Datasheet

1
2 x S x fz x R3
C4 =
1
2 x S x fp
MAX
x R3
C5 =
R3 =
R1 + R2
R2
©
§
¹
·
B
gm
x
FB
COMP
To Vout
R2
R3
C4
C5
R1
C11
8
9
1
20 x S x Ro x Co
fp =
+
0.5
2 x S x L x fsw x Co
1
2S x Re x Co
fz =
2
fsw
fn =
LM26003
SNVS576D –AUGUST 2008–REVISED MARCH 2013 www.ti.com
The control-output corner frequencies can be determined approximately by the following equations:
(17)
Where Co is the output capacitance, Ro is the load resistance, Re is the output capacitor ESR, and fsw is the
switching frequency.
The effects of slope compensation and current sense gain are included in this equation. However, the equation is
an approximation intended to simplify loop compensation calculations.
Since fp is determined by the output network, it shifts with loading. Determine the range of frequencies
(fpmin/max) across the expected load range. Then determine the compensation values as described below and
shown in Figure 22.
Figure 22. Compensation Network
1. The compensation network automatically introduces a low frequency pole (fpc), which is close to 0 Hz.
2. Once the fp range is determined, R5 should be calculated using:
(18)
Where B is the desired feedback gain in v/v between fp and fz, and gm is the transconductance of the error
amplifier. A gain value around 10 dB (3.3v/v) is generally a good starting point. Bandwidth increases with
increasing values of R3.
3. Next, place a zero (fzc) near fp using C5. C5 can be determined with the following equation:
(19)
The selected value of C5 should place fzc within a decade above or below fpmax and not less than fpmin. A
higher C5 value (closer to fpmin) generally provides a more stable loop, but too high a value will slow the
transient response time. Conversely, a smaller C5 value will result in a faster transient response, but lower phase
margin.
4. A second pole (fpc1) can also be placed at fz. This pole can be created with a single capacitor, C4. The
minimum value for this capacitor can be calculated by:
(20)
C4 may not be necessary in all applications. However if the operating frequency is being synchronized below the
nominal frequency, C4 is recommended. Although it is not required for stability, C4 is very helpful in suppressing
noise.
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