO-8
FDC606P
FDC638P
FDW2502P
FDS6875
Dual P
HAT1054R
NTMD6P02R2-D
Dual P
SO-8
On Semi
Dual P
SO-8
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (QG)
3736f
15
Linear [ Linear ]
