ltc3729euh-trpbf Linear Technology Corporation, ltc3729euh-trpbf Datasheet - Page 14

• Manufacturer Part Number: ltc3729euh-trpbf
• Description: 550khz, Polyphase, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3729
14
where is the temperature dependency of R
constant inversely related to the gate drive current and N
is the number of stages.
Both MOSFETs have I
equation includes an additional term for transition losses,
which peak at the highest input voltage. 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.
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
low voltage MOSFETs. C
MOSFET 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 diodes, D1 and D2 shown in Figure 1 conduct
during the dead-time between the conduction of the two
large power MOSFETs. This helps prevent the body diode
of the bottom MOSFET from turning on, storing charge
during the dead-time, and requiring a reverse recovery
period which would reduce efficiency. A 1A to 3A (depend-
ing on output current) Schottky diode 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.
= 0.005/ C can be used as an approximation for
P
P
MAIN
SYNC
k V
V
V
V
OUT
IN
RSS
IN
IN
–
V
2
IN
V
actual provides higher efficiency. The
U
I
I
OUT
MAX
MAX
N
N
2
IN
R losses but the topside N-channel
DS(ON)
> 20V the transition losses rapidly
I
2
MAX
U
RSS
C
N
1
RSS
vs. Temperature curve, but
is usually specified in the
2
R
1
f
DS ON
W
(
R
)
DS ON
(
DS(ON)
DS(ON)
IN
U
)
< 20V the
device
, k is a
These worst-case conditions are commonly used for
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours of life.
C
In continuous mode, the source current of each top
N-channel MOSFET is a square wave of duty cycle V
V
RMS current must be used. The details of a close form
equation can be found in Application Note 77. Figure 4
shows the input capacitor ripple current for different
phase configurations with the output voltage fixed and
input voltage varied. The input ripple current is normalized
against the DC output current. The graph can be used in
place of tedious calculations. The minimum input ripple
current can be achieved when the product of phase num-
ber and output voltage, N(V
the input voltage V
So the phase number can be chosen to minimize the input
capacitor size for the given input and output voltages.
In the graph of Figure 4, the local maximum input RMS
capacitor currents are reached when:
IN
IN
. A low ESR input capacitor sized for the maximum
V
V
and C
V
V
OUT
OUT
IN
IN
Figure 4. Normalized Input RMS Ripple Current vs
Duty Factor for 1 to 6 Output Stages
OUT
N
2
k
k
2
Selection
N
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
1
IN
where k = 1, 2, …, N – 1
0.2
or:
where k = 1, 2, …, N
0.3
DUTY FACTOR (V
OUT
0.4
), is approximately equal to
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
0.5
OUT
0.6
/V
IN
0.7
)
0.8
3729
F04
0.9
sn3729 3729fas
OUT
/