LP8501 datasheet(16/51 Pages) TI1

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REG

1.5X

1.5 x V

, ROUT

VIN

VOUT

LP8501

SNVS548D – SEPTEMBER 2008 – REVISED AUGUST 2013

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Charge Pump Operational Description

Overview

The LP8501 includes a pre-regulated switched-capacitor charge pump with a programmable voltage

multiplications of 1 and 1.5x. The 1.5x mode combines the principles of a switched-capacitor charge pump and a

linear regulator, generating a regulated 4.5V 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 (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 LP8501

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 18 below.

Figure 18. 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 a 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 c harge pump is controlled with two CP_MODE bits in CONFIG register (address 36h). When both of the bits

are low, 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 that 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

defined by current source outputs saturation as described in the next section.

LED Forward Voltage Monitoring

When the charge-pump automatic mode selection is enabled, voltages over LED drivers connected to charge

pump are monitored. If the 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. Charge pump gain is set to 1x when battery voltage is high enough to supply all LEDs. Note that in order

to get the automatic gain change work correctly, power config bits in register 05H must be set according to where

the VDD1_6 and VDD7_9 are connected.

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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