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参数资料
| 型号： | LTC3405ES6#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/16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
top and bottom MOSFET R DS(ON) and the duty cycle
(DC) as follows:
R SW = (R DS(ON)TOP )(DC) + (R DS(ON)BOT )(1 – DC)
The R DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Charateristics
curves. Thus, to obtain I 2 R losses, simply add R SW to
R L and multiply the result by the square of the average
output current.
Other losses including C IN and C OUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
Thermal Considerations
In most applications the LTC3405 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3405 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum junction temperature of the part. If the junction
temperature reaches approximately 150 ° C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3405 from exceeding the maximum
junction temperature, the user will need to do a thermal
analysis. The goal of the thermal analysis is to determine
whether the operating conditions exceed the maximum
junction temperature of the part. The temperature rise is
given by:
T R = (P D )( θ JA )
where P D is the power dissipated by the regulator and θ JA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T J , is given by:
T J = T A + T R
where T A is the ambient temperature.
As an example, consider the LTC3405 in dropout at an
ambient temperature of 70 ° C. From the typical perfor-
mance graph of switch resistance, the R DS(ON) of the
P-channel switch at 70 ° C is approximately 0.94 ? . There-
fore, power dissipated by the part is:
P D = I LOAD2 ? R DS(ON) = 84.6mW
For the SOT-23 package, the θ JA is 250 ° C/ W. Thus, the
junction temperature of the regulator is:
T J = 70 ° C + (0.0846)(250) = 91.15 ° C
which is well below the maximum junction temperature of
125 ° C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (R DS(ON) ).
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V OUT immediately shifts by an amount
equal to ( ? I LOAD ? ESR), where ESR is the effective series
resistance of C OUT . ? I LOAD also begins to charge or
discharge C OUT , which generates a feedback error signal.
The regulator loop then acts to return V OUT to its steady-
state value. During this recovery time V OUT can be moni-
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1 μ F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C OUT , causing a rapid drop in V OUT . No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 ? C LOAD ).
Thus, a 10 μ F capacitor charging to 3.3V would require a
250 μ s rise time, limiting the charging current to about
130mA.
input voltage of 2.7V, a load current of 300mA and an
3405fa
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