LP3933 Datasheet, PDF (8/18 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and Two RGB or FLASH LEDs

SPI Interface (Continued)
SPI Timing Diagram
20080513
SPI Timing Parameters
V_DD1,2 = V_{DD_IO} = 2.775V
Symbol
Parameter
1
Cycle Time
2
Enable Lead Time
3
Enable Lag Time
4
Clock Low Time
5
Clock High Time
6
Data Setup Time
7
Data Hold Time
8
Data Access Time
9
Output Disable Time
10 Output Data Valid
11 Output Data Hold Time
12 SS Inactive Time
Note: Data guaranteed by design.
Limit
Min
Max
70
35
35
35
35
0
20
20
10
20
0
10
Units
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
Magnetic Boost DC/DC Converter
The LP3933 boost DC/DC Converter generates 4.1V–5.3V supply voltage for the LEDs from single Li-Ion battery (3V…4.5V). The
output voltage is controlled with 8-bit register in 9 steps. The converter is a magnetic switching PWM mode DC/DC converter with
a current limit. The converter has 1 MHz switching frequency when timing resistor RT is 82 KΩ.
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 control changes
the resistor divider in the feedback loop.
The following figure shows the boost topology with the protection circuitry. Three 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. Duty cycle limiting, done with digital control.
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