Datasheet

LTC3708
14
3708fb
APPLICATIONS INFORMATION
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3708 is operating in continuous
mode, the duty cycles for the MOSFETs are:
D
V
V
D
VV
V
TOP
OUT
IN
BOT
IN OUT
IN
=
=
–
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
PDI R
VI C f
R
DRV V
V
PDI R
TOP TOP OUT MAX T TOP DS ON
IN OUT MAX RSS
DR
CC GS TH
GS TH
BOT BOT OUT MAX T BOT DS ON
=+
()
+
⎛
⎝
⎜
⎞
⎠
⎟
=
•••
(.)• • • • •
•
–
•••
() () ()
()
()
()
() () ()
2
2
2
05
11
ρ
ρ
Both MOSFETs have I
2
R losses and the top MOSFET in-
cludes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest when the bottom duty cycle is near 100%,
during a short circuit or at high input voltage.
Operating Frequency
The choice of operating frequency is a trade-off between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching and
driving losses but requires larger inductance and/or ca-
pacitance to maintain low output ripple voltage.
The operating frequency of LTC3708 applications is deter-
mined implicitly by the one-shot timer that controls the on
time, t
ON
, of the top MOSFET switch. The on time is set
by the current into the I
ON
pin according to:
t
I
pF
ON
ION
=
()
07
10
.
Tying a resistor, R
ON
, from V
IN
to the I
ON
pin yields an
on time inversely proportional to V
IN
. For a step-down
converter, this results in approximately constant frequency
operation as the input supply varies:
f
V
RpF
OUT
ON
=
()
07 10.•
Figure 4 shows how R
ON
relates to switching frequency
for several common output voltages.
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3708 F03
0.5
0
0
50
100
2.0
Figure 3. R
DS(ON)
vs Temperature
R
ON
(kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3708 F04
V
OUT
= 3.3V
V
OUT
= 1.5V
V
OUT
= 2.5V
Figure 4. Switching Frequency vs R
ON