MIC2208_10 Datasheet, PDF (17/18 Pages) Micrel Semiconductor – 3mm x 3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
From this graph, you can see the effects of bandwidth
and output capacitance. For frequencies <100KHz, the
output impedance is dominated by the gain and
inductance. For frequencies >100KHz, the output
impedance is dominated by the capacitance. A good
approximation for transient response can be calculated
from determining the frequency of the load step in amps
per second:
f = ^A/sec
2π
Then, determine the output impedance by looking at the
output impedance vs. frequency graph. Then calculating
the voltage deviation times the load step:
∆V_OUT = ∆I_OUT × Z_OUT
The output impedance graph shows the relationship
between supply voltage and output impedance. This is
caused by the lower R_DSON of the high side MOSFET
and the increase in gain with increased supply voltages.
This explains why higher supply voltages have better
transient response.
↓ Z _TOTAL
=
↓ R_DSON + DCR + X_L
↑ GAIN
|| X_COUT
MIC2208
Ripple measurements
To properly measure ripple on either input or output of a
switching regulator, a proper ring in tip measurement is
required. Standard oscilloscope probes come with a
grounding clip, or a long wire with an alligator clip.
Unfortunately, for high frequency measurements, this
ground clip can pick-up high frequency noise and
erroneously inject it into the measured output ripple.
The standard evaluation board accommodates a home
made version by providing probe points for both the
input and output supplies and their respective grounds.
This requires the removing of the oscilloscope probe
sheath and ground clip from a standard oscilloscope
probe and wrapping a non-shielded bus wire around the
oscilloscope probe. If there does not happen to be any
non shielded bus wire immediately available, the leads
from axial resistors will work. By maintaining the shortest
possible ground lengths on the oscilloscope probe, true
ripple measurements can be obtained
May 2010
17
M9999-051410-D