LM3075 datasheet(16/18 Pages) NSC

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Loop Compensation (Continued)

The control-output corner frequencies, and thus the desired

compensation corner frequencies, can be determined ap-

proximately by the following equations:

(20)

(21)

Since f

p is determined by the output network, it shifts with

loading (R

o) and duty cycle. First determine the range of

frequencies (f

pmin/max) of the pole across the expected load

range and then place the first compensation zero within that

range.

Example:R

esr

= 20m

Ω,C

= 100uF, R

omax

= 5V/

100mA=50

Ω,R

omin = 5V/5A = 1

Ω,L =8µH.

(22)

(23)

(24)

Once the f

p range is determined, Rc1 should be calculated

using:

(25)

Where B is the desired gain in V/V at f

p (fz1), gm is the

transconductance of the error amplifier, and R

1 and R2 are

the feedback resistors.

A gain value around 10dB (3.3V/V) is generally a good

starting point.

Example: B = 3.3 V/V, gm = 0.650 µmho, R

1 =20k

Ω,R

2 =

60.4 k

Ω:

(26)

Bandwidth varies proportionally to the value of R

c1. Next,

c1 can be determined with the following equation :

(27)

Example: fpmin = 313Hz, R

c1 = 20k

Ω:

(28)

The value of C

c1 should be within the range determined by

pmin/max. A higher value generally provides a more stable

loop, but too high a value slows the transient response time.

The compensation network also introduces a low frequency

pole that is close to 0Hz. A second pole should also be

placed at f

z. This pole can be created with a single capacitor

c2 and a shorted Rc2 (see Figure 9). The minimum value for

this capacitor can be calculated by:

(29)

c2 may not be necessary, however it does create a more

stable control loop. This is especially important with high

load currents and in current sharing mode.

Example: f

z = 36kHz, Rc1 = 20k

Ω:

20162331

FIGURE 6. Control-Output Transfer Function

20162332

FIGURE 7. Output-Control Transfer Function

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