LM26001BMH NSC [National Semiconductor], LM26001BMH Datasheet - Page 14

• Manufacturer Part Number: LM26001BMH
• Description: 1.5A Switching Regulator with High Efficiency Sleep Mode
• Manufacturer: NSC [National Semiconductor]

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COMPENSATION
The purpose of loop compensation is to ensure stable oper-
ation while maximizing dynamic performance. Stability can be
analyzed with loop gain measurements, while dynamic per-
formance is analyzed with both loop gain and load transient
response. Loop gain is equal to the product of control-output
transfer function (power stage) and the feedback transfer
function (the compensation network).
For stability purposes, our target is to have a loop gain slope
that is -20dB /decade from a very low frequency to beyond
the crossover frequency. Also, the crossover frequency
should not exceed one-fifth of the switching frequency, i.e. 60
kHz in the case of 300 kHz switching frequency.
For dynamic purposes, the higher the bandwidth, the faster
the load transient response. A large DC gain means high DC
regulation accuracy (i.e. DC voltage changes little with load
or line variations). To achieve this loop gain, the compensa-
tion components should be set according to the shape of the
control-output bode plot. A typical plot is shown in Figure 8
below.
The control-output transfer function consists of one pole (fp),
one zero (fz), and a double pole at fn (half the switching fre-
quency).
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.
FIGURE 8. Control-Output Transfer Function
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The control-output corner frequencies can be determined ap-
proximately by the following equations:
Where Co is the output capacitance, Ro is the load resistance,
Re is the output capacitor ESR, and fsw is the switching fre-
quency. 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 cal-
culations. To derive the exact transfer function, use 0.2V/V
sense amp gain and 36mVp-p slope compensation.
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:
FIGURE 9. Feedback Transfer Function
FIGURE 10. Compensation Network
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