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| 型號： | LTC1929IG-PG |
| 英文描述： | Analog IC |
| 中文描述： | 模擬IC |
| 文件頁數(shù)： | 14/28頁 |
| 文件大?。?/td> | 389K |
| 代理商： | LTC1929IG-PG |
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LTC1929/LTC1929-PG
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 +
δ
) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs. Temperature curve, but
δ
= 0.005/
°
C can be used as an approximation for low
voltage MOSFETs. C
RSS
is usually specified in the MOS-
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 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.
C
IN
and C
OUT
Selection
In continuous mode, the source current of each top
N-channel MOSFET is a square wave of duty cycle V
OUT
/
V
IN
. A low ESR input capacitor sized for the maximum
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 a 2-phase
configuration 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 input voltage is twice the
output voltage. The minimum is not quite zero due to
inductor ripple current.
In the graph of Figure 4, the local maximum input RMS
capacitor currents are reached when:
V
V
k
OUT
IN
=
4
2
1
where k = 1, 2.
These worst-case conditions are commonly used for de-
sign 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. This
makes it advisable to further derate the capacitor, or to
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design. Always consult
the capacitor manufacturer if there is any question.
It is important to note that the efficiency loss is propor-
tional to the input RMS current squaredand therefore a
2-stage implementation results in 75% less power loss
when compared to a single phase design. Battery/input
protection fuse resistance (if used), PC board trace and
connector resistance losses are also reduced by the re-
duction of the input ripple current in a 2-phase system. The
required amount of input capacitance is further reduced by
the factor, 2, due to the effective increase in the frequency
of the current pulses.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment has been met, the RMS current rating generally far
exceeds the I
RIPPLE(P-P)
requirements. The steady state
output ripple (
V
OUT
) is determined by:
V
I
ESR
fC
OUT
RIPPLE
OUT
≈
+
1
16
APPLICATIU
W
U
U
Figure 4. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
DUTY FACTOR (V
OUT
/V
IN
)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.6
0.5
0.4
0.3
0.2
0.1
0
1929 F04
R
D
2-PHASE
1-PHASE
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