Datasheet

15
LTC3776
3776fa
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 LTC3776
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
LTC3776 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 LTC3776
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
3776 F07
0.5
0
0
50
100
2.0
Figure 5. R
DS(ON)
vs Temperature
The power dissipated in the top and bottom MOSFETs
strongly depends on their respective duty cycles and load
current. When the LTC3776 is operating in continuous
mode, the duty cycles for the MOSFETs are:
Top P-Channel Duty Cycle =
V
Bottom N-Channel Duty Cycle =
V
OUT
IN
V
V
V
IN
OUT
IN
–
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 LTC3776 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 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.