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参数资料
| 型号： | LTC3803IS6#TRMPBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR FLYBK PWM TSOT23-6 |
| 标准包装： | 500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 240kHz |
| 占空比： | 90% |
| 电源电压： | 5.7 V ~ 8 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 ~ 125°C |
| 封装/外壳： | SOT-23-6 细型，TSOT-23-6 |
| 包装： | 带卷 (TR) |
| 相关产品： | 732-2263-6-ND - TRANS FLYBACK LTC3803 16.9UH 732-2263-1-ND - TRANS FLYBACK LTC3803 16.9UH 732-2263-2-ND - TRANS FLYBACK LTC3803 16.9UH |
| 其它名称： | LTC3803IS6#PBF LTC3803IS6#PBF-ND |
LTC3803
APPLICATIONS INFORMATION
and the V CC node begins to charge via R START back up to
V IN
the V CC turn-on threshold. Depending on the particular
situation, this may result in either several on-off cycles
before proper operation is reached or permanent relaxation
oscillation at the V CC node.
R VCC
C VCC
LTC3803
V CC
GND
Component selection is as follows:
Resistor R START should be made small enough to yield a
worst-case minimum charging current greater than the
maximum rated LTC3803 start-up current, to ensure there
is enough current to charge C VCC to the V CC turn-on thresh-
old. It should be made large enough to yield a worst-case
maximum charging current less than the minimum rated
LTC3803 supply current, so that in operation, most of the
LTC3803’s supply current is delivered through the third
winding. This results in the highest possible ef?ciency.
Capacitor C VCC should then be made large enough to avoid
the relaxation oscillation behavior described above. This
is complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The third transformer winding should be designed so that
its output voltage, after accounting for the D2’s forward
voltage drop, exceeds the maximum V CC turn-off threshold.
Also, the third winding’s nominal output voltage should
be at least 0.5V below the minimum rated V CC clamp volt-
age to avoid running up against the LTC3803’s V CC shunt
3803 F03
Figure 3. Powering the LTC3803 Via the Internal Shunt Regulator
The shunt regulator can draw up to 25mA through the
V CC pin to GND to drop enough voltage across R VCC to
regulate V CC to around 9.5V. For applications where V IN
is low enough such that the static power dissipation in
R VCC is acceptable, using the V CC shunt regulator is the
simplest way to power the LTC3803.
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows a third way to power the
LTC3803. An external series preregulator consisting of
series pass transistor Q1, Zener diode D1, and bias resis-
tor R B brings V CC to at least 7.6V nominal, well above the
maximum rated V CC turn-off threshold. Resistor R START
momentarily charges the V CC node up to the V CC turn-on
threshold, enabling the LTC3803.
V IN
regulator, needlessly wasting power.
R B
Q1
R START
LTC3803
V CC
V CC SHUNT REGULATOR
D1
8.2V
C VCC
GND
In applications including a third transformer winding,
the internal V CC shunt regulator serves to protect the
LTC3803 from overvoltage transients as the third wind-
3803 F04
Figure 4. Powering the LTC3803 with an External Preregulator
ing is powering up.
In applications where a third transformer winding is
undesirable or unavailable, the shunt regulator allows
the LTC3803 to be powered through a single dropping
resistor from V IN to V CC , in conjunction with a bypass
capacitor, C VCC , that closely decouples V CC to GND (see
Figure 3). This simplicity comes at the expense of reduced
ef?ciency due to the static power dissipation in the R VCC
dropping resistor.
3803fc
11
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