LP3958 Datasheet, PDF (8/36 Pages) National Semiconductor (TI) – Lighting Management Unit with High Voltage Boost Converter

LP3958
SNVS423C – JANUARY 2006 – REVISED MARCH 2013
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Power-Up Sequence
When powering up the device, V_DD1 and V_DD2 should be greater than V_DDIO to prevent any damage to the device.
VDD
VDD1,2
0V
VDDIO
VDDIO
0V
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 82kΩ. Timing resistor defines the internal oscillator frequency and thus directly affects boost frequency and
KEY timings.
EMI filter (R_SW and C_SW) 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.
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