TPS7A92
www.ti.com.cn
ZHCSGF3B –JULY 2017–REVISED JANUARY 2019
The TPS7A92 features a monotonic, voltage-controlled soft-start that is set by the user with an external capacitor
(CNR/SS). This soft-start helps reduce inrush current, minimizing load transients to the input power bus that can
cause potential start-up initialization problems when powering FPGAs, digital signal processors (DSPs), or other
high current loads.
To achieve a monotonic start-up, the TPS7A92 error amplifier tracks the voltage ramp of the external soft-start
capacitor until the voltage exceeds approximately 97% of the internal reference. The final 3% of VNR/SS is
charged through the noise-reduction resistor (RNR), creating an RC delay. RNR is approximately 280 kΩ and
applications that require the highest accuracy when using a large value CNR/SS must take this RC delay into
account.
The soft-start ramp time depends on the soft-start charging current (INR/SS), the soft-start capacitance (CNR/SS),
and the internal reference (VREF). The approximate soft-start ramp time (tSS) can be calculated with 公式 5:
tSS = (VREF × CNR/SS) / INR/SS
(5)
The value for INR/SS is determined by the state of the SS_CTRL pin. When the SS_CTRL pin is connected to
GND, the typical value for the INR/SS current is 6.2 µA. Connecting the SS_CTRL pin to IN increases the typical
soft-start charging current to 100 µA. The larger charging current for INR/SS is useful if shorter start-up times are
needed (such as when using a large noise-reduction capacitor).
8.1.3 Capacitor Recommendation
The TPS7A92 is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the
input, output, and noise-reduction pin. Multilayer ceramic capacitors have become the industry standard for these
types of applications and are recommended, but must be used with good understanding of their limitations.
Ceramic capacitors that employ X7R-, X5R-, and COG-rated dielectric materials provide relatively good
capacitive stability across temperature, whereas the use of Y5V-rated capacitors is discouraged precisely
because the capacitance varies so widely. In all cases, ceramic capacitors vary a great deal with operating
voltage and temperature and the design engineer must be aware of these characteristics. As a rule of thumb,
ceramic capacitors are recommended to be derated by 50%. The input and output capacitors recommended
herein account for a capacitance derating of 50%.
8.1.3.1 Input and Output Capacitor Requirements (CIN and COUT
)
The TPS7A92 is designed and characterized for operation with ceramic capacitors of 10 µF or greater at the
input and 22 µF or greater at the output. Locate the input and output capacitors as near as practical to the input
and output pins to minimize the trace inductance from the capacitor to the device.
Attention must be given to the input capacitance to minimize transient input droop during startup and load current
steps. Simply using very large ceramic input capacitances can cause unwanted ringing at the output if the input
capacitor (in combination with the wire-lead inductance) creates a high-Q peaking effect during transients, which
is why short, well-designed interconnect traces to the upstream supply are needed to minimize ringing. Damping
of unwanted ringing can be accomplished by using a tantalum capacitor, with a few hundred milliohms of ESR, in
parallel with the ceramic input capacitor. The UVLO circuit responds quickly to glitches on VIN and disables the
output of the device if this rail starts to collapse too quickly. Use an input capacitor that is large enough to slow
input transients to less then two volts per microsecond.
8.1.3.1.1 Load-Step Transient Response
The load-step transient response is the output voltage response by the LDO to a step change in load current.
The depth of charge depletion immediately after the load step is directly proportional to the amount of output
capacitance. However, although larger output capacitances decrease any voltage dip or peak occurring during a
load step, the control-loop bandwidth is also decreased, thereby slowing the response time.
The LDO cannot sink charge, therefore when the output load is removed or greatly reduced, the control loop
must turn off the pass-FET and wait for any excess charge to deplete.
8.1.3.2 Feed-Forward Capacitor (CFF)
Although a feed-forward capacitor (CFF), from the FB pin to the OUT pin is not required to achieve stability, a
10-nF, feed-forward capacitor improves the noise and PSRR performance. A higher capacitance CFF can be
used; however, the startup time is longer and the power-good signal can incorrectly indicate that the output
voltage has settled. For a detailed description, see the Pros and Cons of Using a Feedforward Capacitor with a
Low-Dropout Regulator application report.
版权 © 2017–2019, Texas Instruments Incorporated
19