TPS7A14
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ZHCSPG4B –DECEMBER 2021 –REVISED MAY 2022
8 Application and Implementation
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以下应用部分中的信息不属于TI 器件规格的范围,TI 不担保其准确性和完整性。TI 的客 户应负责确定
器件是否适用于其应用。客户应验证并测试其设计,以确保系统功能。
8.1 Application Information
Successfully implementing an LDO in a system depends on the system requirements. This section discusses
key device features and how to best implement them to achieve a reliable design.
8.1.1 Recommended Capacitor Types
The regulator is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the
input, output, and bias pins. Multilayer ceramic capacitors are the industry standard for use with LDOs, but must
be used with good judgment. Ceramic capacitors that use X7R-, X5R-, and COG-rated dielectric materials
provide relatively good capacitive stability across temperature, whereas the use of Y5V-rated capacitors is
discouraged because of large variations in capacitance. Regardless of the ceramic capacitor type selected,
ceramic capacitance varies with operating voltage and temperature. Generally, assume that effective
capacitance decreases by as much as 50%. The input, output, and bias capacitors recommended in the
Recommended Operating Conditions table account for an effective capacitance of approximately 50% of the
nominal value.
8.1.2 Input, Output, and Bias Capacitor Requirements
A minimum input ceramic capacitor is required for stability. A minimum output ceramic capacitor is also required
for stability, see the Recommended Operating Conditions table for the minimum capacitors values.
The input capacitor counteracts reactive input sources and improves transient response, input ripple, and PSRR.
A higher-value input capacitor may be necessary if large, fast rise-time load or line transients are anticipated, or
if the device is located several inches from the input power source. Dynamic performance of the device is
improved with the use of an output capacitor larger than the minimum value specified in the Recommended
Operating Conditions table.
Although a bias capacitor is not required, good design practice is to connect a 0.1-µF ceramic capacitor from
BIAS to GND. This capacitor counteracts reactive bias source if the source impedance is not sufficiently low.
Place the input, output, and bias capacitors as close as possible to the device to minimize trace parasitics.
If the BIAS source is susceptible to fast voltage drops (for example, a 2-V drop in less than 1 µs) when the LDO
load current is near the maximum value, the BIAS voltage drop can cause the output voltage to fall briefly. In
such cases, use a BIAS capacitor large enough to slow the voltage ramp rate to less than 0.5 V/µs. For smaller
or slower BIAS transients, any output voltage dips must be less than 5% of the nominal voltage.
8.1.3 Dropout Voltage
Dropout voltage (VDO) is defined as the input voltage minus the output voltage (VIN – VOUT) at the rated output
current (IRATED), where the pass transistor is fully on. IRATED is the maximum IOUT listed in the Recommended
Operating Conditions table. The pass transistor is in the ohmic or triode region of operation, and acts as a
switch. The dropout voltage indirectly specifies a minimum input voltage greater than the nominal programmed
output voltage at which the output voltage is expected to stay in regulation. If the input voltage falls to less than
the nominal output regulation, then the output voltage falls as well.
For a CMOS regulator, the dropout voltage is determined by the drain-source on-state resistance (RDS(ON)) of the
pass transistor. Therefore, if the linear regulator operates at less than the rated current, the dropout voltage for
that current scales accordingly. Use 方程式1 to calculate the RDS(ON) of the device.
VDO
RDS(ON)
=
IRATED
(1)
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