LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 14

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

LTC3708
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:
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
Both MOSFETs have I
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.
14
D
D
P
P
TOP
BOT
TOP
BOT
=
=
=
=
D
D
( . ) •
R
2.0
1.5
1.0
0.5
0 5
V
V
0
BOT
DR
–50
TOP
V
OUT
IN
IN
Figure 3. R
–
V
•
•
IN
•
⎛
⎜
⎝
V
V
I
I
OUT MAX
OUT MAX
(
JUNCTION TEMPERATURE (°C)
OUT
IN
DRV
0
2
(
(
2
•
R losses and the top MOSFET in-
I
DS(ON)
CC
OUT MAX
)
–
)
1
2
2
50
(
V
•
•
vs Temperature
GS TH
ρ
ρ
T BOT
T TOP
(
)
(
(
•
100
C
)
)
RSS
)
)
+
•
•
3708 F03
R
R
V
• •
GS TH
DS ON
DS ON
150
f
1
(
(
(
)
)
)
⎞
⎟
⎠
+
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
by the current into the I
Tying a resistor, R
on time inversely proportional to V
converter, this results in approximately constant frequency
operation as the input supply varies:
Figure 4 shows how R
for several common output voltages.
t
f
ON
=
ON
0 7
=
. •
, of the top MOSFET switch. The on time is set
I
0 7
1000
ION
100
.
R
Figure 4. Switching Frequency vs R
V
ON
(
OUT
10
(
100
10
pF
V
OUT
ON
pF
)
= 1.5V
, from V
)
ON
ON
R
relates to switching frequency
ON
pin according to:
1000
(kΩ)
V
IN
OUT
V
to the I
= 3.3V
OUT
= 2.5V
IN
. For a step-down
3708 F04
ON
10000
ON
pin yields an
3708fb