LP3950 Datasheet, PDF (13/31 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

Magnetic Boost DC/DC Converter
The boost DC/DC converter generates a 4.1V–5.3V output voltage to drive LEDs from a single Li-Ion battery (3.0V to 4.5V). The
output voltage is controlled with an eight-bit register in nine steps. The converter is a magnetic switching PWM mode DC/DC
converter with a current limit. The converter has three options for switching frequency, 1.0 MHz, 1.67 MHz and 2.0 MHz (default),
when the timing resistor RT is 82 kΩ.
The LP3950 boost converter uses an unique pulse-skipping elimination method to stabilize the noise spectrum. Even with light
load or no load a minimum length current pulse is fed to the inductor. An internal active load is used to remove the excess charge
from the output capacitor when needed (Note 16). The boost converter should be disabled when there is no load to avoid idle
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 output voltage control changes the resistor divider in the feedback loop.
Figure 11 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 the 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 (FB) protection for no connection
4. Duty cycle limit function, done with digital control
Note 16: When the battery voltage is close to the output voltage, the output voltage may rise slightly over programmed value if the load on output is small and
pulse-skipping elimination is active.
20129315
FIGURE 11. Boost Converter Functional Block Diagram
Magnetic Boost DC/DC Converter Electrical Characteristics
Limits in standard typeface are for T_J = +25˚C. Limits in boldface type apply over the operating ambient temperature range
(−40˚C ≤ T_A ≤ +85˚C). Unless otherwise noted, specifications apply to Figure 1 with: V_DD1 = V_DD2 = V_DDA = 2.8V, C_VDD1 =
C_VDD2 = C_VDDA = C_VDDIO = 100 nF, C_OUT = C_IN = 10 µF, C_VREF = 100 nF, L₁ = 4.7 µH and f_BOOST = 2.0 MHz (Note 12).
Symbol
Parameter
Conditions
Min
Typ
Max Units
I_LOAD
V_OUT
Load Current
Output Voltage Accuracy
(FB Pin)
Output Voltage
(FB Pin)
3.0V ≤ V_IN ≤ 4.5V
0
V_OUT = 5.0V
1.0 mA ≤ I_LOAD ≤ 300 mA
3.0V ≤ V_IN ≤ 4.5V
−5
V_OUT = 5.0V (target value), autoload OFF
1.0 mA ≤ I_LOAD ≤ 300 mA
3.0V < V_IN < 5.0V + V_(SCHOTTKY),
autoload OFF
300 mA
+5
%
5.0
V
1.0 mA ≤ I_LOAD ≤ 300 mA
> V_IN 5V + V_(SCHOTTKY)
V_IN– V_(SCHOTTKY)
V
13
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