Magnetic Boost DC/DC Converter

The LP55271 boost DC/DC converter generates a 4.00 –

5.40V output voltage to drive the LEDs from a single Li-Ion

battery (3.0V to 4.5V). The output voltage is controlled with a

4-bit register in 8 steps. The converter is a magnetic switch-

ing PWM mode DC/DC converter with a current limit. The

converter has 2.0 MHz / 1.0 MHz selectable switching fre-

quency operation, when the timing resistor RT is 82 k

Ω.

The LP55271 boost converter uses pulse-skipping elimina-

tion method to stabilize the noise spectrum. Even with light

load or no load a minimum length current pulse is fed to the

inductor. An internal active load is used to remove the ex-

cess charge from the output capacitor when needed.

The topology of the magnetic boost converter is called CPM

control, current programmed mode, where the inductor cur-

rent is measured and controlled with the feedback. The

output voltage control changes the resistor divider in the

feedback loop.

The following figure shows the boost topology with the pro-

tection circuitry. Four different protection schemes are imple-

mented:

1.

Over voltage protection, limits the maximum output volt-

age.

— Keeps the output below breakdown voltage.

— Prevents boost operation if battery voltage is much

higher than desired output.

2.

Over current protection, limits the maximum inductor

current.

— Voltage over switching NMOS is monitored; too high

voltages turn the switch off.

3.

Feedback (FB) protection for no connection.

4.

Duty cycle limiting, done with digital control.

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Boost Converter Topology

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