lm26001bmhx National Semiconductor Corporation, lm26001bmhx Datasheet - Page 14

• Manufacturer Part Number: lm26001bmhx
• Description: 1.5a Switching Regulator With High Efficiency Sleep Mode
• Manufacturer: National Semiconductor Corporation

www.national.com
Design Information
The control-output transfer function consists of one pole (fp),
one zero (fz), and a double pole at fn (half the switching
frequency).
Referring to Figure 8, the following should be done to create
a -20dB /decade roll-off of the loop gain:
1. Place a pole at 0Hz (fpc)
2. Place a zero at fp (fzc)
3. Place a second pole at fz (fpc1)
The resulting feedback (compensation) bode plot is shown
below in Figure 9. Adding the control-output response to the
feedback response will then result in a nearly continuous
-20db/decade slope.
The control-output corner frequencies can be determined
approximately by the following equations:
FIGURE 8. Control-Output Transfer Function
FIGURE 9. Feedback Transfer Function
(Continued)
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Where Co is the output capacitance, Ro is the load resis-
tance, Re is the output capacitor ESR, and fsw is the switch-
ing frequency. The effects of slope compensation and cur-
rent sense gain are included in this equation. However, the
equation is an approximation intended to simplify loop com-
pensation calculations. To derive the exact transfer function,
use 0.2V/V sense amp gain and 36mVp-p slope compensa-
tion.
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 com-
pensation values as described below and shown in Figure
10.
1. The compensation network automatically introduces a low
frequency pole (fpc), which is close to 0Hz.
2. Once the fp range is determined, R5 should be calculated
using:
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 10dB (3.3v/v) is generally a good starting
point. Bandwidth increases with increasing values of R5.
3. Next, place a zero (fzc) near fp using C8. C8 can be
determined with the following equation:
The selected value of C8 should place fzc within a decade
above or below fpmax, and not less than fpmin. A higher C8
value (closer to fpmin) generally provides a more stable
loop, but too high a value will slow the transient response
time. Conversely, a smaller C8 value will result in a faster
transient response, but lower phase margin.
FIGURE 10. Compensation Network
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