LTC3776EUF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3776EUF
Linear Technology
'LTC3776EUF' PDF : 28 Pages View PDF
LTC3776
APPLICATIO S I FOR ATIO
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3776 F07
Figure 5. RDS(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 = VOUT
VIN
Bottom N-Channel Duty Cycle = VIN – VOUT
VIN
The MOSFET power dissipations at maximum output
current are:
PTOP
=
VOUT
VIN
• IOUT(MAX)2
• ρT
• RDS(ON)
+
2 • VIN2
• IOUT(MAX) • CRSS • fOSC
PBOT
=
VIN
– VOUT
VIN
• IOUT(MAX)2
• ρT
• RDS(ON)
Both MOSFETs have I2R losses and the PTOP 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.
Reasonable starting criteria for selecting the P-channel
MOSFET are that it must typically have a gate charge (QG)
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) with gate drive (VIN) 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.,
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
LTC3776 applications.
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.
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
P/N
TSSOP-8
FDW2521C
Fairchild
Complementary
P/N
TSSOP-8
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
3776f
15
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