LM3311SQ National Semiconductor Corporation, LM3311SQ Datasheet - Page 22

• 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

www.national.com
be optimized by choosing values within the range 5kΩ
≤
Output Capacitor ESR Compensation) and 68pF
4.7nF. Refer to the Applications Information section for rec-
ommended values for specific circuits and conditions. Refer
to the Compensation section for other design requirement.
COMPENSATION
This section will present a general design procedure to help
insure a stable and operational circuit. The designs in this
datasheet are optimized for particular requirements. If differ-
ent conversions are required, some of the components may
need to be changed to ensure stability. Below is a set of gen-
eral guidelines in designing a stable circuit for continuous
conduction operation, in most all cases this will provide for
stability during discontinuous operation as well. The power
components and their effects will be determined first, then the
compensation components will be chosen to produce stabili-
ty.
INDUCTOR AND DIODE SELECTION
Although the inductor sizes mentioned earlier are fine for most
applications, a more exact value can be calculated. To ensure
stability at duty cycles above 50%, the inductor must have
some minimum value determined by the minimum input volt-
age and the maximum output voltage. This equation is:
where fs is the switching frequency, D is the duty cycle, and
R
equation is only good for duty cycles greater than 50%
(D>0.5), for duty cycles less than 50% the recommended val-
ues may be used. The value given by this equation is the
inductance necessary to supress sub-harmonic oscillations.
In some cases the value given by this equation may be too
small for a given application. In this case the average inductor
current and the inductor current ripple must be considered.
The corresponding inductor current ripple, average inductor
current, and peak inductor current as shown in Figure 5 (a) is
given by:
Continuous conduction mode occurs when Δi
average inductor current and discontinuous conduction mode
occurs when Δi
Care must be taken to make sure that the switch will not reach
its current limit during normal operation. The inductor must
also be sized accordingly. It should have a saturation current
rating higher than the peak inductor current expected. The
output voltage ripple is also affected by the total ripple current.
The output diode for a boost regulator must be chosen cor-
rectly depending on the output voltage and the output current.
The typical current waveform for the diode in continuous con-
duction mode is shown in Figure 5 (b). The diode must be
DSON
100kΩ (R
is the ON resistance of the internal power switch. This
C
can be higher values if C
L
is greater than the average inductor current.
C2
is used, see High
L
is less than the
≤
C
≤
C
R
≤
C
22
rated for a reverse voltage equal to or greater than the output
voltage used. The average current rating must be greater than
the maximum load current expected, and the peak current
rating must be greater than the peak inductor current. During
short circuit testing, or if short circuit conditions are possible
in the application, the diode current rating must exceed the
switch current limit. Using Schottky diodes with lower forward
voltage drop will decrease power dissipation and increase ef-
ficiency.
DC GAIN AND OPEN-LOOP GAIN
Since the control stage of the converter forms a complete
feedback loop with the power components, it forms a closed-
loop system that must be stabilized to avoid positive feedback
and instability. A value for open-loop DC gain will be required,
from which you can calculate, or place, poles and zeros to
determine the crossover frequency and the phase margin. A
high phase margin (greater than 45°) is desired for the best
stability and transient response. For the purpose of stabilizing
the LM3311, choosing a crossover point well below where the
right half plane zero is located will ensure sufficient phase
margin.
To ensure a bandwidth of ½ or less of the frequency of the
RHP zero, calculate the open-loop DC gain, A
value is known, you can calculate the crossover visually by
placing a −20dB/decade slope at each pole, and a +20dB/
decade slope for each zero. The point at which the gain plot
crosses unity gain, or 0dB, is the crossover frequency. If the
crossover frequency is less than ½ the RHP zero, the phase
margin should be high enough for stability. The phase margin
can also be improved by adding C
section. The equation for A
equations required for the calculation:
where R
input voltage, g
found in the Electrical Characteristics table, and R
value chosen from the graph "NMOS R
in the Typical Performance Characteristics section.
INPUT AND OUTPUT CAPACITOR SELECTION
The switching action of a boost regulator causes a triangular
voltage waveform at the input. A capacitor is required to re-
duce the input ripple and noise for proper operation of the
regulator. The size used is dependant on the application and
board layout. If the regulator will be loaded uniformly, with
very little load changes, and at lower current outputs, the input
L
is the minimum load resistance, V
m
is the error amplifier transconductance
mc
≊
0.072fs (in V/s)
DC
is given below with additional
C2
as discussed later in this
DSON
IN
vs. Input Voltage"
is the minimum
DC
. After this
DSON
is the