參數(shù)資料
型號(hào)： LTC3853IUJ#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 0.15 A DUAL SWITCHING CONTROLLER, 830 kHz SWITCHING FREQ-MAX, PQCC40
封裝： 6 X 6 MM, PLASTIC, QFN-40
文件頁(yè)數(shù)： 14/36頁(yè)
文件大?。?/td> 1780K
代理商： LTC3853IUJ#TR
LTC3853
21
3853fa
Topside MOSFET Driver Supply (CB, DB)
Externalbootstrapcapacitors,CB,connectedtotheBOOST
pinssupplythegatedrivevoltagesforthetopsideMOSFETs.
Capacitor CB in the Functional Diagram is charged though
external diode, DB, from INTVCC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the CB voltage across the gate source of the
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
VIN and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: VBOOST
= VIN + VINTVCC. The value of the boost capacitor, CB,
needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than VIN(MAX).
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Undervoltage Lockout
The LTC3853 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTVCCvoltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTVCC is below
3.35V. To prevent oscillation when there is a disturbance
on the INTVCC, the UVLO comparator has 500mV of preci-
sion hysteresis.
Anotherwaytodetectanundervoltageconditionistomoni-
tor the VIN supply. Because the RUN pins have a precision
turn-on reference of 1.2V, one can use a resistor divider
to VIN to turn on the IC when VIN is high enough. An extra
4.5A of current flows out of the RUN pin once the RUN
pinvoltagepasses1.2V.Onecanprogramthehysteresisof
the run comparator by adjusting the values of the resistive
divider. For accurate VIN undervoltage detection using the
RUN pin, VIN needs to be higher then 4V.
CIN and COUT Selection
The selection of CIN is simplified by the 3-phase architec-
ture and its impact on the worst-case RMS current drawn
throughtheinputnetwork(battery/fuse/capacitor).Itcanbe
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (VOUT)(IOUT) product needs to be used in the
formula below to determine the maximum RMS capacitor
current requirement. Increasing the output current drawn
from the other controllers will actually decrease the input
RMS ripple current from its maximum value. The out-of-
phasetechniquetypicallyreducestheinputcapacitor’sRMS
ripple current by a factor of 30% to 70% when compared
to a single phase power supply solution.
Incontinuousmode,thesourcecurrentofthetopMOSFET
is a square wave of duty cycle (VOUT)/(VIN). 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:
CIN Required IRMS ≈
IMAX
VIN
VOUT
(
) VIN – VOUT
(
)
1/2
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offermuchrelief.Notethatcapacitormanufacturers’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 LTC3853, ceramic capacitors
can also be used for CIN. Always consult the manufacturer
if there is any question.
The benefit of the LTC3853 3-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 all controller channels switched on at the same
APPLICATIONS INFORMATION
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