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参数资料
| 型号： | LTC3803MPS6#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 240 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | PLASTIC, MO-193, TSOT-23, 6 PIN |
| 文件页数： | 3/16页 |
| 文件大小： | 164K |
| 代理商： | LTC3803MPS6#TR |
LTC3803
11
3803fc
APPLICATIONS INFORMATION
and the VCC node begins to charge via RSTART back up to
the VCC 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 VCC node.
Component selection is as follows:
Resistor RSTART 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 CVCC to the VCC 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 efciency.
Capacitor CVCC 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 VCCturn-offthreshold.
Also, the third winding’s nominal output voltage should
be at least 0.5V below the minimum rated VCC clamp volt-
age to avoid running up against the LTC3803’s VCC shunt
regulator, needlessly wasting power.
VCC SHUNT REGULATOR
In applications including a third transformer winding,
the internal VCC shunt regulator serves to protect the
LTC3803 from overvoltage transients as the third wind-
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 VIN to VCC, in conjunction with a bypass
capacitor, CVCC, that closely decouples VCC to GND (see
Figure 3). This simplicity comes at the expense of reduced
efciency due to the static power dissipation in the RVCC
dropping resistor.
The shunt regulator can draw up to 25mA through the
VCC pin to GND to drop enough voltage across RVCC to
regulate VCC to around 9.5V. For applications where VIN
is low enough such that the static power dissipation in
RVCC is acceptable, using the VCC 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 RB brings VCC to at least 7.6V nominal, well above the
maximum rated VCC turn-off threshold. Resistor RSTART
momentarily charges the VCC node up to the VCC turn-on
threshold, enabling the LTC3803.
Figure 3. Powering the LTC3803 Via the Internal Shunt Regulator
LTC3803
VCC
RVCC
CVCC
3803 F03
VIN
GND
Figure 4. Powering the LTC3803 with an External Preregulator
LTC3803
VCC
RSTART
RB
D1
8.2V
Q1
CVCC
3803 F04
VIN
GND
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