LP5523 datasheet(13/56 Pages) TI1

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REG

1.5X

1.5 x V

, ROUT

VIN

VOUT

LP5523

www.ti.com

SNVS550D – SEPTEMBER 2009 – REVISED MAY 2013

Charge Pump Operational Description

Overview

The LP5523 includes a pre-regulated switched-capacitor charge pump with a programmable voltage

multiplication of 1 and 1.5x. In 1.5x mode, by combining the principles of a switched-capacitor charge pump and

a linear regulator, a regulated 4.5V output is generated from the Li-Ion input voltage range. A two-phase non-

overlapping clock generated internally controls the operation of the charge pump. During the charge phase, both

flying capacitors (C1 and C2) are charged from input voltage. In the pump phase that follows, the flying

capacitors are discharged to output. A traditional switched-capacitor charge pump operating in this manner will

use switches with very low on-resistance, ideally 0

Ω, to generate an output voltage that is 1.5x the input voltage.

The LP5523 regulates the output voltage by controlling the resistance of the input-connected pass-transistor

switches in the charge pump.

Output Resistance

At lower input voltages, the charge pump output voltage may degrade due to effective output resistance (ROUT) of

the charge pump. The expected voltage drop can be calculated by using a simple model for the charge pump

illustrated in Figure 16 below.

Figure 16. Charge Pump Output Resistance Model

The model shows a linear pre-regulation block (REG), a voltage multiplier (1.5x), and an output resistance

(ROUT). Output resistance models the output voltage drop that is inherent to switched capacitor converters. The

output resistance is 3.5

Ω (typ.), and it is a function of switching frequency, input voltage, flying capacitors’

capacitance value, internal resistances of the switches and ESR of the flying capacitors. When the output voltage

is in regulation, the regulator in the model controls the voltage V’ to keep the output voltage equal to 4.5V (typ.).

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 4.5V. 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 1.5x 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 x VIN – IOUT x ROUT.

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 (typ.) resistor.

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

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

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

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 1.5x. 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 1x, when 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.

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