LTC3524

10

3524f

OPERATION

has its input connected to V2x and output to VH. The regu-

lated VH voltage is programmed at FBH and can be set to

VH can source depends on charge pump loading and the

output impedance of the doubler and quadrupler stages

(see Typical Performance Characteristics).

output at VN. Regulated VN voltage is set at FBN and can

or VH depending on the negative voltage value required

for the application. Efficiency is improved by using the

pump outputs, the maximum negative voltage that VN can

maintain will depend on loading. Two Schottky diodes are

required to complete the negative charge pump as shown

on the front page and applications circuits.

LCD BIAS Sequencing

Referring to the following text and Figure 2, the LTC3524

power-up and discharge sequence is explained. When input

power is applied and ELCD is active, the boost converter

initializes and charges its output towards the final pro-

has reached approximately 90% of its final value, an internal

signal is asserted which allows the charge pump doubler

Approximately 2ms later, the charge pump inverter (VN)

begins operation toward its programmed value. When the

VN has reached approximately 50% of its final value, a

2ms (nominal) timeout period begins. At the conclusion

of the 2ms timeout period, the charge pump quadrupler

(VH) is allowed to begin operation.

During the initial power-up sequence, the charge pumps

Typical Applications) to ensure proper sequencing.

When ELCD is brought low, internal transistors discharge

the outputs in an orderly fashion. As shown in Figure 2,

VN and V2x are initially discharged, followed by VH, fol-

can enter low current shutdown mode (ELCD, ELED1,

ELED2 must be low, as well).

White LED Boost Driver

The white LED driver portion of the LTC3524 consists of

a nonsynchronous, fixed frequency, current mode boost

converter that generates the voltage required for one or

two LED strings. The converter has an internal feedback

loop and slope compensation circuitry, reducing external

components and simplifying the design. As with the LCD

bias boost converter, the 1.5MHz operation allows tiny

external components to be used. The boost converter

Figure 2. LCD Power-Up and Power-Down Timing Diagram

V2x

VN

VH

V2x

VH

TIME

ELCD

ELCD

VN

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