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参数资料
| 型号： | LTC3891HUDC#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/32页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 20-QFN |
| 标准包装： | 91 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 835kHz |
| 占空比： | 99% |
| 电源电压： | 4 V ~ 60 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 ~ 150°C |
| 封装/外壳： | 20-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3891
APPLICATIONS INFORMATION
V OUT
V IN
V IN ? V OUT
V IN
PowerMOSFETandSchottkyDiode(Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3891 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak drive levels are set by the INTV CC
voltage. This voltage is typically 5.1V during start-up
(see EXTV CC Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
Pay close attention to the BV DSS specification for the
MOSFETs as well.
Selection criteria for the power MOSFETs include the on-
resistance, R DS(ON) , Miller capacitance, C MILLER , input
voltage and maximum output current. Miller capacitance,
C MILLER , can be approximated from the gate charge
curve usually provided on the MOSFET manufacturers’
datasheet. 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
current are given by:
where δ is the temperature dependency of R DS(ON) and
R DR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V THMIN 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+ δ ) is generally given for a MOSFET in the
form of a normalized R DS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
A Schottky diode can be inserted in parallel with the bot-
tom MOSFET to conduct during the dead-time between
the conduction of the two power MOSFETs. This prevents
the body diode of the bottom MOSFET 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 operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
C IN and C OUT Selection
V OUT 2
V IN
( V IN ) ? ? MAX ? ? ( R DR ) ( C MILLER ) ?
1
1
? V INTVCC – V THMIN
V THMIN ?
P MAIN =
? ?
( I MAX ) ( 1 + δ ) R DS(ON) +
2 ? I ?
2
? + ? ( f )
The selection of C IN is usually based off the worst-case RMS
input current. The highest (V OUT )(I OUT ) product needs to
be used in the formula shown in Equation 1 to determine
the maximum RMS capacitor current requirement.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V OUT )/(V IN ). To prevent
large voltage transients, a low ESR capacitor sized for the
( I MAX ) ( ) δ R DS(ON)
1 +
P SYNC =
V IN – V OUT
V IN
2
3891fa
17
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