Datasheet

LTC3809
13
3809fc
APPLICATIONS INFORMATION
short-circuit current sense threshold ΔV
SC
is set approxi-
mately 90mV when IPRG is fl oating (60mV when IPRG is
t i e d l o w ; 15 0 m V w h e n I P R G i s t i e d h i g h). T h e o n - r e s i s t a n c e
of N-channel MOSFET is determined by:
R
V
I
DS ON MAX
SC
SC PEAK
()
()
=
Δ
The short-circuit current limit (I
SC(PEAK)
) should be larger
than the I
OUT(MAX)
with some margin to avoid interfering
with the peak current sensing loop. On the other hand,
in order to prevent the MOSFETs from excessive heating
and the inductor from saturation, I
SC(PEAK)
should be
smaller than the minimum value of their current ratings.
A reasonable range is:
I
OUT(MAX)
< I
SC(PEAK)
< I
RATING(MIN)
Therefore, the on-resistance of N-channel MOSFET should
be chosen within the following range:
Δ
<<
ΔV
I
R
V
I
SC
RATING MIN
DS ON
SC
OUT MAX()
()
()
where ΔV
SC
is 90mV, 60mV or 150mV with IPRG being
fl oated, tied to GND or V
IN
respectively.
The power dissipated in the MOSFET strongly depends
on its respective duty cycles and load current. When the
LTC3809 is operating in continuous mode, the duty cycles
for the MOSFETs are:
Top P-Channel Duty Cycle =
Bottom N-Channel Duty Cycle =
V
V
VV
V
OUT
IN
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
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 100%.
The LTC3809 utilizes a non-overlapping, anti-shoot-
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-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 specifi cation 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 LTC3809
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 increases,
if the input supply current increases dramatically, then the
likely cause is shoot-through. Note that some MOSFETs
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3809 F02
0.5
0
0
50
100
2.0
Figure 2. R
DS(ON)
vs Temperature