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LM2585 Datasheet(PDF) 17 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2585
Description  SIMPLE SWITCHER 3A Flyback Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2585 Datasheet(HTML) 17 Page - National Semiconductor (TI)

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Step-Up (Boost) Regulator Operation
Figure 30 shows the LM2585 used as a step-up (boost)
regulator. This is a switching regulator that produces an out-
put voltage greater than the input supply voltage.
A brief explanation of how the LM2585 Boost Regulator
works is as follows (refer to
Figure 30). When the NPN
switch turns on, the inductor current ramps up at the rate of
V
IN/L, storing energy in the inductor. When the switch turns
off, the lower end of the inductor flies above V
IN, discharging
its current through diode (D) into the output capacitor (C
OUT)
at a rate of (V
OUT −VIN)/L. Thus, energy stored in the induc-
tor during the switch on time is transferred to the output dur-
ing the switch off time. The output voltage is controlled by
adjusting the peak switch current, as described in the flyback
regulator section.
DS012515-48
By adding a small number of external components (as shown in
Figure 30), the LM2585 can be used to produce a regulated output voltage that is greater than
the applied input voltage. The switching waveforms observed during the operation of this circuit are shown in
Figure 31. Typical performance of this regulator is
shown in
Figure 32.
FIGURE 30. 12V Boost Regulator
DS012515-49
A: Switch Voltage, 10 V/div
B: Switch Current, 2 A/div
C: Inductor Current, 2 A/div
D: Output Ripple Voltage,
100 mV/div, AC-Coupled
Horizontal: 2 µs/div
FIGURE 31. Switching Waveforms
DS012515-50
FIGURE 32. V
OUT Response to Load Current Step
www.national.com
17


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