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參數(shù)資料
| 型號(hào)： | LTC3809EMS#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 6/24頁 |
| 文件大?。?/td> | 266K |
| 代理商： | LTC3809EMS#PBF |
14
LTC3809
3809f
MOSFETs that do not work well at high input voltages (e.g.,
VIN > 5V) may work fine at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given RDS(ON), the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Operating Frequency and Synchronization
The choice of operating frequency, fOSC, 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 LTC3809’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 VIN
selects 750kHz operation; pulling the PLLLPF to GND
selects 300kHz operation.
Alternatively, the LTC3809 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, fOSC, directly determine
the inductor’s peak-to-peak ripple current:
I
V
VV
fL
RIPPLE
OUT
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 IOUT(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
VV
fI
V
IN
OUT
OSC
RIPPLE
OUT
IN
≥
–
Burst Mode Operation Considerations
The choice of RDS(ON) and inductor value also determines
the load current at which the LTC3809 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
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher IRIPPLE)
will reduce the load current at which Burst Mode operation
begins.
The ripple current is normally set so that the inductor
current is continuous during the burst periods. Therefore,
IRIPPLE ≤ IBURST(PEAK)
This implies a minimum inductance of:
L
VV
fI
V
MIN
IN
OUT
OSC
BURST PEAK
OUT
IN
≤
–
()
A smaller value than LMIN could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
IRIPPLE comparable to IBURST(PEAK).
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot afford
the core loss found in low cost powdered iron cores, forc-
ing the use of more expensive ferrite, molypermalloy or Kool
M
cores. Actual core loss is independent of core size for
APPLICATIO S I FOR ATIO
WU
UU
Kool M
is a registered trademark of Magnetics, Inc.
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