Texas Instruments TPS7A57EVM-056, Руководство пользователя

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2.4 Optional Load Transient Circuit Operation
The TPS7A57EVM-056 evaluation module contains an optional high-performance load transient circuit to allow
efficient testing of the TPS7A57 LDO load transient performance. To use the optional load transient circuit, install
the correct components in accordance with the application. Modify the input and output capacitance connected
to the TPS7A57 LDO to match the expected operating conditions. As given by
Equation 1 , determine the desired
peak current to test and modify the parallel resistor combination of R10, 11, R12, R13, and R14.
I
Peak
=
VOUT
R10 ∥ R11 ∥ R12 ∥ R13 ∥ R14
(1)
The slew rate of the load step can be adjusted by C11, R15, and R21. In this section, only R21 is adjusted to
set the slew rate. For a 0-mA to 5-A load step, use
Table 2-1 to help select a value of R21 that results within a
desired rise or fall time.
Table 2-1. Suggested Ramp Rate Resistor Values
R21 Rise Time Fall Time
49.9 kΩ 11.5 µs 31 µs
30.9 kΩ 7.4 µs 19.7 µs
24.9 kΩ 6 µs 15.1 µs
21.5 kΩ 3.9 µs 14.8 µs
4.12 kΩ 980 ns 2.5 µs
After the EVM is modified (if needed), and if the LMG1020 gate driver is used, connect a power supply to
banana connectors J7(VCC) and J8 (GND) with a 5-V DC supply and a 1-A DC current limit. As illustrated in
Figure 2-3 , the TPS7A57 transient response is very fast and the output voltage recovers in well under 1 ms after
the initial load transient. Therefore, use a load transient pulse duration limit of 1 ms to prevent excessive heating
of the pulsed resistors (R10, R11, R12, R13, and R14). Configure a function generator for the 50-Ω output, in
a 0-V DC to 5-V DC square pulse. If necessary, burst mode can be configured in the function generator for
repetitive, low duty cycle, load transient testing.
Figure 2-3 provides example test data with R21 = 21.5 kΩ. The blue trace is the output voltage and the green
trace is the output current. R10, R11, R12, R13, and R14 provide 5-A of pulsed load. The resulting test data
shows a 0-mA to 5-A load step on VOUT of the LDO, with only a 22-μF capacitor on the output of the LDO.
The load transient circuit also provides footprints to install a damping network as needed. Install R23 and C15 to
create a damping network if needed.
Alternatively, load transient measurements can be achieved by solely using the pre-installed Q1 load transient
MOSFET. J15 can be used to provide a signal to Q1 using a function generator.
Figure 2-4 depicts results using
the MOSFET without the LMG1020 gate driver at a 5-A load.
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SBVU075 – APRIL 2022
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