LP5521 datasheet(12/51 Pages) TI2

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Charge Pump

REG

1.5 x

9¶

1.5

[ 9¶

ROUT

VOUT

VIN

LP5521

SNVS441I – JANUARY 2007 – REVISED NOVEMBER 2016

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7.3 Feature Description

7.3.1 Charge Pump Operational Description

The LP5521 includes a pre-regulated switched-capacitor charge pump with a programmable voltage

multiplication of 1 and 1.5×.

In 1.5× mode by combining the principles of a switched-capacitor charge pump and a linear regulator, the device

generates a regulated 4.5-V output from 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

(CFLY1 and CFLY2) 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 uses switches with

very low on-resistance, ideally 0

Ω, to generate an output voltage that is 1.5× the input voltage. The LP5521

regulates the output voltage by controlling the resistance of the input-connected pass-transistor switches in the

charge pump.

7.3.1.1 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

shown in Figure 14.

Figure 14. Charge Pump Block Diagram

The model shows a linear pre-regulation block (REG), a voltage multiplier (1.5×), 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 (typical) and is function of switching frequency, input voltage, capacitance value of the

flying capacitors, internal resistances of switches, and ESR of 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 4.5 V (typical).

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.

7.3.1.2 Controlling Charge Pump

The charge pump is controlled with two CP_MODE bits in register 08H. When both bits are low, the charge pump

is disabled, and the output voltage is pulled down with 300 k

Ω. Charge pump can be forced to bypass mode, so

that battery voltage is going directly to RGB outputs. In 1.5× mode output voltage is boosted to 4.5 V. In

automatic mode, charge pump operation mode is defined by LED outputs saturation described in LED Forward

Voltage Monitoring. Table 1 lists operation modes and selection bits.

Table 1. CONFIG Register (08H)

NAME

BIT

DESCRIPTION

CP_MODE

4:3

Charge pump operation mode

00b = OFF

01b = Forced to bypass mode (1×)

10b = Forced to 1.5× mode

11b = Automatic mode selection

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