LP39542 Datasheet, PDF (8/58 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

Magnetic Boost DC/DC Converter
The LP39542 Boost DC/DC Converter generates a 4.0 – 5.3V
voltage for the LEDs from single Li-Ion battery (3V…4.5V).
The output voltage is controlled with an 8-bit register in 9
steps. The converter is a magnetic switching PWM mode DC/
DC converter with a current limit. The converter has three op-
tions for switching frequency, 1 MHz, 1.67 MHz and 2 MHz
(default), when timing resistor RT is 82 kΩ. Timing resistor
defines the internal oscillator frequency and thus directly af-
fects boost frequency and all circuit's internally generated
timing (RGB, Flash, WLED fading).
The LP39542 Boost Converter uses pulse-skipping elimina-
tion 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. At very light load and when
input and output voltages are very close to each other, the
pulse skipping is not completely eliminated. Output voltage
should be at least 0.5V higher than input voltage to avoid
pulse skipping. Reducing the switching frequency will also
reduce the required voltage difference.
Active load can be disabled with the en_autoload bit. Dis-
abling will increase the efficiency at light loads, but the down-
side is that pulse skipping will occur. The Boost Converter
should be stopped when there is no load to minimise the cur-
rent 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 feed-
back 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
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.
Boost Converter Topology
30008508
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