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参数资料
| 型号： | LTC3405ES6#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ .3A SOT23-6 |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 6 V |
| 输入电压： | 2.5 V ~ 5.5 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 1.5MHz |
| 电流 - 输出： | 300mA |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | SOT-23-6 细型，TSOT-23-6 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | TSOT-23-6 |
LTC3405
APPLICATIO S I FOR ATIO
where f = operating frequency, C OUT = output capacitance
and ? I L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ? I L increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
Another solution is to connect the feedback resistor to the
SW pin as shown in Figure 4. Taking the feedback informa-
tion at the SW pin removes the phase lag due to the output
capacitor resulting in a very stable loop. This configuration
lowers the load regulation by the DC resistance of the
inductor multiplied by the load current. This slight shift in
load regulation actually helps reduce the overshoot and
undershoot of the output voltage during a load transient.
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
V IN
2.7V
TO 4.2V
C IN
2.2 μ F
CER
4
1
6
V IN SW
LTC3405
RUN
MODE V FB
GND
2
3
5
4.7 μ H
887k
22pF
1M
V OUT
1.5V
C OUT
4.7 μ F
CER
3405 F04
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
the output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, V IN . At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V IN , large enough to damage the part.
When ceramic capacitors are used at the output, their low
Figure 4. Using All Ceramic Capacitors
A third solution is to use a high value resistor to inject a
feedforward signal at V FB mimicking the ripple voltage of
a high ESR output capacitor. The circuit in Figure 5 shows
how this technique can be easily realized. The feedforward
resistor, R2B, is connected to SW as in the previous
example. However, in this case, the feedback information
is taken from the resistive divider, R2A and R1, at the
output. This eliminates most of the load regulation degra-
dation due to the DC resistance of the inductor while
providing a stable operation similar to that obtained from
a high ESR tantalum type capacitor. Using this technique,
the extra feedforward resistor, R2B, must be accounted
for when calculating the resistive divider as follows:
ESR cannot provide sufficient phase lag cancellation to
stabilize the loop. One solution is to use a tantalum
capacitor, with its higher ESR, to provide the bulk capaci-
R 2 = R 2 A || R 2 B =
R 2 A ? R 2 B
R 2 A + R 2 B
= 0 . 8 V ? 1 +
R 1 ?
tance and parallel it with a small ceramic capacitor to
reduce the ripple voltage as shown in Figure 3.
V OUT
?
?
R 2 ?
?
V IN
2.7V
TO 4.2V
C IN
2.2 μ F
CER
4
1
6
V IN SW
LTC3405
RUN
MODE V FB
3
5
4.7 μ H
22pF
C OUT1 +
1 μ F
CER
V OUT
1.5V
C OUT2
22 μ F
TANT
V IN
2.7V
TO 4.2V
C IN
2.2 μ F
CER
4
1
6
V IN SW
LTC3405
RUN
MODE V FB
3
5
4.7 μ H
R2B 22pF
1M
V OUT
1.5V
C OUT1
4.7 μ F
CER
GND
2
887k
1M
GND
2
R1
200k
R2A
215k
3405 F03
Figure 3. Paralleling a Ceramic with a Tantalum Capacitor
Figure 5. Feedforward Injection in an
All Ceramic Capacitor Application
3405 F05
3405fa
9
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