Datasheet

PSRR (dB) = 20 Log
10
V (f)
S(IN)
V (f)
S(OUT)
TPS7A4700
TPS7A4701
SBVS204E –JUNE 2012–REVISED JANUARY 2014 www.ti.com
AC PERFORMANCE
LDO ac performance is typically understood to include power-supply rejection ratio, load step transient response,
and output noise. These metrics are primarily a function of open-loop gain and bandwidth, phase margin, and
reference noise.
Power-Supply Rejection Ratio (PSRR)
PSRR is a measure of how well the LDO control loop rejects ripple noise from the input source to make the dc
output voltage as noise-free as possible across the frequency spectrum (usually 10 Hz to 10 MHz). Even though
PSRR is therefore a loss in noise signal amplitude (the output ripple relative to the input ripple), the PSRR
reciprocal is plotted in the Electrical Characteristics as a positive number in decibels (dB) for convenience.
Equation 6 gives the PSRR calculation as a function of frequency where input noise voltage [V
S(IN)
(f)] and output
noise voltage [V
S(OUT)
(f)] are understood to be purely ac signals.
(6)
Noise that couples from the input to the internal reference voltage for the control loop is also a primary
contributor to reduced PSRR magnitude and bandwidth. This reference noise is greatly filtered by the noise
reduction capacitor at the NR pin of the LDO in combination with an internal filter resistor (R
SS
) for optimal
PSRR.
The LDO is often employed not only as a dc/dc regulator, but also to provide exceptionally clean power-supply
voltages that are free of noise and ripple to power-sensitive system components. This usage is especially true for
the TPS7A470x.
Load Step Transient Response
The load step transient response is the output voltage response by the LDO to a step change in load current
whereby output voltage regulation is maintained. The worst-case response is characterized for a load step of
10 mA to 1 A (at 1 A per microsecond) and shows a classic critically-damped response of a very stable system.
The voltage response shows a small dip in the output voltage when charge is initially depleted from the output
capacitor and then the output recovers as the control loop adjusts itself. The depth of the charge depletion
immediately after the load step is directly proportional to the amount of output capacitance. However, to some
extent, the speed of recovery is inversely proportional to that same output capacitance. In other words, larger
output capacitances act to decrease any voltage dip or peak occurring during a load step but also decrease the
control-loop bandwidth, thereby slowing response.
The worst-case off-loading step characterization occurs when the current step transitions from 1 A to 0 mA.
Initially, the LDO loop cannot respond fast enough to prevent a small increase in output voltage charge on the
output capacitor. Because the LDO cannot sink charge current, the control loop must turn off the main pass-FET
to wait for the charge to deplete, thus giving the off-load step its typical monotonic decay (which appears
triangular in shape).
Noise
The TPS7A470x is designed, in particular, for system applications where minimizing noise on the power-supply
rail is critical to system performance. This scenario is the case for phase-locked loop (PLL)-based clocking
circuits for instance, where minimum phase noise is all important, or in-test and measurement systems where
even small power-supply noise fluctuations can distort instantaneous measurement accuracy. Because the
TPS7A470x is also designed for higher voltage industrial applications, the noise characteristic is well designed to
minimize any increase as a function of the output voltage.
LDO noise is defined as the internally-generated intrinsic noise created by the semiconductor circuits alone. This
noise is the sum of various types of noise (such as shot noise associated with current-through-pin junctions,
thermal noise caused by thermal agitation of charge carriers, flicker noise or 1/f noise that is a property of
resistors and dominates at lower frequencies as a function of 1/f, burst noise, and avalanche noise).
To calculate the LDO RMS output noise, a spectrum analyzer must first measure the spectral noise across the
bandwidth of choice (typically 10 Hz to 100 kHz in units of µV/√Hz). The RMS noise is then calculated in the
usual manner as the integrated square root of the squared spectral noise over the band, then averaged by the
bandwidth.
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