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POWER MANAGEMENT

SC606

Detailed Description

The SC606 contains a fractional charge pump, mode selection

circuit, serial I/O logic, serial data registers and current regulation

circuitry for 6 LED outputs. All are depicted in the Block Diagram

on page 6.

The fractional charge pump multiplies the input voltage by 1, 1.5

or 2 times the input voltage. The charge pump switches at a fixed

frequency that is bit selectable to 1.33MHz or 250kHz. The default

frequency is 1.33MHz. 250kHz may require additional input

capacitance of up to 10μF. The charge pump does not switch

during 1x mode, saving power and improving efficiency.

The mode selection circuit automatically selects the mode as 1x,

1.5x or 2x based on circuit conditions such as LED voltage, input

voltage and load current. 1x is the most efficient mode, followed

by 1.5x and 2x modes. At lower voltages a stronger mode may be

needed to maintain regulation. If so , the mode will change first to

1.5x and then later to 2x. 2x mode usually operates for a much

shorter run time compared to 1x mode, and 2x mode maintains

the output until the battery is discharged to 2.85V or less. The LED

requiring the highest voltage drop will determine the output voltage

needed to drive all outputs with sufficient anode voltage.

Comparing all cathodes and regulating VOUT for the LED with the

lowest cathode voltage ensures sufficient bias for all LEDs.

The LED outputs are controlled through the serial data registers,

found on page 10 in Table 1. LED on/off functions are

independently controlled, so that any combination of LEDs may be

switched on. LED current is set per LED pair. For example, when

ILEDA1 and ILEDA2 are both on, ILEDA1 = ILEDA2. But ILEDA1 and

ILEDA2 do not have to be on at the same time.

Six (6) current regulating circuits sink matched currents from the

LEDs on a per pair basis. LEDs with matched forward voltage will

produce the best possible matched currents. For best matching

performance it is recommended that the LED to LED difference,

ΔV

Designing for Lowest Possible Battery Current

The battery current and efficiency of the SC606 are mostly

dependent on the charge pump mode of operation. To get the

best performance from the SC606 it is better to use LEDs with

consistantly lower V

in 1x mode longer and will use less battery current, extending the

run time of the battery.

Mode transition voltages V

by the following equations:

Applications Information

V

I

V

I

where, V

at the ILED pin. Typically V

Power efficiency can be estimated for the intended battery voltage

range.

where, VOUT = V

and

I

I

Quiescent Current

I

now described which will demonstrate the effect that the device

state has on I

(1) After power up (Enable High) and before any serial

communication, the I

the output defaults to 5V and the charge pump is active to support

the 5V. The charge pump also defaults to 1.33MHz.

(2) After power up if the clock is changed to 250kHz, the I

decrease to 1mA.

(3) If one LED is set on at 0.5mA, the input current will be 1.5mA.

For a typical white LED, the 5V output is much higher than

necessary, so the charge pump will switch off and the output will

adjust to a normal value for the LED, typically around 3.5V.

(4) Turn off the LED and I

the lowest power state while Enable = High.

I

I

The tolerance of the I

For example, if the data registers are configured so that each LED

current should be 10mA, the actual LED currents would be between

9.35mA and 10.65mA (+/-6.5%). All 6 outputs meet this

requirement over the industrial temperature range. To calculate

the error I

I

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