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| 型号： | LTC3850IUF#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/38页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-QFN |
| 产品培训模块： | LTC3850 Dual Output DC/DC Switching Regulator Controller |
| 标准包装： | 2,500 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 860kHz |
| 占空比： | 97.2% |
| 电源电压： | 4 V ~ 30 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
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LTC3850/LTC3850-1
APPLICATIONS INFORMATION
V OUT
V IN
V IN – V OUT
V IN
( I MAX ) 2 ( ) R DS(ON) +
P MAIN =
V OUT
( V IN ) 2 ? ? MAX ? ? ( R DR ) ( C MILLER ) ?
( I MAX ) 2 ( 1 + δ ) R DS(ON)
P SYNC = IN
t SOFTSTART = 0.8 ?
voltageandmaximumoutputcurrent.Millercapacitance,
C MILLER , can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C MILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in V DS . This result is
then multiplied by the ratio of the application applied V DS
to the gate charge curve specified V DS . When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
The MOSFET power dissipations at maximum output cur-
rent are given by:
1 + δ
V IN
? I ?
2
? 1 1 ?
? ?
? + ? ? f OSC
? V INTVCC – V TH(MIN) V TH(MIN) ?
V – V OUT
V IN
where d is the temperature dependency of R DS(ON) and
R DR (approximately 2?) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V TH(MIN) is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I 2 R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V IN < 20V
the high current efficiency generally improves with larger
MOSFETs, while for V IN > 20V the transition losses rapidly
increase to the point that the use of a higher R DS(ON) device
with lower C MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + d ) is generally given for a MOSFET in the
form of a normalized R DS(ON) vs Temperature curve, but
d = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes conduct during the dead
time between the conduction of the two power MOSFETs.
These prevent the body diodes of the bottom MOSFETs
from turning on, storing charge during the dead time and
requiring a reverse recovery period that could cost as much
as 3% in efficiency at high V IN . A 1A to 3A Schottky is
generally a good compromise for both regions of opera-
tion 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 LTC3850 has the ability to either soft-start by itself
with a capacitor or track the output of another channel or
external supply. When one particular channel is configured
to soft-start by itself, a capacitor should be connected to
its TK/SS pin. This channel is in the shutdown state if its
RUN pin voltage is below 1.2V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.2V, the channel pow-
ers up. A soft-start current of 1.3μA 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
defined to be the voltage range from 0V to 0.8V on the TK/
SS pin. The total soft-start time can be calculated as:
C SS
1.3μA
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.74V, it
will operate in forced continuous mode and revert to the
38501fc
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