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| 型号： | LT3509EMSE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 0.7A DL 16MSOP |
| 标准包装： | 37 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 0.8 V ~ 35.3 V |
| 输入电压： | 3.6 V ~ 36 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 260kHz ~ 2.15MHz |
| 电流 - 输出： | 700mA |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TFSOP（0.118"，3.00mm 宽）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 16-MSOP，裸露焊盘 |
| 产品目录页面： | 1332 (CN2011-ZH PDF) |
LT3509
APPLICATIONS INFORMATION
When the switch is off, the potential across the inductor is
the output voltage plus the catch diode forward voltage. This
gives the peak-to-peak ripple current in the inductor:
There is a limit to the actual minimum duty cycle imposed
by the minimum on-time of the switch. For a robust design
it is important that inductor that will not saturate when
Δ I L = (1– DC)
V OUT + V F
L ? f SW
the switch is at its minimum on-time, the input voltage
is at maximum and the output is short circuited. In this
case the full input voltage, less the drop in the switch, will
Δ I L
I SWPK = I LPK = I OUT +
V IN ON ( MIN )
I PEAK = + 1 . 1 A
where:
DC = Duty Cycle
f SW = Switching Frequency
L = Inductor Value
V F = Diode Forward Voltage
The peak inductor and switch current is:
2
To maintain output regulation, this peak current must be
less than the LT3509’s switch current limit I LIM . This is
dependent on duty cycle due to the slope compensation.
For I LIM is at least 1.4A at low duty cycles and decreases
linearly to 1.0A at DC = 0.8.
The theoretical minimum inductance can now be calcu-
lated as:
appear across the inductor. This doesn’t require an actual
short, just starting into a capacitive load will provide the
same conditions. The Diode current sensing scheme will
ensure that the switch will not turn-on if the inductor
current is above the DA current limit threshold, which has
a maximum of 1.1A. The peak current under short-circuit
conditions can then be calculated from:
? t
L
The inductor should have a saturation current greater than
this value. For safe operation with high input voltages this
can often mean using a physically larger inductor as higher
value inductors often have lower saturation currents for
a given core size. As a general rule the saturation current
should be at least 1.8A to be short-circuit proof. However, it’s
generally better to use an inductor larger than the minimum
value. For robust operation at input voltages greater than
30V, use an inductor with a value of 4.2μH or greater, and
1– DC MIN V OUT + V F
I LIM OUT
L MIN =
f – I
?
a saturation current rating of 1.8A or higher. The minimum
inductor has large ripple currents which increase core
losses and require large output capacitors to keep output
where DC MIN is the minimum duty cycle called for by the
application i.e.:
voltage ripple low. Select an inductor greater than L MIN
that keeps the ripple current below 30% of I LIM .
DC MIN =
V OUT ( MAX ) + V F
V IN ( MIN ) – V SW + V F
3509fc
14
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