VDD1,2

VDDIO

0V

0V

VDD

VDDIO

LP3958

SNVS423C – JANUARY 2006 – REVISED MARCH 2013

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Power-Up Sequence

When powering up the device, VDD1 and VDD2 should be greater than VDDIO to prevent any damage to the device.

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 register in 10 steps. The converter is a magnetic

switching 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 (RSW 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 current 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. Figure 3 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 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 break protection. Prevents uncontrolled operation if FB pin gets disconnected.

4. Duty cycle limiting, done with digital control.

8

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