MIC2207 datasheet(13/21 Pages) MICREL

13 / 21 page

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 1

µH

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;

⎟

⎠

⎞

⎜

⎝

⎛

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 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 (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;

200kHz

CFF

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

demonstrates

how

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

M9999-040705

MIC2207 Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC2207 Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL