Simplified Switching Circuit

Product Description

OVERVIEW

The LM2707 is a magnetic switch-mode boost converter that

has been designed specifically for driving white LEDs. The

device is an asynchronous boost regulator that uses a low-

resistance internal NFET power transistor and an external

rectifier diode. An internal high-power gate driver quickly

turns the power switch ON and OFF.

Operation of the LM2707 can be best understood by refer-

ring to the block diagram of Figure 1, the simplified switching

circuit in Figure 2, and the switching waveforms in Figure 3.

The part regulates the feedback voltage with pulse-

frequency-modulated (PFM) control. The key blocks in this

control circuit are the R-S latch, the oscillator, and the feed-

back error amplifier. Several sense-and-control circuit

blocks, including the oscillator and the error amplifier, are

inputs to the R-S latch. The output of the R-S latch is the

control signal for the power transistor gate driver. The power

transistor turns ON and OFF at a frequency and duty cycle

that is generated by the oscillator. The oscillator frequency

can be programmed with an external capacitor (C

part switches continuously until one of the LM2707 sense

circuits asserts a reset signal on the R-S latch.

The error amplifier resets the R-S latch when the output

feedback voltage is above the 515mV (typ.) reference volt-

age. The part will idle in a low-power state until the feedback

voltage falls below the reference voltage. At this point, the

oscillator signal again becomes the output signal of the R-S

latch, and switching resumes.

In addition to the feedback circuit, a few other internal pro-

tection and control circuits stop switching by overriding the

oscillator signal at the R-S latch. A current limit circuit brings

switching to a halt when current through the power switch

becomes excessive. Similar interrupts in switching are trig-

gered by an over-voltage protection circuit on the output and

an under-voltage lockout circuit on the input. An external

shutdown signal can also be applied to place the LM2707 in

a low-power shutdown mode.

PROGRAMMABLE OSCILLATOR

The LM2707 contains an oscillator with an internally fixed

duty cycle. The frequency of the oscillator is programmed

externally with capacitor C

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FIGURE 2. LM2707 Simplified Switching Circuit

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FIGURE 3. C

Operation

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