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参数资料
| 型号： | LTC3809IMSE-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 10-MSOP |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 600kHz |
| 占空比： | 100% |
| 电源电压： | 2.75 V ~ 9.8 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： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽）裸露焊盘 |
| 包装： | 管件 |
LTC3809-1
APPLICATIONS INFORMATION
V OUT V IN OUT
I RIPPLE =
V IN OUT
? OUT
L MIN ≤
f OSC BURST ( PEAK )
V IN
that do not work well at high input voltages (e.g., V IN >
5V) may work ?ne at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R DS(ON) , the gate charge and turn-on and turn-off
delays are much smaller than for a P-channel MOSFET.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f OSC , directly determine
the inductor’s peak-to-peak ripple current:
– V
?
V IN f OSC ? L
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest ef?ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I OUT(MAX) . Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a speci?ed maximum,
the inductor should be chosen according to:
The corresponding average current depends on the
amount of ripple current. Lower inductor values (higher
I RIPPLE ) will reduce the load current at which Burst Mode
operation begins.
The ripple current is normally set so that the inductor current
is continuous during the burst periods. Therefore,
I RIPPLE ≤ I BURST(PEAK)
This implies a minimum inductance of:
– V V
? I
A smaller value than L MIN could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
ef?ciency. In general, though, it is a good idea to keep
I RIPPLE comparable to I BURST(PEAK) .
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
?xed inductor value, but is very dependent on the induc-
tance selected. As inductance increases, core losses go
down. Unfortunately, increased inductance requires more
V IN OUT
f OSC RIPPLE
? OUT
L ≥
– V
? I
V
V IN
turns of wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
1 Δ V SENSE ( MAX )
I BURST ( PEAK ) = ?
R DS ( ON )
Burst Mode Operation Considerations
The choice of R DS(ON) and inductor value also determines
the load current at which the LTC3809-1 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
4
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. Core saturation results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
38091fc
14
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