參數(shù)資料
型號： LTC3737EUF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： Dual 2-Phase, No RSENSE DC/DC Controller with Output Tracking
中文描述： 1 A DUAL SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PQCC24
封裝： 4 X 4 MM, PLASTIC, QFN-24
文件頁數(shù)： 15/28頁
文件大?。?/td> 372K
代理商： LTC3737EUF
15
LTC3808
3808f
V
IN
> 5V) may work fine at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R
DS(ON)
, the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Using a Sense Resistor
A sense resistor R
SENSE
can be connected between SENSE
+
and SENSE
–
to sense the output load current. In this case,
the source of the P-channel MOSFET is connected to
SENSE
–
pin and the drain is connected to SW pin of
LTC3808. Therefore the current comparator monitors the
voltage developed across R
SENSE
instead of V
DS
of the
P-channel MOSFET. The output current that the LTC3808
can provide in this case is given by:
I
V
R
I
OUT MAX
(
SENSE MAX
SENSE
RIPPLE
2
)
(
)
–
=
Setting ripple current as 40% of I
OUT(MAX)
and using
Figure 1 to choose SF, the value of R
SENSE
is:
R
SF
V
I
SENSE
SENSE MAX
OUT MAX
(
=
5
6
(
)
)
See the P-channel R
DS(ON)
selection in Power MOSFET
Selection.
Variation in the resistance of a sense resistor is much
smaller than the variation in on-resistance of the external
MOSFET. Therefore the load current is well controlled with
a sense resistor. However the sense resistor causes extra
I
2
R losses in addition to the I
2
R losses of the MOSFET.
Therefore, using a sense resistor lowers the efficiency of
LTC3808, especially for large load current.
Operating Frequency and Synchronization
The choice of operating frequency, f
OSC
, is a trade-off
between efficiency and component size. Low frequency
operation improves efficiency by reducing MOSFET switch-
ing losses, both gate charge loss and transition loss.
However, lower frequency operation requires more induc-
tance for a given amount of ripple current.
The internal oscillator for the LTC3808’s controller runs at
a nominal 550kHz frequency when the PLLLPF pin is left
floating and the SYNC/MODE pin is not configured for
spread spectrum operation. Pulling the PLLLPF to V
IN
selects 750kHz operation; pulling the PLLLPF to GND
selects 300kHz operation.
Alternatively, the LTC3808 will phase-lock to a clock signal
applied to the SYNC/MODE pin with a frequency between
250kHz and 750kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
To further reduce EMI, the nominal 550kHz frequency will
be spread over a range with frequencies between 460kHz
and 635kHz when spread spectrum modulation is
enabled (see Spread Spectrum Modulation with
SYNC/MODE and PLLLPF Pins).
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f
OSC
,
directly determine
the inductor’s peak-to-peak ripple current:
I
V
V
V
V
f
L
RIPPLE
OUT
IN
IN
OUT
OSC
=
–
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX).
Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
V
V
f
I
V
V
IN
OUT
RIPPLE
OSC
OUT
IN
≥
–
Burst Mode Operation Considerations
The choice of R
DS(ON)
and inductor value also determines
the load current at which the LTC3808 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I
V
R
BURST PEAK
SENSE MAX
DS ON
(
(
)
(
)
)
=
1
4
APPLICATIU
W
U
U
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