lm27241 National Semiconductor Corporation, lm27241 Datasheet - Page 20

• Manufacturer Part Number: lm27241
• Description: Synchronous Buck Regulator Controller For Mobile Systems
• Manufacturer: National Semiconductor Corporation

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Application Information
Zero Frequency due to R
Complex Poles due to output inductor and output capacitors:
Type III Compensator (PID)
Type III Compensator Transfer Function
The Type III compensation is needed in designs where
output ceramic capacitors are used, and therefore is very
small. The Zero frequency, due to R
FIGURE 16. Type III Compensator (PID)
ESR
:
ESR
, is usually much
(Continued)
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20
larger than the crossover frequency when ceramic capaci-
tors are used. Rule of thumb is if F
compensation.
ω
ω
quency.
ω
Must be present to in the compensation to ensure the gain
rolls off at high frequency and prevent switching ripple from
disrupting the regulation of the converter.
The Loop Bandwidth ( F
frequency (F
basic guideline:
COMPENSATION DESIGN STEPS
Output Filter:
1) Choose crossover frequency
2) Determine complex pole frequency due to output filter:
3) Determine zero frequency due to R
Type III Compensator:
1) Frequency at which maximum phase boost occurs is at
the average between the frequency of F
Place this at the Crossover frequency:
where 60˚ ≥ Φ
Z1
Z2
PZ1
: Adds phase lead in the vicinity of the crossover fre-
and ω
and ω
Z2
P2
: +20dB/DEC and +45DEG/DEC phase.
CROSS
: -20dB/DEC and -45DEG/DEC phase.
MAX
F
≥ 45˚
SW
) is usually set by the designer, with the
/10 ≤ F
BW
) or known as the crossover
BW
≤ F
ZERO
SW
ESR
/5
P2
>
and F
F
BW
use type III
Z2