LP3958 datasheet(8/28 Pages) NSC

8 / 28 page

Power-Up Sequence

When powering up the device, V

DD1 and VDD2 should be

greater than V

DDIO to prevent any damage to the device.

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Magnetic Boost DC/DC Converter

The LP3958 Boost DC/DC Converter generates an 8…18V

supply voltage for the LEDs from single Li-Ion battery

(3V…4.5V). The output voltage is controlled with an 8-bit

PWM mode DC/DC converter with a current limit. Switching

frequency is 1MHz, when timing resistor RT is 82k

Ω. Timing

resistor defines the internal oscillator frequency and thus

directly affects boost frequency and KEY timings.

EMI filter (R

SW and CSW) on the SW pin can be used to

suppress EMI caused by fast switching. These components

should be as near as possible to the SW pin to ensure

reliable operation. The LP3958 Boost Converter uses pulse-

skipping elimination to stabilize the noise spectrum. Even

with light load or no load a minimum length current pulse is

fed to the inductor. An active load is used to remove the

excess charge from the output capacitor at very light loads.

Active load can be disabled with the EN_AUTOLOAD bit.

Disabling active load will increase slightly the efficiency at

light loads, but the downside is that pulse skipping will occur.

The Boost Converter should be stopped when there is no

load to minimise the current consumption.

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 user

can program the output voltage of the boost converter. The

output voltage control changes the resistor divider in the

feedback loop. The following figure shows the boost topol-

ogy with the protection circuitry. Four different protection

schemes are implemented:

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.

Over current protection, limits the maximum inductor

current

— Voltage over switching NMOS is monitored; too high

voltages turn the switch off.

Feedback break protection. Prevents uncontrolled op-

eration if FB pin gets disconnected.

Duty cycle limiting, done with digital control.

20175577

Boost Converter Topology

LP3958 Datasheet(PDF) 8 Page - National Semiconductor (TI)

LP3958 Datasheet(HTML) 8 Page - National Semiconductor (TI)

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