參數(shù)資料
型號(hào)： LM3311
廠商： National Semiconductor Corporation
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： Step-Up PWM DC/DC Converter with Integrated LDO, Op-Amp, and Gate Pulse Modulation Switch
中文描述： 升壓的PWM直流/直流集成穩(wěn)壓器，運(yùn)算放大器和開(kāi)關(guān)門脈沖調(diào)制器
文件頁(yè)數(shù)： 21/29頁(yè)
文件大?。?/td> 2052K
代理商： LM3311
Operation
(Continued)
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
criteria. 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 applica-
tions, while a 4.7μH inductor may be used for most 1.28 MHz
applications. 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
detailed inductor sizing.
The LM3311 provides a compensation pin (V
) to customize
the voltage loop feedback. It is recommended that a series
combination of R
and C
be used for the compensation
network, as shown in the typical application circuit. For any
given application, there exists a unique combination of R
C
and C
that will optimize the performance of the LM3311
circuit in terms of its transient response. The series combi-
nation of R
and C
introduces a pole-zero pair according to
the following equations:
where R
is the output impedance of the error amplifier,
approximately 900k
. For most applications, performance
can be optimized by choosing values within the range 5k
≤
R
≤
100k
(R
can be higher values if C
is used, see
High Output Capacitor ESR Compensation
) and 68pF
≤
C
C
≤
4.7nF. Refer to the
Applications Information
section for
recommended values for specific circuits and conditions.
Refer to the
Compensation
section for other design require-
ment.
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
general guidelines in designing a stable circuit for continu-
ous 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 pro-
duce stability.
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 voltage and the maximum output voltage. This equa-
tion is:
where fs is the switching frequency, D is the duty cycle, and
R
is the ON resistance of the internal power switch.
This equation is only good for duty cycles greater than 50%
(D
>
0.5), for duty cycles less than 50% the recommended
values 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 induc-
tor current and the inductor current ripple must be consid-
ered.
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
i
is less than
the average inductor current and discontinuous conduction
mode occurs when
i
is greater than the average inductor
current. Care must be taken to make sure that the switch will
not reach its current limit during normal operation. The in-
ductor must also be sized accordingly. It should have a
saturation current rating higher than the peak inductor cur-
rent expected. The output voltage ripple is also affected by
the total ripple current.
20126305
FIGURE 5. (a) Inductor current. (b) Diode current.
L
www.national.com
21
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