lm2711 National Semiconductor Corporation, lm2711 Datasheet - Page 13

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

Operation
CONTINUOUS CONDUCTION MODE
The LM2711 is a current-mode, PWM boost regulator. A
boost regulator steps the input voltage up to a higher output
voltage. In continuous conduction mode (when the inductor
current never reaches zero at steady state), the boost regu-
lator operates in two cycles.
In the first cycle of operation, shown in Figure 1 (a), the
transistor is closed and the diode is reverse biased. Energy
is collected in the inductor and the load current is supplied by
C
The second cycle is shown in Figure 1 (b). During this cycle,
the transistor is open and the diode is forward biased. The
energy stored in the inductor is transferred to the load and
output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D' will be
required for design calculations
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the feedback pin and a
resistor divider connected to the output as shown in the
typical operating circuit. The feedback pin voltage is 1.265V,
so the ratio of the feedback resistors sets the output voltage
according to the following equation:
OUT
.
(a) First Cycle of Operation (b) Second Cycle Of Operation
FIGURE 1. Simplified Boost Converter Diagram
13
SOFT-START CAPACITOR
The LM2711 has patented internal circuitry that is used to
limit the inductor inrush current on start-up. This inrush
current limiting circuitry serves as a soft-start. However,
many applications may require much more soft-start than
what is available with the internal circuitry. The external SS
pin is used to tailor the soft-start for a specific application. A
11µA current charges the external soft-start capacitor, Css.
The soft-start time can be estimated as:
The minimum soft-start time is set by the internal soft-start
circuitry, typically 7ms for 600kHz operation and approxi-
mately half that for 1.25MHz operation. Only longer soft-start
times may be implemented using the SS pin and a capacitor
C
tion, the internal soft-start circuitry will override it.
Due to the unique nature of the dual internal/external soft-
start, care was taken in the design to ensure temperature
stable operation. As you can see with the Iss data in the
Electrical Characterisitcs table and the graph "Soft-Start
Current vs. V
section, the soft start curent has a temperature coefficient
and would lead one to believe there would be significant
variation with temperature. Though the current has a tem-
perature coefficient the actual programmed external soft
start time does not show this extreme of a temperature
variation. As you can see in the following transient plots:
SS
. If a shorter time is designed for using the above equa-
IN
" in the Typical Performance Characterisitcs
Tss = Css*0.6V/11µA
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