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参数资料
| 型号： | LT3825EFE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/32页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR FLYBK ISO 16-TSSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 250kHz |
| 占空比： | 88% |
| 电源电压： | 11 V ~ 18 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 是 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 带卷 (TR) |
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LT3825
APPLICATIONS INFORMATION
4. Compute:
300
R CMP = K1 ?
R SENSE
R S(OUT)
? R1 ? N SF
200
R S(OUT)CMP ?
? ?? 1 +
R S(OUT) ? ?
5. Verifythisresultbyconnectingaresistorofthisvalue
from the R CMP pin to ground.
6. Disconnect the ground short to C CMP and connect a 0.1μF
filter capacitor to ground. Measure the output imped-
ance R S(OUT) = ?V OUT /?I OUT with the new compensation
in place. R S(OUT) should have decreased significantly.
Fine tuning is accomplished experimentally by slightly
altering R CMP . A revised estimate for R CMP is:
?
′
R CMP = R CMP ?
?
where R ′ CMP is the new value for the load compensation
resistor, R S(OUT)CMP is the output impedance with R CMP
in place and R S(OUT) is the output impedance with no
load compensation (from step 2).
Setting Frequency
The switching frequency of the LT3825 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF, yielding switching frequencies between 50kHz and
250kHz. Figure 2 shows the nominal relationship between
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
trace length and area to minimize stray capacitance and
potential noise pickup.
You can synchronize the oscillator frequency to an external
frequency. This is done with a signal on the SYNC pin.
Set the LT3825 frequency 10% slower than the desired
external frequency using the OSC pin capacitor, then use
a pulse on the SYNC pin of amplitude greater than 2V
and with the desired frequency. The rising edge of the
SYNC signal initiates an OSC capacitor discharge forcing
primary MOSFET off (PG voltage goes low). If the oscilla-
tor frequency is much different from the sync frequency,
problems may occur with slope compensation and system
stability. Keep the sync pulse width greater than 500ns.
100
50
30 100 200
C OSC (pF)
3825 F02
Figure 2. f OSC vs OSC Capacitor Values
Selecting Timing Resistors
There are three internal “one-shot” times that are pro-
grammed by external application resistors: minimum
on-time, enable delay time and primary MOSFET turn-on
delay. These are all part of the isolated flyback control
technique, and their functions are previously outlined in
the Theory of Operation section.
The following information should help in selecting and/or
optimizing these timing values.
Minimum On-Time (t ON(MIN) )
Minimum on-time is the programmable period during which
current limit is blanked (ignored) after the turn on of the
primary-side switch. This improves regulator performance
by eliminating false tripping on the leading edge spike in
the switch, especially at light loads. This spike is due to
both the gate/source charging current and the discharge
of drain capacitance. The isolated flyback sensing requires
a pulse to sense the output. Minimum on-time ensures
that there is always a signal to close the loop.
The LT3825 does not employ cycle skipping at light loads.
Therefore, minimum on-time along with synchronous
rectification sets the switch over to forced continuous
mode operation.
3825fe
17
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| LT3837EFE#PBF | 功能描述:IC REG CTRLR FLYBK ISO 16-TSSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 特色产品:LM3753/54 Scalable 2-Phase Synchronous Buck Controllers 标准包装:1 系列:PowerWise® PWM 型:电压模式 输出数:1 频率 - 最大:1MHz 占空比:81% 电源电压:4.5 V ~ 18 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:-5°C ~ 125°C 封装/外壳:32-WFQFN 裸露焊盘 包装:Digi-Reel® 产品目录页面:1303 (CN2011-ZH PDF) 其它名称:LM3754SQDKR |
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