參數(shù)資料
型號(hào)： LTC3851EUD-1#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC16
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, MO-220WEED-6, QFN-16
文件頁(yè)數(shù)： 6/28頁(yè)
文件大?。?/td> 337K
代理商： LTC3851EUD-1#TRPBF
LTC3851-1
14
38511f
The peak-to-peak drive levels are set by the INTVCC voltage.
This voltage is typically 5V during start-up. Consequently,
logic-level threshold MOSFETs must be used in most ap-
plications. The only exception is if low input voltage is ex-
pected (VIN < 5V); then, sub-logic level threshold MOSFETs
(VGS(TH) < 3V) should be used. Pay close attention to the
BVDSS specication for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, RDS(ON), Miller capacitance, CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
at divided by the specied change in VDS. This result is
then multiplied by the ratio of the application applied VDS
to the gate charge curve specied VDS. When the IC is
operating in continuous mode, the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle =
V
Synchronous
OUT
VIN
S
Switch Duty Cycle =
VIN –V
V
OUT
IN
The MOSFET power dissipations at maximum output
current are given by:
P
V
IR
V
I
MAIN
OUT
IN
MAX
DS ON
IN
MAX
=
() +
()
+
()
2
1
δ
()
2
11
()(
)
+
RC
VV
V
DR
MILLER
INTVCC
TH MIN
–
()
T
TH MIN
SYNC
IN
OUT
IN
MAX
f
P
VV
V
I
()
–
=
()2 11+
()δR
DS ON
()
where
δ is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTH(MIN) is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I2R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For VIN < 20V,
the high current efciency generally improves with larger
MOSFETs, while for VIN > 20V, the transition losses rapidly
increase to the point that the use of a higher RDS(ON) device
with lower CMILLER actually provides higher efciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 +
δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diode conducts during the dead time
between the conduction of the two power MOSFETs. This
prevents the body diode of the bottom MOSFET from turn-
ing on, storing charge during the dead time and requiring
a reverse recovery period that could cost as much as 3%
in efciency at high VIN. A 1A to 3A Schottky is generally
a good size due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
Soft-Start and Tracking
The LTC3851-1 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or external supply. When the LTC3851-1 is congured
to soft-start by itself, a capacitor should be connected to
the TK/SS pin. The LTC3851-1 is in the shutdown state if
the RUN pin voltage is below 1.25V. TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.25V, the LTC3851-1
powers up. A soft-start current of 1μA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
APPLICATIONS INFORMATION
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LTC3851IGN#TRPBF 功能描述:IC REG CTRLR BUCK PWM CM 16-SSOP RoHS:是 類(lèi)別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無(wú) 升壓:是 回掃:無(wú) 反相:無(wú) 倍增器:無(wú) 除法器:無(wú) Cuk:無(wú) 隔離:無(wú) 工作溫度:-40°C ~ 125°C 封裝/外殼:24-WQFN 裸露焊盤(pán) 包裝:帶卷 (TR)
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