LTC3524_15 Datasheet, PDF (10/16 Pages) Linear Technology – Adjustable TFT Bias Supply with WLED Driver

LTC3524
OPERATION
The doubler is internally connected to V_OUT and generates
a voltage of approximately 2X V_OUT at V2x. The quadrupler
has its input connected to V2x and output to VH. The regu-
lated VH voltage is programmed at FBH and can be set to
produce a voltage up to 4X V_OUT. The maximum voltage
VH can source depends on charge pump loading and the
output impedance of the doubler and quadrupler stages
(see Typical Performance Characteristics).
The inverting charge pump has its input at VN_IN and
output at VN. Regulated VN voltage is set at FBN and can
be programmed to a minimum negative voltage of VN_IN
minus diode drops. VN_IN can be connected to V_OUT, V2x,
or VH depending on the negative voltage value required
for the application. Efficiency is improved by using the
lowest voltage possible on VN_IN. As with the other charge
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-
grammed value. When the boost converter output (V_OUT)
has reached approximately 90% of its final value, an internal
signal is asserted which allows the charge pump doubler
(V2x) to begin operation toward its final goal of 2X V_OUT.
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
run at half speed. If VN_IN is connected to VH, a diode-OR
circuit is needed between V2x, VH, and VN_IN (see the
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-
lowed by V_OUT. V_OUT must be discharged before the part
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
VH
VH
V2x
V_OUT
ELCD
V2x
ELCD
10
VN
VN
Figure 2. LCD Power-Up and Power-Down Timing Diagram
V_OUT
TIME
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