15
LTC3736
3736fa
The MOSFET power dissipations at maximum output
current are:
P
V
V
I
R
V
I
C
f
P
V
V
V
I
R
TOP
OUT
IN
OUT MAX
T
DS ON
IN
OUT MAX
RSS
OSC
BOT
IN
OUT
IN
OUT MAX
T
DS ON
=
+
=
•
•
•
•
•
•
•
–
•
•
•
(
)
(
)
(
)
(
)
(
)
2
2
2
2
r
r
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 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 (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
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 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
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
Operating Frequency and Synchronization
The choice of operating frequency, f
OSC
, is a trade-off
between efficiency and component size. Low frequency
APPLICATIO S I FOR ATIO
W
U
U
U
JUNCTION TEMPERATURE (
°
C)
– 50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3736 F04
0.5
0
0
50
100
2.0
Figure 4. R
DS(ON)
vs Temperature
