20080513

SPI Timing Diagram

SPI Timing Parameters

V

Symbol

Parameter

Limit

Units

Min

Max

1

Cycle Time

70

ns

2

Enable Lead Time

35

ns

3

Enable Lag Time

35

ns

4

Clock Low Time

35

ns

5

Clock High Time

35

ns

6

Data Setup Time

0

ns

7

Data Hold Time

20

ns

8

Data Access Time

20

ns

9

Output Disable Time

10

ns

10

Output Data Valid

20

ns

11

Output Data Hold Time

0

ns

12

SS Inactive Time

10

ns

Note: Data guaranteed by design.

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 K

Ω.

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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