LM3311SQ National Semiconductor Corporation, LM3311SQ Datasheet - Page 21

• Manufacturer Part Number: LM3311SQ
• Description: Step-up Pwm Dc/dc Converter With Integrated Ldo, Op-amp, And Gate Pulse Modulation Switch
• Manufacturer: National Semiconductor Corporation

INTRODUCTION TO COMPENSATION (BOOST
CONVERTER)
The LM3311 is a current mode PWM boost converter. The
signal flow of this control scheme has two feedback loops,
one that senses switch current and one that senses output
voltage.
To keep a current programmed control converter stable
above duty cycles of 50%, the inductor must meet certain cri-
teria. The inductor, along with input and output voltage, will
determine the slope of the current through the inductor (see
Figure 5 (a)). If the slope of the inductor current is too great,
the circuit will be unstable above duty cycles of 50%. A 10µH
inductor is recommended for most 660 kHz applications,
while a 4.7µH inductor may be used for most 1.28 MHz ap-
plications. If the duty cycle is approaching the maximum of
85%, it may be necessary to increase the inductance by as
much as 2X. See Inductor and Diode Selection for more de-
tailed inductor sizing.
The LM3311 provides a compensation pin (V
the voltage loop feedback. It is recommended that a series
FIGURE 5. (a) Inductor current. (b) Diode current.
C
) to customize
21
combination of R
work, as shown in the typical application circuit. For any given
application, there exists a unique combination of R
that will optimize the performance of the LM3311 circuit in
terms of its transient response. The series combination of
R
lowing equations:
where R
proximately 900kΩ. For most applications, performance can
C
and C
O
C
is the output impedance of the error amplifier, ap-
introduces a pole-zero pair according to the fol-
C
and C
C
be used for the compensation net-
20126305
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C
and C
C