LP3954 Datasheet, PDF (9/60 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

LP3954
www.ti.com
SNVS340D – JUNE 2005 – REVISED MARCH 2013
Magnetic Boost DC/DC Converter
The LP3954 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 options for switching
frequency, 1MHz, 1.67MHz and 2MHz (default), when timing resistor RT is 82kohm. Timing resistor defines the
internal oscillator frequency and thus directly affects boost frequency and all circuit's internally generated timing
(RGB, Flash, WLED fading).
The LP3954 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. 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. Disabling will increase 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.
The following figure 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.
2 MHz clock
Duty control
V_IN
SW
V_OUT
FBNCCOMP
+
-
FB
R
S
OVPCOMP
+
-
-
+
R
R
RESETCOMP
+
-
R
SWITCH
ERRORAMP
+
-
ACTIVE
R
LOAD
SLOPER
+ OLPCOMP
-
LOOPC
Figure 2. Boost Converter Topology
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