參數(shù)資料
型號(hào)： LTC3851IMSE-1#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 0.05 A SWITCHING CONTROLLER, 530 kHz SWITCHING FREQ-MAX, PDSO16
封裝： LEAD FREE, PLASTIC, MSOP-16
文件頁(yè)數(shù)： 9/28頁(yè)
文件大?。?/td> 352K
代理商： LTC3851IMSE-1#PBF
LTC3851-1
17
38511fa
APPLICATIONS INFORMATION
is low. When the topside MOSFET is to be turned on, the
driver places the CB voltage across the gate source of the
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.
The reverse breakdown of the external Schottky diode
must be greater than VIN(MAX).
Undervoltage Lockout
The LTC3851-1 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTVCC voltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTVCC is below
3.2V. To prevent oscillation when there is a disturbance
on the INTVCC, the UVLO comparator has 400mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the VINsupply.BecausetheRUNpinhasaprecision
turn-on reference of 1.25V, one can use a resistor divider
to VIN to turn on the IC when VIN is high enough.
CIN Selection
In continuous mode, the source current of the top N-channel
MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
II
V
RMS
O MAX
OUT
IN
OUT
()
/
–1
12
This formula has a maximum at VIN = 2VOUT, where IRMS =
IO(MAX)/2. This simple worst-case condition is commonly
used for design because even signicant 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 also be paralleled to meet
size or height requirements in the design. Always consult
the manufacturer if there is any question.
COUT Selection
The selection of COUT is primarily determined by the
effective series resistance, ESR, to minimize voltage
ripple. The output ripple, ΔVOUT, in continuous mode is
determined by:
ΔΔ
V
I ESR
fC
OUT
L
OUT
≈
+
1
8
where f = operating frequency, COUT = output capacitance
and ΔIL = ripple current in the inductor. The output ripple is
highest at maximum input voltage since ΔIL increases with
input voltage. Typically, once the ESR requirement for COUT
has been met, the RMS current rating generally far exceeds
the IRIPPLE(P-P) requirement. With ΔIL = 0.3IOUT(MAX) and
allowing 2/3 of the ripple to be due to ESR, the output
ripple will be less than 50mV at maximum VIN and:
C
Required ESR < 2.2R
OUT
SENSE
C
fR
OUT
SENSE
>
1
8
The rst condition relates to the ripple current into the ESR
of the output capacitance while the second term guarantees
that the output capacitance does not signicantly discharge
during the operating frequency period due to ripple current.
The choice of using smaller output capacitance increases
the ripple voltage due to the discharging term but can be
compensated for by using capacitors of very low ESR to
maintain the ripple voltage at or below 50mV. The ITH pin
OPTI-LOOP compensation components can be optimized
to provide stable, high performance transient response
regardless of the output capacitors selected.
The selection of output capacitors for applications with
large load current transients is primarily determined by the
voltage tolerance specications of the load. The resistive
component of the capacitor, ESR, multiplied by the load
current change, plus any output voltage ripple must be
within the voltage tolerance of the load.
相關(guān)PDF資料
PDF描述
LTC3851IMSE#TRPBF 0.05 A SWITCHING CONTROLLER, 790 kHz SWITCHING FREQ-MAX, PDSO16
LTC3851IMSE#PBF 0.05 A SWITCHING CONTROLLER, 790 kHz SWITCHING FREQ-MAX, PDSO16
LTC3851IUD#TRPBF 0.05 A SWITCHING CONTROLLER, 790 kHz SWITCHING FREQ-MAX, PQCC16
LTC3851IGN#TRPBF 0.05 A SWITCHING CONTROLLER, 790 kHz SWITCHING FREQ-MAX, PDSO16
LTC3851EUD#TRPBF 0.05 A SWITCHING CONTROLLER, 790 kHz SWITCHING FREQ-MAX, PQCC16
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