Magnetic Boost DC/DC Converter

The boost DC/DC converter generates a 4.1V–5.3V output voltage to drive LEDs from a single Li-Ion battery (3.0V to 4.5V). The

output voltage is controlled with an eight-bit register in nine steps. The converter is a magnetic switching PWM mode DC/DC

converter with a current limit. The converter has three options for switching frequency, 1.0 MHz, 1.67 MHz and 2.0 MHz (default),

when the timing resistor RT is 82 k

Ω.

The LP3950 boost converter uses an unique pulse-skipping elimination 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 excess charge

from the output capacitor when needed (Note 16). The boost converter should be disabled when there is no load to avoid idle

current consumption.

The topology of the magnetic boost converter is called CPM control, current programmed mode, where the inductor current is

measured and controlled with the feedback. The output voltage control changes the resistor divider in the feedback loop.

Figure 11 shows the boost topology with the protection circuitry. Four different protection schemes are implemented:

1.

Over voltage protection, limits the maximum output voltage

— Keeps the output below breakdown voltage

— Prevents boost operation if the 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 limit function, done with digital control

Note 16: When the battery voltage is close to the output voltage, the output voltage may rise slightly over programmed value if the load on output is small and

pulse-skipping elimination is active.

Magnetic Boost DC/DC Converter Electrical Characteristics

Limits in standard typeface are for T

(−40˚C

≤ T

A

≤ +85˚C). Unless otherwise noted, specifications apply to Figure 1 with: V

C

Symbol

Parameter

Conditions

Min

Typ

Max

Units

I

LOAD

Load Current

3.0V

≤ V

IN

≤ 4.5V

V

0

300

mA

V

OUT

Output Voltage Accuracy

(FB Pin)

1.0 mA

≤ I

LOAD

≤ 300 mA

3.0V

≤ V

IN

≤ 4.5V

V

−5

+5

%

Output Voltage

(FB Pin)

1.0 mA

≤ I

LOAD

≤ 300 mA

autoload OFF

5.0

V

1.0 mA

≤ I

LOAD

≤ 300 mA

V

V

V

20129315

FIGURE 11. Boost Converter Functional Block Diagram

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