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参数资料
| 型号： | LTC3806EDE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR FLYBACK PWM 12-DFN |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 290kHz |
| 占空比： | 94% |
| 电源电压： | 10 V ~ 75 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： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 12-WFDFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
LTC3806
APPLICATIO S I FOR ATIO
The rising threshold voltage on the RUN pin is equal to the
internal reference voltage of 1.230V. The comparator has
For a 50% duty factor, this reduces to:
91mV of hysteresis to increase noise immunity.
The turn-on and turn-off input voltage thresholds are
N IDEAL =
V OUT 1
V IN
programmed using a resistor divider according to the
following formulas:
If N IDEAL is integer, use this for your turns ratio. If not, find
a ratio of small integers that comes close to N IDEAL . If these
V IN ( OFF ) = 1 . 139 V ? ? 1 +
?
?
R2 ?
R 1 ? ?
conditions are met, bifilar winding techniques can be used
that will improve coupling coefficient. Cross regulation
will be better and primary-side snubbing may be reduced
V IN ( ON ) = 1 . 230 V ? ? 1 +
?
?
R2 ?
R 1 ? ?
or eliminated.
The selected turns ratio doesn’t have to be perfectly equal
The resistor R1 is typically chosen to be less than 1M. For
applications where the RUN pin is only to be used as a
logic input, the user should be aware of the 7V Absolute
Maximum Rating for this pin! The RUN pin can be
connected to the input voltage through an external 1M
resistor, as shown in Figure 3c, for “always on” operation.
Application Circuits
A basic LTC3806 application circuit is shown in Figure 1.
External component selection is driven by the character-
istics of the load and the input supply.
Duty Cycle Considerations
Current and voltage stress on the power switch and
to N IDEAL because a flyback converter’s output voltage is
not set through transformer action. Instead, the trans-
former stores energy when the primary-side switch turns
on and transfers this energy to the output(s) by flyback
action when the primary-side switch turns off.
Cross regulation may be improved by using a target duty
factor which is less than 50%. This improves cross
regulation because the secondary-side MOSFETs (syn-
chronous rectifiers) will be on a larger percentage of the
time (thereby increasing the average coupling between the
outputs). Duty factor is reduced by proportionately in-
creasing all turns ratios.
Reduced duty factor has the following effect on MOSFET
stresses:
synchronous rectifiers, input and output capacitor RMS
MOSFET
MOSFET
currents and transformer utilization (size vs power) are
impacted by duty factor. Unfortunately duty factor cannot
be adjusted to simultaneously optimize all of these re-
LOCATION
Primary
Secondary
CURRENT STRESS
Increased
Reduced
VOLTAGE STRESS
Reduced
Increased
quirements. In general, avoid extreme duty factors since
this severely impacts the current stress on most of the
The duty factor with the selected turns ratio will equal:
components. A reasonable target for duty factor is 50% at
nominal input voltage. Using this rule of thumb, calculate
the ideal transformer turns ratio:
D =
V OUT 1
V OUT 1 + ( N ? V IN )
? D ? ?
? ?
N IDEAL =
V OUT 1 ? 1– D ?
V IN
While the output(s)/input turns ratio are not critical, the
turns ratio between outputs are critical and affect the
accuracy of the slave output voltages.
3806fb
11
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