SP7683 datasheet(5/8 Pages) SIPEX

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Date: 12/21/04

SP7683 Four Channel Charge Pump LED Driver

The SP7683 is comprised of two stages. First is a regulated change pump to convert the input

voltage V

IN into a stable output voltage or current depending on the mode used. The second stage

is a low dropout current matching regulator that provides stable identical currents in all four

channels to drive LED1 - LED4. The first stage can drive an additional four LEDs that are

connected in parallel to the V

OUT pin. It is recommended to use resistors in series with these

additional diodes for current matching.

The SP7683 provides dimming control for the LEDs, with a constant ratio of brightness between

them. It can also be used to turn off the LEDs completely.

The Charge Pump

The regulated charge pump can operate in two modes 1X V

IN and 1.5X VIN to optimize efficiency

over the entire battery voltage range. The input voltage trip point at which the charge pump will

automatically switch from a 1X mode to 1.5X mode is defined by an external voltage divider

connected to V

MODE pin. Cycle-by-cycle regulation ensures that no mode change occurs during

a switching cycle. To achieve better efficiency it is recommended that the V

IN_TRIP is set at

IN_TRIP = VF + 0.31 + MILED *ROUT where VF is the LED forward voltage and (M) is the number of LEDs

connected to V

OUT . ROUT is the output resistance. ILED is the current per LED.

The charge pump may be configured in voltage mode or as a current source. When used as a

voltage source the output voltage is determined by an external resistive divider connected to FB

pin as: V

OUT = (1+R10/R5)*VFB (see Figure 2) where VFB = 0.31V.

When used as a current source the current in one single LED (D5 - D8) may be set by series

resistor R

S as ILED = VFB / RS. The current in the remaining LEDs will track based on the ballast

resistors. Three-wire LED modules with internal series resistors can also be driven in this way.

The flying capacitors C1 an C2 of the charge pump control the tade-off between the output voltage

ripple and the output current capability. Decreasing the flying capacitors will reduce the output

voltage ripple because less charge will be delivered to the output capacitor. However, smaller

flying capacitors lead to larger output resistance, thus decreasing the output current handling

capability and circuit efficiency. We recommend using ceramic capacitors in the range of 1µF -

4.7µF with low ESR, which helps reduce peak-to-peak output ripple and reduce high frequency

noise spikes.

Dimming control can be achieved by applying a PWM control signal to the EN/PWM pin. The

brightness of the LEDs is controlled by varying the duty cycle of the PWM signal. PWM repetition

rate from 60Hz - 700Hz. A repetition rate of at least 60Hz is required to prevent visible flickering.

Applying a logic low signal to EN/PWM control input turns of the device resulting in supply current

below 1µA and high impedance state of the V

OUT pin.

CIRCUIT DESCRIPTION

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