ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 16

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

LTC1876
APPLICATIO S I FOR ATIO
then, sub-logic level threshold MOSFETs (V
should be used. Pay close attention to the BV
cation for the MOSFETs as well; most of the logic level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage and maximum output current. When the
LTC1876 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period.
16
P
P
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
MAIN
SYNC
k V
V
V
RSS
DS(ON)
V
OUT
IN
IN
IN
–
V
2
actually provides higher efficiency. The
IN
V
I
I
MAX
MAX
OUT
, reverse transfer capacitance C
U
2
IN
R losses while the topside N-channel
> 20V the transition losses rapidly
2
I
MAX
C
U
1
RSS
2
V
V
OUT
R
1
f
IN
DS ON
W
(
R
V
)
DS ON
IN
(
–
V
DS(ON)
GS(TH)
IN
DS(ON)
V
DSS
IN
)
OUT
U
< 20V the
specifi-
device
< 3V)
and k
RSS
,
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
voltage MOSFETs. C
FET characteristics. The constant k = 1.7 can be used to
estimate the contributions of the two terms in the main
switch dissipation equation.
The Schottky diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead-
time and requiring a reverse recovery period that could
cost as much as 3% in efficiency at high V
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
C
The selection of C
tecture and its impact on the worst-case RMS current
drawn through the input network (battery/fuse/capacitor).
It can be shown that the worst case RMS current occurs
when only one controller is operating. The controller with
the highest (V
formula below to determine the maximum RMS current
requirement. Increasing the output current, drawn from
the other out-of-phase controller, will actually decrease
the RMS ripple current from this maximum value (see
Figure 4). The out-of-phase technique typically reduces
the input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
The type of input capacitor, value and ESR rating have
efficiency effects that need to be considered in the selec-
tion process. The capacitance value chosen should be
sufficient to store adequate charge to keep high peak
battery currents down. 20 F to 40 F is usually sufficient
for a 25W output supply operating at 200kHz. The ESR of
the capacitor is important for capacitor power dissipation
as well as overall battery efficiency. All of the power (RMS
ripple current • ESR) not only heats up the capacitor but
wastes power from the battery.
IN
= 0.005/ C can be used as an approximation for low
Selection
OUT
)(I
IN
OUT
is simplified by the multiphase archi-
RSS
DS(ON)
) product needs to be used in the
is usually specified in the MOS-
vs temperature curve, but
IN
. A 1A to 3A
1876fa