AN017
4
the BOOST pin current and BD pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not
exceeded. Efficiency is not affected by the capacitor value, but the capacitor should have an
ESR of less than 1
Ω
to ensure that it can be recharged fully
under the worst-case condition
of minimum input voltage. Almost any type of film or ceramic capacitor will work fine.
For maximum efficiency, switch rise and fall times are made as short as possible. To prevent
radiation and high frequency resonance problems, proper layout of the components
connected to the switch node is essential.
Loop Stability
The evaluation board compensation consists of a 150pF capacitor in parallel with a series
RC consisting of a .015µF capacitor and a 18.2k
resistor. This compensation was selected
for use with the default components on this evaluation board. New values may have to be
selected if different components are used. The values for loop compensation components
depend on parameters which are not always well controlled. These include inductor value
(±30% due to production tolerance, load current and ripple current variations), output
capacitance (±20% to ±50% due to production tolerance, temperature, aging and changes at
the load), output capacitor ESR (±200% due to production tolerance, temperature and
aging), and finally, DC input voltage and output load current. This makes it important to
check out the final design to ensure that it is stable and tolerant of all these variations.
Phase margin and gain margin are measures of stability in closed loop systems. Phase
margin indicates relative stability, and whether or not there is a tendency to oscillate during
its damped response to an input change such as a step function. Moreover, the phase margin
measures how much phase variation is needed at the gain crossover frequency to lose
stability. Gain margin is also an indication of relative stability. Gain margin measures how
much the gain of the system can increase before the system becomes unstable. Together,
these two numbers give an estimate of the safety margin for closed-loop stability. The
smaller the stability margins, the more likely the circuit will become unstable.
One method for measuring the stability of a feedback circuit is a network analyzer. Use an
isolation transformer / adapter to isolate the grounded output analyzer from the feedback
network.
Remove the jumper across R4 and connect the output of the isolation transformer
across R4 using TP1 and TP2 terminals. Use 1M-ohm or greater probes to connect the
inputs of the analyzer to TP1 and TP2. Use GND
3
f
or the ground reference for the network
analyzer inputs.
Inject a swept frequency signal into the feedback loop, and plot the loop’s
gain and phase response between 1 kHz and 1 MHz. This provides a full picture of the
frequency response on both sides of the unity gain frequency (18 kHz in this case). Figure 2
illustrates typical results for the default configuration. The phase margin is the phase value
at the unity gain frequency, or about 81.6 Deg. The gain margin is the gain at the 0° phase
shift frequency, or approximately 32.4dB.
