LP55231 datasheet(13/59 Pages) TI2

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LP55231

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SNOSCR5D – MARCH 2013 – REVISED DECEMBER 2016

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Feature Description (continued)

With increased output current, the voltage drop across ROUT increases. To prevent drop in output voltage, the

voltage drop across the regulator is reduced, V’ increases, and VOUT remains at 4.5 V. When the output current

increases to the point that there is zero voltage drop across the regulator, V’ equals the input voltage, and the

output voltage is on the edge of regulation. Additional output current causes the output voltage to fall out of

regulation, so that the operation is similar to a basic open-loop 1.5× charge pump. In this mode, output current

results in output voltage drop proportional to the output resistance of the charge pump. The out-of-regulation

output voltage can be approximated by: VOUT = 1.5 × VIN – IOUT × ROUT.

7.3.5.3 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 output voltage is pulled down with an internal 300-k

Ω (typical) resistor.

The charge pump can be forced to bypass mode, so the battery voltage is connected directly to the current

sources; in 1.5× mode output voltage is boosted to 4.5 V. In automatic mode, charge-pump operation mode is

determined by saturation of constant current drivers, as described in LED Forward Voltage Monitoring.

7.3.5.4 LED Forward Voltage Monitoring

When the charge-pump automatic mode selection is enabled, voltages over LED drivers D1 to D6 are monitored.

NOTE

Power input for current source outputs D7, D8, and D9 are internally connected to the

VDD pin.

If the D1 to D6 drivers do not have enough headroom, charge-pump gain is set to 1.5×. Driver saturation monitor

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

pump gain is set to 1×, when battery voltage is high enough to supply all LEDs.

In automatic gain change mode, the charge pump is switched to bypass mode (1×), when LEDs are inactive for

over 50 ms.

7.3.5.5 Gain Change Hysteresis

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 could occur due to LED driver) and charge-pump

voltage drop in 1× mode. The hysteresis of the gain change is user-configurable; default setting is factory-

programmable. Flexible configuration ensures that 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 15.

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