LM2700MT-ADJ National Semiconductor, LM2700MT-ADJ Datasheet - Page 9

• Manufacturer Part Number: LM2700MT-ADJ
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2700MT-ADJ

• No. Of Pins: 14
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Package / Case: 14-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Operation
The LM2700 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 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 600 kHz appli-
cations, while a 2.2µH inductor may be used for most 1.25
MHz applications. If the duty cycle is approaching the maxi-
mum 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 LM2700 provides a compensation pin (V
the voltage loop feedback. It is recommended that a series
combination of R
network, as shown in Figure 3. For any given application,
there exists a unique combination of R
optimize the performance of the LM2700 circuit in terms of
its transient response. The series combination of R
introduces a pole-zero pair according to the following equa-
tions:
where R
approximately 850kΩ. For most applications, performance
can be optimized by choosing values within the range 5kΩ ≤
R
Output Capacitor ESR Compensation) and 680pF ≤ C
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
general guidelines in designing a stable circuit for continu-
ous conduction operation (loads greater than approximately
100mA), 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 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:
C
≤ 20kΩ (R
O
is the output impedance of the error amplifier,
C
can be up to 200kΩ if C
C
(Continued)
and C
C
be used for the compensation
C2
C
is used, see High
and C
C
) to customize
C
C
that will
and C
C
C
≤
9
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 voltage ripple is 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 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 condi-
tions 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 dissipa-
tion and increase efficiency.
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 feed-
back 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 LM2700, choosing a crossover point well be-
low where the right half plane zero is located will ensure
sufficient phase margin. A discussion of the right half plane
zero and checking the crossover using the DC gain will
follow.
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
reduce 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
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
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L
L
.