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参数资料
| 型号： | LTC1266IS#TR-5 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, PLASTIC, SO-16 |
| 文件页数： | 3/20页 |
| 文件大小： | 245K |
| 代理商： | LTC1266IS#TR-5 |
11
LTC1266
LTC1266-3.3/LTC1266-5
From the duty cycles, the required RDS(ON) for each
MOSFET can be derived:
TS RDS(ON) =
VIN PT
VOUT IMAX2 (1 + δT)
BS RDS(ON) =
VIN PB
(VIN – VOUT) IMAX2 (1 + δB)
where PT and PB are the allowable power dissipations and
δTandδBarethetemperaturedependenciesofRDS(ON).PT
and PB will be determined by efficiency and/or thermal
requirements(seeEfficiencyConsiderations).Fora MOSFET,
(1 +
δ) is generally given in the form of a normalized
RDS(ON) vs temperature curve, but δPCH = 0.007/°C and
δNCH = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The minimum input voltage determines whether standard
threshold or logic-level threshold MOSFETs must be used.
For VIN > 8V, standard threshold MOSFETs (VGS(TH) < 4V)
may be used. If VIN is expected to drop below 8V, logic-
level threshold MOSFETs (VGS(TH) < 2.5V) are strongly
recommended. The LTC1266 series Power VIN must al-
ways be less than the absolute maximum VGS ratings for
the MOSFETs.
The Schottky diode D1 shown in Figure 1 only conducts
during the deadtime between the conduction of the two
power MOSFETs. D1’s sole purpose in life is to prevent the
body diode of the bottom-side MOSFET from turning on
and storing charge during the deadtime, which could cost
as much as 1% in efficiency (although there are no other
harmful effects if D1 is omitted). Therefore, D1 should be
selected for a forward voltage of less than 0.7V when
conducting IMAX.
CIN and COUT Selection
In continuous mode, the current through the topside
MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR (Effective
Series Resistance) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
CIN Required IRMS ≈ IMAX
[VOUT(VIN – VOUT)]1/2
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly used for design because even significant devia-
tions 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. Always consult the
manufacturer if there is any question. An additional 0.1
F
to 1
F ceramic capacitor is also required on Power VIN
(Pin 2) for high frequency decoupling.
The selection of COUT is driven by the required ESR. The
ESR of COUT must be less than twice the value of RSENSE
for proper operation of the LTC1266 series:
COUT Required ESR < 2RSENSE
Optimum efficiency is obtained by making the ESR equal
to RSENSE. As the ESR is increased up to 2RSENSE, the
efficiency degrades by less than 1%. If the ESR is greater
than 2RSENSE, the voltage ripple on the output capacitor
will prematurely trigger Burst Modeoperation, resulting in
disruption of continuous mode and an efficiency hit which
can be several percent. If Burst Mode operation is dis-
abled, the ESR requirement can be relaxed and is limited
only by the allowable output voltage ripple.
Manufacturers such as Nichicon and United Chemicon
should be considered for high performance capacitors.
The OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR/size ratio of any aluminum
electrolytic at a somewhat higher price. Once the ESR
requirement for COUT has been met, the RMS current
rating generally far exceeds the IRIPPLE(P-P) requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirements of the application. An
excellent choice is the AVX TPS series of surface mount
tantalums.
At low supply voltages, a minimum capacitance at COUT
is needed to prevent an abnormal low frequency oper-
ating mode (see Figure 4). When COUT is made too
small, the output ripple at low frequencies will be large
enough to trip the voltage comparator. This causes
Burst Modeoperation to be activated when the LTC1266
APPLICATIO S I FOR ATIO
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