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參數資料
| 型號： | LFC789D25CPWRE4 |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 模擬信號調理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, PDSO8 |
| 封裝： | GREEN, PLASTIC, TSSOP-8 |
| 文件頁數： | 11/15頁 |
| 文件大?。?/td> | 422K |
| 代理商： | LFC789D25CPWRE4 |
LFC789D25
DUAL LINEAR FET CONTROLLER
SLLS565B MARCH 2003 REVISED SEPTEMBER 2004
5
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
FUNCTIONAL DESCRIPTION
A linear voltage regulator can be broken down into four essential building blocks: a pass transistor, a voltage
reference, a feedback network, and a control circuit to drive the pass element, based on the comparison
between the output voltage (as sampled by the feedback network) and the voltage reference. With the exception
of the pass transistor, the -ADJ provides the other three building blocks needed. Thus, with minimal external
components and low overall solution cost, a designer can create two independent, tightly regulated output
voltages capable of delivering high currents in excess of 3 A (as limited by the external pass transistor). One
output is fixed at 2.5 V. The other output can be adjusted to any desired voltage via an externally applied signal
to the VREF pin. Because the output of the regulator always tracks any changes to this VREF pin, it is relatively
easy to implement a tracking regulator. See the typical application circuit (Figure 1).
internal reference
The fixed 2.5-V output controller uses an internal temperature-compensated bandgap reference centered at
1.2 V. Its tolerance is designed to be
<±2% over the specified temperature range, which, when coupled with the
low offset of the driver circuit, allows the 2.5-V output to have a tolerance of 2% over the specified temperature
range and full load.
external reference pin (VREF)
For the adjustable output controller, the VREF pin allows great flexibility for the designer. Taking a simple resistor
divider tied to an external voltage source and connecting the divider to the VREF pin allows the controller to
regulate an output voltage that is some fraction of the external voltage source. And, because any changes in
the external voltage source are sensed by the voltage divider, the regulated output tracks those changes.
If a tracking regulator is not desired, a fixed voltage can be achieved by applying a constant voltage to the VREF
pin. This signal can be provided by a simple device such as the TL431 adjustable shunt regulator.
The VREF pin typically sources a current of 20 nA and, as such, has a minimal loading effect on the resistor
divider or the shunt regulator. The accuracy of the adjustable output depends on the accuracy of the signal
applied to the VREF pin. Using high-precision resistors or a TL431A (1% output tolerance) helps achieve good
accuracy.
feedback network (SENSE pins)
The 2.5-V controller senses the output voltage via the SEN_V25 pin. This pin is tied to an internal resistor divider
that essentially halves the sensed output voltage and feeds it back to the controller for comparison to the internal
bandgap reference.
For the adjustable output controller, the SEN_VADJ pin provides direct feedback of the output voltage to the
controller for comparison to the externally applied VREF signal.
controller/driver
Both drivers essentially are error amplifiers that can output a worst-case minimum of 9 V (10.5 V at 25
°C) when
the LFC789D25 is powered by 12 V. This allows the controllers to regulate a large range of output
voltages, as limited by the threshold voltages of the external NMOS. Both drivers sample the output voltage via
a SEN pin. For the adjustable version, this SEN pin typically sources a current of 20 nA and, thus, has minimal
loading on the output voltage. For the 2.5-V version, this SEN pin sinks a current of approximately 125
A
(including the currents through the internal resistor divider); this results in minimal loading on the output voltage.
Although not tested, both of these controllers are designed with very low offset (typically less than 4 mV),
resulting in very accurate control of the drive signals.
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