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参数资料
| 型号： | LT3439EFE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 7/12页 |
| 文件大小： | 0K |
| 描述： | IC REG PSH-PLL CTRLR ISO 16TSSOP |
| 标准包装： | 95 |
| 类型： | 推挽式控制器，隔离式 |
| 输出数： | 1 |
| 输入电压： | 2.8 V ~ 17.5 V |
| 频率 - 开关： | 20kHz ~ 250kHz |
| 电流 - 输出： | 1A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 16-TSSOP-EP |
| 产品目录页面： | 1331 (CN2011-ZH PDF) |
LT3439
APPLICATIO S I FOR ATIO
V ON = ? A B ? ? V SHDN
Oscillator SYNC
The oscillator can be synchronized to an external clock.
Set the RC timing components for an oscillator frequency
? R + R ?
? R B ?
10% below the desired sync frequency.
V IN
R A
SHDN
It is recommended that the SYNC pin be driven with a
square wave that has an amplitude greater than 2V, a pulse
width greater than 1 μ s and a rise time less than 500ns. The
rising edge of the sync waveform triggers the change in
the state of the outputs.
R B
3439 AI01
V ON is the input voltage at which the supply will turn on and
V SHDN is the SHDN pin turn-on threshold, typically 1.3V.
? ? V SHDN
V HYST A ? ?
+ I SHDN ?
Slew Rate Setting
Setting the LT3439 maximum slew rate is easy. The
= R
? R A || R B
?
?
external resistor to ground on the R SL pin sets the maxi-
mum slew rate. To determine the maximum slew rate
connect a 50k resistor pot with a 3.4k series resistance to
the R SL pin. Start at the lowest resistance setting and
increase the pot until the noise level meets your require-
ments. Note that slower slewing waveforms will lower the
power supply efficiency. Consult Linear Technology Appli-
V HYST is the actual hysteresis voltage seen at the input
voltage. I SHDN is the current hysteresis sourced by the IC
at the turn-on threshold, typically 20 μ A. ? V SHDN is the
voltage hysteresis seen at the SHDN pin at the turn-on
threshold, typically 35mV.
The resistors can be calculated as follows:
I SHDN SHDN
cation Note 70, “A Monolithic Switching Regulator with
100 μ V Output Noise” for recommended noise measure-
ment techniques.
Shutdown
The SHDN pin is used to shut down the part. Grounding
R A =
R B =
( V HYST ? V SHDN – V ON ? ? V SHDN )
? V
( V HYST ? V SHDN – V ON ? ? V SHDN )
I SHDN ? ( V ON – V SHDN )
R A = ( 0 . 5 V ? 1 . 3 V – 5 V ? 35 mV ) = 18 . 27 k
R B = ( 0 . 5 V ? 1 . 3 V – 5 V ? 35 mV ) = 6 . 42 k
this pin will disable all internal circuitry.
Increasing the SHDN voltage above the turn-on threshold,
approximately 1.3V, will enable the part. At the turn-on
threshold approximately 20 μ A of current is sourced out of
the pin. This current, in conjunction with the Thevenin
resistance on the pin, sets up the amount of hysteresis.
This allows the user to set the turn-on voltage of the supply
and the start-up hysteresis with a resistor divider. The
hysteresis can be used to prevent the part from shutting
down due to input voltage sag from an initial high current
draw. When the SHDN pin is greater than 2.1V, the
hysteresis current is reduced to zero.
In addition to the current hysteresis, there is also approxi-
mately 35mV of voltage hysteresis on the SHDN pin.
If a resistor divider is used to set the turn on threshold the
resistors are determined by the following equations:
For example if the turn-on voltage was to be set at 5V with
0.5V of hysteresis:
20 μ A ? 1 . 3 V
20 μ A ? ( 5 V – 1 . 3 V )
The nearest 1% values would be 18.2k and 6.49k.
A resistor in series with the SHDN pin could further change
hysteresis without changing the turn-on voltage.
Thermal Considerations
Decreasing the noise by lowering the slew rate of the
output switches does not come for free. Lower slew rates
sn3439 3439fs
7
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