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參數(shù)資料
| 型號： | LTC3776EUF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual 2-Phase, No RSENSE Synchronous Controller for DDR/QDR Memory Termination |
| 中文描述： | 1 A DUAL SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封裝： | 4 X 4 MM, PLASTIC, MO-220-WGGD, QFN-24 |
| 文件頁數(shù)： | 17/28頁 |
| 文件大?。?/td> | 429K |
| 代理商： | LTC3776EUF |
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LTC3776
3776f
size for most LTC3776 applications, since it conducts a
relatively small average current. Larger diodes result in
additional transition losses due to their larger junction
capacitance. This diode may be omitted if the efficiency
loss can be tolerated.
C
IN
and C
OUT
Selection
The selection of C
IN
is simplified by the 2-phase architec-
ture 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 capacitor RMS current
occurs when only one controller is operating. The control-
ler with the highest (V
OUT
)(I
OUT
) product needs to be used
in the formula below to determine the maximum RMS
capacitor current requirement. Increasing the output cur-
rent drawn from the other controller will actually decrease
the input RMS ripple current from its maximum value. 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.
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
OUT
)/(V
IN
). To
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current of one channel must be
used. The maximum RMS capacitor current is given by:
C
I
V
V
V
V
IN
MAX
IN
OUT
IN
OUT
Required I
RMS
≈
(
)(
)
[
]
–
/2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’
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 be
paralleled to meet size or height requirements in the
design. Due to the high operating frequency of the LTC3776,
ceramic capacitors can also be used for C
IN
. Always
consult the manufacturer if there is any question.
The benefit of the LTC3776 2-phase operation can be cal-
culated by using the equation above for the higher power
controller and then calculating the loss that would have
resulted if both controller channels switched on at the
same time. The total RMS power lost is lower when both
controllers are operating due to the reduced overlap of
current pulses required through the input capacitor’s ESR.
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the
dual controller design. Also, the input protection fuse re-
sistance, battery resistance, and PC board trace resistance
losses are also reduced due to the reduced peak currents
in a 2-phase system. The overall benefit of a multiphase
design will only be fully realized when the source imped-
ance of the power supply/battery is included in the effi-
ciency testing. The sources of the P-channel MOSFETs
should be placed within 1cm of each other and share a
common C
IN
(s). Separating the sources and C
IN
may pro-
duce undesirable voltage and current resonances at V
IN
.
A small (0.1
μ
F to 1
μ
F) bypass capacitor between the chip
V
IN
pin and ground, placed close to the LTC3776, is also
suggested. A 10
resistor placed between C
IN
(C1) and
the V
IN
pin provides further isolation between the two
channels.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The
output ripple (
V
OUT
) is approximated by:
≈
+
V
I
ESR
fC
OUT
RIPPLE
OUT
1
8
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increases with input voltage.
Setting Output Voltage
The LTC3776’s channel 1 output voltage is set by an
external feedback resistor divider carefully placed across
the output, as shown in Figure 6. The regulated output
voltage is determined by:
V
V
R
R
OUT
B
A
1
0 6
.
1
=
+
APPLICATIU
W
U
U
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