lm2715sqx-adj National Semiconductor Corporation, lm2715sqx-adj Datasheet - Page 16

• Manufacturer Part Number: lm2715sqx-adj
• Description: Tft Panel Module
• Manufacturer: National Semiconductor Corporation

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Operation
INTRODUCTION TO COMPENSATION
The LM2715 contains 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 2 (a)). If the slope of the inductor current is too
great, the circuit will be unstable above duty cycles of 50%.
A 4.7µH inductor is recommended for most 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 LM2715 provides a compensation pin (V
the voltage loop feedback. It is recommended that a series
combination of R
network, as shown in the typical application circuit. For any
given application, there exists a unique combination of R
and C
circuit in terms of its transient response. The series combi-
nation of R
the following equations:
where R
approximately 1MΩ. For most applications, performance can
be optimized by choosing values within the range 5kΩ ≤ R
≤ 40kΩ (R
Output Capacitor ESR Compensation) and 680pF ≤ C
FIGURE 2. (a) Inductor current. (b) Diode current.
C
that will optimize the performance of the LM2715
O
C
C
is the output impedance of the error amplifier,
and C
can be up to 200kΩ if C
C
C
(Continued)
and C
introduces a pole-zero pair according to
C
be used for the compensation
C2
is used, see High
C
) to customize
20058405
C
C
C
≤
16
4.7nF. Refer to the Typical Application Circuit and the Appli-
cations Information section for recommended values for spe-
cific circuits and conditions. Refer to the Compensation sec-
tion for other design requirement.
COMPENSATION FOR BOOST DC/DC
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 (Inductor current never reaches
zero), 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 stability.
INDUCTOR AND DIODE SELECTION
Although the inductor size mentioned earlier is fine for most
applications, a more exact value can be calculated. To en-
sure 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
the graph "R
acteristics section. This equation is only good for duty cycles
greater than 50% (D
recommended values may be used. The corresponding in-
ductor current ripple as shown in Figure 2 (a) is given by:
The inductor ripple current is important for a few reasons.
One reason is because the peak switch current will be the
average inductor current (input current or I
As a side note, discontinuous operation occurs when the
inductor current falls to zero during a switching cycle, or ∆i
is greater than the average inductor current. Therefore, con-
tinuous conduction mode occurs when ∆i
average inductor current. 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 and input voltage
ripples are also affected by the total ripple current.
The output diode for a boost regulator must be chosen
correctly depending on the output voltage and the output
current. The typical current waveform for the diode in con-
tinuous conduction mode is shown in Figure 2 (b). The diode
must be rated for a reverse voltage 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 cur-
rent. During short circuit testing, or if short circuit conditions
are possible in the application, the diode current rating must
DSON
is the ON resistance of the internal switch taken from
DSON
vs. V
>
0.5), for duty cycles less than 50% the
IN
" in the Typical Performance Char-
L
LOAD
is less than the
/D’) plus ∆i
L
L
.