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參數資料
| 型號： | LT3957EUHE#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, PQCC36 |
| 封裝： | 5 X 6 MM, LEAD FREE, PLASTIC, QFN-36 |
| 文件頁數： | 3/28頁 |
| 文件大小： | 359K |
| 代理商： | LT3957EUHE#TRPBF |
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LT3957
11
3957f
APPLICATIONS INFORMATION
Operating Frequency and Synchronization
The choice of operating frequency may be determined by
on-chip power dissipation (a low switching frequency may
be required to ensure IC junction temperature does not
exceed 125°C), otherwise it is a trade-off between efciency
and component size. Low frequency operation improves
efciency by reducing gate drive current and MOSFET
and diode switching losses. However, lower frequency
operation requires a physically larger inductor. Switching
frequency also has implications for loop compensation.
The LT3957 uses a constant-frequency architecture that
can be programmed over a 100kHz to 1000kHz range
with a single external resistor from the RT pin to SGND,
as shown in Figure 1. A table for selecting the value of RT
for a given operating frequency is shown in Table 1.
Table 1. Timing Resistor (RT) Value
SWITCHING FREQUENCY (kHz)
RT (kΩ)
100
140
200
63.4
300
41.2
400
30.9
500
24.3
600
19.6
700
16.5
800
14
900
12.1
1000
10.5
The operating frequency of the LT3957 can be synchronized
to an external clock source. By providing a digital clock
signal into the SYNC pin, the LT3957 will operate at the
SYNC clock frequency. The LT3957 detects the rising edge
of each clock cycle. If this feature is used, an RT resistor
should be chosen to program a switching frequency 20%
slower than SYNC pulse frequency. It is recommended that
the SYNC pin has a minimum pulse width of 200ns. Tie
the SYNC pin to SGND if this feature is not used.
Duty Cycle Consideration
Switching duty cycle is a key variable dening con-
verter operation. As such, its limits must be considered.
Minimum on-time is the smallest time duration that the
LT3957 is capable of turning on the power MOSFET. This
time is typically about 240ns (see Minimum On-Time in
the Electrical Characteristics table). In each switching
cycle, the LT3957 keeps the power switch off for at least
220ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time, minimum off-time and the switching
frequency dene the minimum and maximum switching
duty cycles a converter is able to generate:
Minimum duty cycle = minimum on-time frequency
Maximum duty cycle = 1 – (minimum off-time frequency)
Programming the Output Voltage
The output voltage VOUT is set by a resistor divider, as
shown in Figure 1. The positive and negative VOUT are set
by the following equations:
VOUT,POSITIVE =1.6V 1+
R2
R1
VOUT,NEGATIVE = –0.8V 1+
R2
R1
The resistors R1 and R2 are typically chosen so that
the error caused by the current owing into the FBX pin
during normal operation is less than 1% (this translates
to a maximum value of R1 at about 158k).
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| LT3957IUHE#PBF | 功能描述:IC REG MULTI CONFIG ADJ 36VQFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 開關穩(wěn)壓器 系列:- 標準包裝:2,500 系列:- 類型:降壓（降壓） 輸出類型:固定 輸出數:1 輸出電壓:1.2V，1.5V，1.8V，2.5V 輸入電壓:2.7 V ~ 20 V PWM 型:- 頻率 - 開關:- 電流 - 輸出:50mA 同步整流器:是 工作溫度:-40°C ~ 125°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬）裸露焊盤 包裝:帶卷 (TR) 供應商設備封裝:10-MSOP 裸露焊盤 |
| LT3957IUHE#TRPBF | 功能描述:IC REG MULTI CONFIG ADJ 36VQFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 開關穩(wěn)壓器 系列:- 標準包裝:2,500 系列:- 類型:降壓（降壓） 輸出類型:固定 輸出數:1 輸出電壓:1.2V，1.5V，1.8V，2.5V 輸入電壓:2.7 V ~ 20 V PWM 型:- 頻率 - 開關:- 電流 - 輸出:50mA 同步整流器:是 工作溫度:-40°C ~ 125°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬）裸露焊盤 包裝:帶卷 (TR) 供應商設備封裝:10-MSOP 裸露焊盤 |
| LT3957IUHEPBF | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch |
| LT3957IUHETRPBF | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch |
| LT3958 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Monolithic High Voltage Isolated Flyback Converter No External Start-Up Resistor |
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