AS3661 datasheet(19/87 Pages) AMSCO

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AS3661

Datasheet - D e t a i l e d D e s c r i p t i o n

8.6.3 Controlling the Charge Pump

The charge pump is controlled with two CP_MODE bits in MISC register (address 36H). When both of the bits are low,

the charge pump is disabled and the output voltage is pulled down with an internal 300 k

Ω (typ.) resistor. The charge

pump can be forced to bypass mode, so that the battery voltage is connected directly to the current sources. In 1.5x

mode the output voltage is boosted to 4.5V. In automatic mode the charge pump operation mode is determined by

saturation of constant current drivers, like described in chapter LED Forward Voltage Monitoring.

8.6.4 LED Forward Voltage Monitoring

When the charge pump automatic mode selection is enabled, the voltages over the LED drivers LED1 to LED6 are

monitored.

Note: Power input for current source outputs LED7, LED8 and LED9 are internally connected to the VBAT pin.

If the LED1 to LED6 drivers do not have enough headroom, the charge pump gain is set to 1.5x. Driver saturation

monitor does not have a fixed voltage limit, since saturation voltage is a function of temperature and current. The

charge pump gain is set to 1x, when the battery voltage is high enough to supply all LEDs. In automatic gain change

mode, the charge pump is switched to bypass mode (1x), when LEDs are inactive for over 50 ms.

8.6.5 Gain Change Hysteresis

The charge pump gain control utilizes digital filtering to prevent supply voltage disturbances (for example, the transient

voltage on the power supply during the GSM burst) from triggering unnecessary gain changes. Hysteresis is provided

to prevent periodic gain changes, which would occur due to LED driver and charge pump voltage drop in 1x mode. The

hysteresis of the gain change is user configurable, default setting is factory programmable. Flexible configuration

ensures, that the hysteresis can be minimized or set to desired level in each application. LED forward voltage

monitoring and gain control block diagram is shown in Figure 21.

Figure 21. Forward Voltage Monitoring and Gain Control Block

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