MIC2208 Datasheet, PDF (17/19 Pages) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
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 Rdson 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
↑ GAIN
+
X_L
X _COUT
Ripple measurements
To properly measure ripple on either
input or output of a switching regulator,
MIC2208
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.
September 2005
17
M9999-092905
www.micrel.com