MIC2207 Datasheet, PDF (13/21 Pages) Micrel Semiconductor – 3mmx3mm 2MHz 3A PWM Buck Regulator

Micrel
MIC2207
recommended due to there lower equivalent series
ESR and ESL.
The MIC2207 utilizes type III voltage mode internal
compensation and utilizes an internal zero to
compensate for the double pole roll off of the LC
filter. For this reason, larger output capacitors can
create instabilities. In cases where a 4.7uF output
capacitor is not sufficient, the MIC2208 offers the
ability to externally control the compensation,
allowing for a wide range of output capacitor types
and values.
Inductor Selection
The MIC2207 is designed for use with a 1µH
inductor. Proper selection should ensure the
inductor can handle the maximum average and peak
currents required by the load. Maximum current
ratings of the inductor are generally given in two
methods; permissible DC current and saturation
current. Permissible DC current can be rated either
for a 40°C temperature rise or a 10% to 20% loss in
inductance. Ensure the inductor selected can handle
the maximum operating current. When saturation
current is specified, make sure that there is enough
margin that the peak current will not saturate the
inductor.
Diode Selection
Since the MIC2207 is non-synchronous, a free-
wheeling diode is required for proper operation. A
schottky diode is recommended due to the low
forward voltage drop and their fast reverse recovery
time. The diode should be rated to be able to handle
the average output current. Also, the reverse voltage
rating of the diode should exceed the maximum
input voltage. The lower the forward voltage drop of
the diode the better the efficiency. Please refer to
the layout recommendations to minimize switching
noise.
Feedback Resistors
The feedback resistor set the output voltage by
dividing down the output and sending it to the
feedback pin. The feedback voltage is 1.0V.
Calculating the set output voltage is as follows;
V_OUT
=
V_FB
⎜⎛
⎝
R1
R2
+
1⎟⎞
⎠
Where R1 is the resistor from VOUT to FB and R2 is
the resistor from FB to GND. The recommended
feedback resistor values for common output
voltages is available in the bill of materials on page
19. Although the range of resistance for the FB
resistors is very wide, R1 is recommended to be
10K. This minimizes the effect the parasitic
capacitance of the FB node.
Feedforward Capacitor (C_FF)
A capacitor across the resistor from the output to the
feedback pin (R1) is recommended for most
designs. This capacitor can give a boost to phase
margin and increase the bandwidth for transient
response. Also, large values of feedforward
capacitance can slow down the turn-on
characteristics, reducing inrush current. For
maximum phase boost, C_FF can be calculated as
follows;
C_FF
=
2π
1
× 200kHz × R1
Bias filter
A small 10 Ohm resistor is recommended from the
input supply to the bias pin along with a small 0.1uF
ceramic capacitor from bias to ground. This will
bypass the high frequency noise generated by the
violent switching of high currents from reaching the
internal reference and control circuitry. Tantalum
and electrolytic capacitors are not recommended for
the bias, these types of capacitors lose their ability
to filter at high frequencies.
Loop Stability and Bode Analysis
Bode analysis is an excellent way to measure small
signal stability and loop response in power supply
designs. Bode analysis monitors gain and phase of
a control loop. This is done by breaking the
feedback loop and injecting a signal into the
feedback node and comparing the injected signal to
the output signal of the control loop. This will require
a network analyzer to sweep the frequency and
compare the injected signal to the output signal. The
most common method of injection is the use of
transformer. Figure 7 demonstrates how a
transformer is used to inject a signal into the
feedback network.
Figure 7. Transformer Injection
A 50 ohm resistor allows impedance matching from
the network analyzer source. This method allows the
DC loop to maintain regulation and allow the
April 2005
13
M9999-040705
www.micrel.com