Application Information

CIRCUIT DESCRIPTION

The LM2759 is an adaptive CMOS charge pump with gains

of 1x, 1.5x, and 2x, optimized for driving Flash LEDs in cam-

era phones and other portable applications. It provides a

constant current of up to 1A (typ.) for Flash mode and 180 mA

(typ.) for Torch mode.

The LM2759 has selectable modes including Flash, Torch,

Indicator and Shutdown. Flash mode for the LM2759 can also

be enabled via the Strobe input pin. The LED is driven from

V

interface. The LM2759 adaptively selects the next highest

gain mode when needed to maintain the programmed LED

current level.

To prevent a high battery load condition during a simultane-

ous RF PA transmission and Flash event, LM2759 has a

Flash interrupt pin (TX) to reduce the LED current to the pro-

grammed Torch current level for the duration of the RF PA

transmission pulse.

CHARGE PUMP AND GAIN TRANSITIONS

The input to the 1x, 1.5x, 2x charge pump is connected to the

V

is connected to the V

voltage is less than 4.7V (typ.), the output voltage is approx-

imately equal to the input voltage. When the input voltage is

over 4.7V (typ.) the output voltage is regulated to 4.7V (typ.).

In 1.5x mode, the output voltage is regulated to 4.7V (typ.)

over entire input voltage range. For the gain of 2x, the output

voltage is regulated to 5.1V (typ.). When under load, the volt-

age at V

while the charge pump is still in closed loop operation. This is

due to the load regulation topology of the LM2759.

The charge pump’s gain is selected according to the head-

room voltage across the current sink of LM2759. When the

headroom voltage V

350 mV (typ.) the charge pump gain transitions to the next

available higher gain mode. Once the charge pump transi-

tions to a higher gain, it will remain at that gain for as long as

the device remains enabled. Shutting down and then re-en-

abling the device resets the gain mode to the minimum gain

required to maintain the load.

SOFT START

The LM2759 contains internal soft-start circuitry to limit inrush

currents when the part is enabled. Soft start is implemented

internally with a controlled turn-on of the internal voltage ref-

erence.

CURRENT LIMIT PROTECTION

The LM2759 charge pump contains current limit protection

circuitry that protects the device during V

where excessive current is drawn. Output current is limited to

1.4A typically.

LOGIC CONTROL PINS

LM2759 has two asynchronous logic pins, Strobe and TX.

These logic inputs function according to the table below:

TX STROBE

FUNCTION

0

0

Flash, Indicator)

1

0

and TX is logic High, the LED current will be

at the programmed Torch level.

0

1

Flash Mode (Total LED "ON" Duration limited

by Flash Timeout)

1

1

Torch Mode (Total LED "ON" Duration

limited by Flash Timeout)

START AND STOP CONDITIONS

START and STOP conditions classify the beginning and the

signal transitioning from HIGH to LOW while SCL line is

HIGH. A STOP condition is defined as the SDA transitioning

sidered to be busy after a START condition and free after a

can generate repeated START conditions. First START and

repeated START conditions are equivalent, function-wise.

30069311

FIGURE 1. Start and Stop Conditions

DATA VALIDITY

The data on SDA line must be stable during the HIGH period

of the clock signal (SCL). In other words, state of the data line

can only be changed when SCL is LOW.

30069325

FIGURE 2. Data Validity Diagram

A pull-up resistor between the controller's VIO line and SDA

must be greater than [ (VIO-V

requirement on SDA. Using a larger pull-up resistor results in

lower switching current with slower edges, while using a

smaller pull-up results in higher switching currents with faster

edges.

TRANSFERING DATA

Every byte put on the SDA line must be eight bits long, with

the most significant bit (MSB) transferred first. Each byte of

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