LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 18

• Manufacturer Part Number: LTC1909-8
• Description: Wide Operating Range,No RSENSE TM Step-Down DC/DC Controller with SMBus Programming
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
one must carefully choose the MOSFET on-resistance as
discussed below.
Power MOSFET Selection
The LTC1909-8 requires two external N-channel power
MOSFETs, one for the top (main) switch and one for the
bottom (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage V
threshold voltage V
transfer capacitance C
The gate drive voltage is set by the 5V INTV
Consequently, logic-level threshold MOSFETs must be
used in LTC1909-8 applications. If the input voltage is
expected to drop below 5V, then sub-logic level threshold
MOSFETs should be considered.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-
resistance. MOSFET on-resistance is typically specified
with a maximum value R
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
The
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/ C as shown in
Figure 2. For a maximum temperature of 100 C, using a
value
LTC1909-8
18
R
DS ON MAX
T
(
T
term is a normalization factor (unity at 25 C)
= 1.3 is reasonable.
)(
2.0
1.5
1.0
0.5
0
– 50
Figure 2. R
)
JUNCTION TEMPERATURE ( C)
(GS)TH
R
U
0
SENSE
RSS
T
DS(ON)
DS(ON)(MAX)
, on-resistance R
and maximum current I
U
50
vs. Temperature
W
100
at 25 C. In this case,
19098 F02
DS(ON)
150
CC
U
, reverse
DS(MAX)
(BR)DSS
supply.
,
.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC1909-8 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
includes an additional term for transition losses, which are
largest at high input voltages. The constant K = 1.7A
be used to estimate the amount of transition loss. 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 tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC1909-8 applications is
determined implicitly by the one-shot timer that controls
the on-time t
set by the current into the I
Tying a resistor R
time inversely proportional to V
converter, this results in approximately constant fre-
quency operation as the input supply varies:
P
P
D
D
t
ON
TOP
BOT
BOT
TOP
= D
= D
V
+ k V
I
ION
VON
V
V
TOP
BOT
V
IN
OUT
ON
IN
–
V
IN
(
of the top MOSFET switch. The on-time is
I
I
10
IN
OUT(MAX)
OUT(MAX)
V
2
ON
OUT
I
pF
OUT(MAX)
from V
)
2
R losses and the top MOSFET
2
2
ON
IN
T(TOP)
T(BOT)
C
to the I
RSS
pin according to:
f
R
R
IN
DS(ON)(MAX)
DS(ON)(MAX)
. For a step-down
ON
pin yields an on-
–1
19098f
can