LM2700_04 NSC [National Semiconductor], LM2700_04 Datasheet - Page 8

• Manufacturer Part Number: LM2700_04
• Description: 600kHz/1.25MHz, 2.5A, Step-up PWM DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

Operation
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FIGURE 1. Simplified Boost Converter Diagram
(a) First Cycle of Operation (b) Second Cycle Of Operation
CONTINUOUS CONDUCTION MODE
The LM2700 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.
INTRODUCTION TO COMPENSATION
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
.
OUT
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 Figure 3.
The feedback pin voltage is 1.26V, so the ratio of the feed-
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back resistors sets the output voltage according to the fol-
lowing equation:
FIGURE 2. (a) Inductor current. (b) Diode current.
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