lm2711 National Semiconductor Corporation, lm2711 Datasheet - Page 14

• Manufacturer Part Number: lm2711
• Description: Step-up Pwm Dc/dc Converter Integrated With 4 Buffers
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation
V
F = 1.25MHz.
Trace:
1) SHDN, 1V/div, DC Coupled
2) I
3) V
When programming the softstart time externally, simply use
the equation given in the Soft-Start Capacitor section above.
This equation uses the typical room temperature value of the
soft start current, 11µA, to set the soft start time.
OUT
L
OUT
, 0.5A/div, DC Coupled
= 8V, V
, 5V/div, DC Coupled
IN
= 2.5V, R
(Continued)
T
T
T
L
A
A
A
= 27Ω, C
= −20˚C
= 27˚C
= 85˚C
SS
= 330nF, T = 4ms/div,
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INTRODUCTION TO COMPENSATION
The LM2711 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 10µH inductor is recommended for most 600 kHz applica-
tions, while a 4.7µH inductor may be used for most 1.25 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 LM2711 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
≤ 60kΩ (R
Output Capacitor ESR Compensation) and 680pF ≤ C
FIGURE 2. (a) Inductor current. (b) Diode current.
C
that will optimize the performance of the LM2711
O
C
C
is the output impedance of the error amplifier,
and C
can be up to 200kΩ if C
C
C
and C
introduces a pole-zero pair according to
C
be used for the compensation
C2
is used, see High
C
) to customize
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C
C
C
≤