MIC4721 datasheet(12/19 Pages) MICREL

12 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

Output Capacitor

The MIC4721 is designed for a 4.7µF output capacitor.

X5R or X7R dielectrics are recommended for the output

capacitor. Y5V dielectrics lose most of their capacitance

over temperature and voltage and are therefore, not

recommended.

Smaller

case

size

capacitors

are

recommended due to their lower ESL. The MIC4721

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

4.7µF 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 MIC4721 is designed for use with a 1µH inductor.

Proper selection should ensure the inductor can handle

the maximum RMS 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

since

higher

temperatures, the inductor will saturate at a lower

current.

Diode Selection

Since the MIC4721 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 fast reverse recovery time. The diode should be

rated to handle the average output current. Also, the

reverse voltage rating of the diode should exceed the

maximum input voltage. 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:

⎟

⎠

⎞

⎜

⎝

⎛

OUT

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 are available in the bill of materials at the

end of this specification. Although the resistance range

of the FB resistors is very wide, R1 is recommended to

10K.

This

minimizes

the

effect

the

parasitic

capacitance of the FB node.

Resistor R2 can be

calculated by:

OUT

−

Feedforward Capacitor (CFF)

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, CFF can be calculated as follows:

200

KHz

CFF

Bias filter

A small 10

Ω resistor is recommended from the input

supply to the bias pin along with a small 0.1µF 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 since these types of capacitors aren’t

as effective at filtering 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 a transformer. Figure 7 demonstrates how a

transformer is used to inject a signal into the feedback

network.

Figure 7. Transformer Injection

A 50

Ω resistor allows impedance matching from the

network analyzer source. This method allows the DC

loop to maintain regulation and allow the network

analyzer to insert an AC signal on top of the DC voltage.

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