LTC3524 - Adjustable TFT Bias Supply with WLED Driver

LTC3524
Adjustable TFT Bias Supply
with WLED Driver
DESCRIPTION
FEATURES
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Generates Three Adjustable, Low Noise Rails for
Small/Medium TFT Displays
Drives Up to Ten White LEDs
LED Dimming and Open-Circuit Protection
Controlled Power-Up/Power-Down Sequencing
1.5MHz Fixed Frequency, Low Noise Operation
VIN Range 2.5V to 6V, VOUT Range 3V to 6V
TFT Supply Efficiency Up to 90%
LED Supply Efficiency Up to 78%
Two Independantly Enabled LED Strings
200 to 1 True Color PWMTM Dimming
Tiny External Solution
24-Lead QFN Package (4mm × 4mm × 0.75mm)
The LTC®3524 is an integrated BIAS and white LED power
converter solution for small/medium-sized polysilicon thin
film transistor (TFT) liquid crystal (LCD) display panels.
The device operates from a single Lithium-Ion/polymer
battery or any voltage source between 2.5V and 6V.
A 1.5MHz synchronous boost converter generates a programmable low noise, high efficiency 25mA TFT supply
of up to 6.0V. Regulated, low ripple charge pumps are
used to generate up to +20V and –20V at 2mA. Output
sequencing is internally controlled to insure proper
initialization and rapid discharge of the LCD panel in
shutdown.
A second 1.5MHz boost converter powers one or two LED
strings with up to five series elements each. LED current
and display brightness can be controlled over a wide range
using analog or digital means up to 25mA.
APPLICATIONS
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PDAs, Palmtop Computers
Digital Still and Video Cameras
Handheld GPS
Portable Instrument Displays
Portable Media Players
The LTC3524 is offered in the 4mm × 4mm 24-pin QFN
package, minimizing the total solution footprint.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
LCD Bias and LED Efficiency
VIN = 3.6V, VOUT = 5V, 8 LEDs
+5V, –7.5V, +12.5V, 8 LED Power Supply
100
10μH
2.2μF
Li-Ion
SW1
+5V
25mA
VIN
324k
LED2
FBVO
VNIN
V2x
C2+
0.47μF
0.1μF
+12.5V
2mA
220k
90
1M
+10V
0.47μF
10μF
SW2
VLED
VOUT
10μF
LED1
PROG
ELED2
ELED1
ELCD
LTC3524
C2–
VH
470k
VN
CH– GND CN+
0.1μF
80
LED
VIN
VOUT
1M
FBH
LCD
70
100k for 20mA
FBN
2M
CH+
VIN = 3.6V
3.3μH
EFFICIENCY (%)
+
–
–7.5V
2mA
60
5
15
20
10
VOUT OR LED STRING CURRENT (mA)
25
3524 TA01b
0.47μF
0.1μF
3524 TA01a
3524f
1
LTC3524
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Referred to GND)
LED1
VLED
SW2
ELED2
ELED1
PROG
TOP VIEW
VIN, SW1, VOUT, C2–....................................... –0.3 to 7V
ELCD, ELED1, ELED2, PROG ......................... –0.3 to 7V
FBN, FBH, FBVO ............................................. –0.3 to 7V
V2x, C2+, CH– .............................................. –0.3 to 13V
LED1, LED2, VLED, SW2 ............................. –0.3 to 22V
VNIN, VH, CH+, CN+ ...................................... –0.3 to 21V
VN .............................................................. –21 to +0.3V
Operating Temperature Range (Note 2) ...–40°C to 85°C
Storage Temperature Range...................–65°C to 125°C
24 23 22 21 20 19
ELCD 1
18 LED2
17 CH–
VIN 2
16 CH+
FBVO 3
25
VOUT 4
15 VH
VN
9 10 11 12
NC
8
CN+
7
VNIN
13 FBN
C2+
14 FBH
C2– 6
V2x
SW1 5
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W
EXPOSED PAD (PIN 25) MUST BE SOLDERED TO PCB
AND CONNECTED TO GND
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3524EUF#PBF
LTC3524EUF#TRPBF
3524
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3524EUF
LTC3524EUF#TR
3524
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V, VOUT = 5.1V, TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
●
Input Voltage Range
TYP
2.5
MAX
6.0
UNITS
V
VIN Quiescent Supply Current LCD
ELCD = 1.5V, ELED1,2 = GND
200
μA
VIN Quiescent Supply Current LED
ELCD = GND, ELED1,2 = 1.5V (LED1 and LED2 Open)
VOUT Quiescent Supply Current LCD
ELCD = 1.5V, ELED1,2 = GND
4
mA
250
μA
VIN Quiescent Current Shutdown
ELCD = ELED1,2 = GND
.02
2
μA
Switching Frequency
LED and LCD Boosts
1
1.5
2
MHz
Maximum Duty Cycle
LED and LCD Boosts
85
94
1.20
1.225
%
VOUT Boost Regulator
●
FBVO Regulation Voltage
VOUT Adjust Range
See Note 3
3.0
1.25
V
6.0
V
3524f
2
LTC3524
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V, VOUT = 5.1V, TA = 25°C, unless otherwise noted.
PARAMETER
CONDITIONS
Switch Current Limit
MIN
TYP
100
150
MAX
UNITS
mA
Charge Pumps
V2x Output Voltage
Load on V2x = 250μA
10
V
Output Impedance V2x
Flying Capacitors = 0.1μF
250
Ω
V2x Maximum Operating Voltage
(Note 3)
12
V
VH Output Voltage (Quadrupler)
Load = 250μA (FBH = 1V)
20
V
Output Impedance (2X + Quadrupler)
Flying Capacitors = 0.1μF
1200
Ω
●
FBH Regulation Voltage
VH Maximum Operating Voltage
(Note 3)
VN Output Voltage
Load on VN = 250μA, VNIN = 10.2V, External Schottkys
1.225
1.30
V
20
–9.7
●
FBN Regulation Voltage
1.15
0.94
1
V
1.06
V
Ω
Output Impedance VN (2X + VN)
Flying Capacitor = 0.1μF
650
VN Minimum Operating Voltage
(Note 3)
–20
V
94
KHz
Switching Frequency Charge Pumps
V2x to VN Delay
(Note 4)
2
ms
VN to VH Delay
(Note 4)
2
ms
LED Boost
LED1,2 Current Accuracy
RPROG = 100k
SW2 Maximum Current Limit
SW2 VCESAT
18
20
500
700
mA
350
mV
ISW = 350mA
22
mA
Logic Inputs
ELED1, ELED2 , ELCD Thresholds
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3524E is guaranteed to meet specifications from 0°C to
85°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization, and statistical process controls.
●
0.4
0.8
1.2
V
Note 3: Specification is guaranteed by design and not 100% tested in
production.
Note 4: Measured from point at which VN crosses –VOUT to point at which
CH+ starts switching.
3524f
3
LTC3524
TYPICAL PERFORMANCE CHARACTERISTICS
LCD Boost Efficiency
vs Load Current
100
85
85
L = 10μH
VOUT = 5V
L = 4.7μH
L = 4.7μH
80
80
70
VIN = 5
VIN = 4.2
VIN = 3.6
VIN = 3.1
VIN = 2.5
50
0
40
30
20
50
VOUT CURRENT (mA)
10
60
75
PER STRING:
5 LEDs
4 LEDs
3 LEDs
2 LEDs
70
65
2.5
70
4
3.5
VIN (V)
4.5
21.5
21.5
21.0
21.0
CURRENT (mA)
22.0
20.5
20.0
19.5
PER STRING:
5 LEDs
4 LEDs
3 LEDs
2 LEDs
3
3.5
4
10
15
20
LED CURRENT (mA)
3524 G03
V2X Output Voltage
vs V2X Load Current
20.5
20.0
19.5
PER STRING:
5 LEDs
4 LEDs
3 LEDs
2 LEDs
18.5
18.0
2.5
5
3
3.5
VIN (V)
4
4.5
9.5
9.0
8.5
5
0
VIN (V)
3524 G04
1
2
4
3
V2X LOAD CURRENT (mA)
3524 G05
VH Voltage vs VH and
VN Load Current (FBH = 0V)
5
3524 G06
VOUT, |VN|, and VH/2
Regulation Overtemperature
VN Voltage vs VN and
VH Load Current (FBN = 1.3V)
20
25
10.0
19.0
4.5
VIN = 5
VIN = 4.2
VIN = 3.6
VIN = 3.1
VIN = 2.5
5
5
LED2 String Current
vs VIN and Number of LEDs
22.0
18.0
2.5
65
3524 G02
LED1 String Current vs VIN
and Number of LEDs
18.5
70
55
3
3524 G01
19.0
75
60
V2X VOLTAGE (V)
60
EFFICIENCY (%)
80
EFFICIENCY (%)
EFFICIENCY (%)
4 LEDs per String Efficiency
vs VIN and LED Current
LED Efficiency vs VIN
90
CURRENT (mA)
TA = 25°C, unless otherwise noted.
8.0
10.0
|VN|
VN = 0mA
9.0
8.5
VOLTAGE (V)
VN = 2mA
18
7.0
VH = 1mA
VN = 1mA
–VN VOLTAGE (V)
VH VOLTAGE (V)
19
7.5
VH = 0mA
9.5
VH = 2mA
VH/2
6.0
5.5
8.0
17
VOUT
5.0
7.5
16
6.5
0
0.5
1.5
1
VH LOAD CURRENT (mA)
2
3524 G07
4.5
7.0
0
0.5
1
1.5
VN LOAD CURRENT (mA)
2
3524 G09
4.0
–40
–15
10
35
TEMPERATURE (°C)
60
85
3524 G10
3524f
4
LTC3524
TYPICAL PERFORMANCE CHARACTERISTICS
LCD Bias Sequencing
TA = 25°C, unless otherwise noted.
SW1 Voltage and 10μH Inductor
Current at 25mA Load
LCD Bias Sequencing
VH
VH
SW1
2V/DIV
VOUT
5V/DIV
V2X
5V/DIV
VOUT
VN
VN
ILCD BOOST INDUCTOR CURRENT
200mA/DIV
3524 G11
5ms/DIV
3524 G12
5ms/DIV
SW1 Voltage and 10μH Inductor
Current at 5mA Load
ILCD BOOST
INDUCTOR
CURRENT
50mA/DIV
SW2 Voltage and 4.7μH Inductor
Current at 20mA
LED Initial Start-Up Waveforms
ILED BOOST
INDUCTOR
CURRENT
SW2
200mV/DIV
SW1
2V/DIV
ILCD BOOST
INDUCTOR
CURRENT
50mA/DIV
ILCD BOOST
INDUCTOR
CURRENT
200mA/DIV
500mV/DIV
SW2
5V/DIV
5V/DIV
VLED
3524 G15
200ns/DIV
LED Burst Dimming Waveforms
LED1 and SW2
ELED1 AND
ELED2
LED1
200mA/DIV
12V
10V/DIV
10V/DIV
SW2
500μs/DIV
3524 G17
50μs/DIV
3524 G16
LED Burst Dimming Waveforms
LED2 and VLED
5V/DIV
ILED BOOST
INDUCTOR
LED1
LED2
3524 G14
200ns/DIV
3524 G13
200ns/DIV
ELED1 AND
ELED2
5V/DIV
ILED BOOST
INDUCTOR
VLED
200mA/DIV
12.5V
10V/DIV
12V
10V/DIV
LED2
500μs/DIV
3524 G18
3524f
5
LTC3524
PIN FUNCTIONS
VIN (Pin 2): Common Input Supply for LCD Bias and White
LED Boost Converters. This pin must be locally bypassed
with a minimum of 2.2μF.
GND/Exposed Pad (Pin 25): Signal and Power Ground for
the LTC3524. Provide a short, direct PCB path between GND
and the (–) side of the boost (VOUT, VLED) filter capacitors, and the (–) side of the charge pump outputs (V2x,
VH, VN) filter capacitors. PCB ground must be soldered
to the Exposed Pad for proper operation.
LCD BIAS PIN FUNCTIONS
ELCD (Pin 1): Enable Input for the LTC3524’s LCD Circuits. LCD bias supplies are actively discharged to GND
when ELCD is low through internal pull down devices. An
optional RC network on ELCD provides a slower ramp-up
of the LCD boost converter inductor current during startup (soft-start). Shutdown mode is activated by driving
ELCD, ELED1, and ELED2 low. Shutdown disables all IC
functions and reduces quiescent current from the battery
to less than 2μA.
FBVO (Pin 3): Feedback Pin for the VOUT Switcher. Reference voltage is 1.225V. Connect resistive divider tap here
with minimum trace area.
R1⎞
⎛
VOUT = 1 . 225 ⎜ 1 + ⎟ (See Block Diagram)
⎝ R2 ⎠
VOUT (Pin 4): Main Output of the LCD Boost Regulator
and Input to the Voltage Doubler (2X) Stage. Bypass
VOUT with a low ESR, ESL ceramic capacitor (X5R type)
between 4.7 and 22μF.
SW1 (Pin 5): Synchronous Boost Switch. Connect a
4.7μH-15μH inductor between SW1 and VIN. Keep PCB
trace lengths as short and wide as possible to reduce EMI
and voltage overshoot. If the inductor current falls to zero,
the PMOS synchronous rectifier is turned off to prevent
reverse charging of the inductor and an internal switch
connects SW1 to VIN to reduce EMI.
C2– (Pin 6): Charge pump doubler flying capacitor negative
node. The charge pump doubler flying capacitor is connected between C2+ and C2–. The voltage on C2– will alternate between GND and VOUT at an approximate 50% duty
cycle while the charge pump is operating. Use a 0.1μF
X5R type ceramic capacitor for best results.
C2+ (Pin 7): Charge pump doubler flying capacitor positive node. The charge pump doubler flying capacitor is
connected between C2+ and C2–. The voltage on C2+ will
alternate between VOUT and V2x at an approximate 50%
duty cycle while the charge pump is operating. Use a 0.1μF
X5R type ceramic capacitor for best results.
V2x (Pin 8): Charge Pump Doubler Output and Input to
the Charge Pump Quadrupler. This output generates 2X
VOUT. V2x should be bypassed to GND with a 0.47μF X5R
type ceramic capacitor. C2+ and C2– should be left open
and V2x connected to VOUT if the doubler is not needed
to generate VH or VN.
VNIN (Pin 9): Positive Voltage Input for the Charge Pump
Inverter. The charge pump inverter can generate a regulated negative voltage up to the voltage applied to VNIN.
Connect VNIN to VOUT, V2x, or VH. If VNIN is connected
to VH, external diodes and a capacitor are required for
sequencing (see the Applications Information section).
CN+ (Pin 10): Charge Pump Inverter Flying Capacitor
Positive Node. The charge pump inverter flying capacitor
is connected between CN+ and external Schottky diodes
(see Typical Application figures). The voltage on CN+ will
alternate between GND and VNIN at an approximate 50%
duty cycle while the inverting charge pump is operating. Use
a 0.1μF X5R type ceramic capacitor for best results.
NC (PIN 11): No Connect. This pin should be connected
to GND.
VN (Pin 12): Negative Charge Pump Converter Output.
VN can be regulated down to approximately –VNIN volts
depending on where VNIN is connected. VN should be
bypassed to GND with at 0.47μF or larger X5R type ceramic capacitor.
3524f
6
LTC3524
LCD BIAS PIN FUNCTIONS
FBN (Pin 13): Feedback Pin for the VN Charge-Pump
Output. Reference voltage is 1.0V. Connect the resistive
divider tap between VOUT and VN here with minimum
trace area.
VN =
−R6 ( VOUT − 1)
+ 1 (See Block Diagram)
R5
FBH (Pin 14): Feedback Pin for the VH Charge-Pump
Output. Reference voltage is 1.225V. Connect resistive
divider tap here with minimum trace area.
⎛ R3 ⎞
VH = 1 . 225 ⎜ 1 + ⎟ (See Block Diagram)
⎝ R4 ⎠
VH (Pin 15): Charge Pump Quadrupler Output. This output
can be regulated to 4X VOUT and is capable of delivering
up to 2mA to a load. VH should be bypassed to GND with
a 0.47μF X5R type ceramic capacitor. Connect V2x to
VOUT for applications requiring a regulated voltage less
than 2X VOUT.
CH+ (Pin 16): Charge Pump Quadrupler Flying Capacitor
Positive Node. The charge pump quadrupler (4X) flying
capacitor is connected between CH+ and CH–. The voltage
on CH+ will alternate between V2x and VH at an approximate
50% duty cycle while the charge pump is operating. Use
a 0.1μF X5R type ceramic capacitor for best results.
CH– (Pin 17): Charge Pump Quadrupler (4X) Flying Capacitor Negative Node. The voltage on CH– will alternate
between GND and V2x at an approximate 50% duty cycle
while the charge pump is operating. Use a 0.1μF X5R type
ceramic capacitor for best results.
WHITE LED DRIVER PIN FUNCTIONS
LED2 (Pin 18): Output for Second LED String. Connect
up to five white LEDs between LED2 (anode) and GND
(cathode). For best current matching and efficiency use
the same number of white LEDs in both strings.
frequency (ie., 500Hz). Driving ELCD, ELED1, and ELED2
low initiates shutdown mode which disables all IC functions and reduces quiescent current from the battery to
less than 2μA.
LED1 (Pin 19): Output for First LED String.
PROG (Pin 23): A single resistor (RPROG) between
PROG and GND sets the current in the LED strings. LED
current in mA is programmed by:
VLED (Pin 20): Output of the LED Switcher. Bypass VLED
with a low ESR, ESL ceramic capacitor (X5R type) of at
least 1μF. Keep PCB trace lengths as short and wide as
possible to minimize EMI and voltage overshoot.
SW2 (Pin 21): White LED Boost Switch. Connect a 3.315μH inductor between SW2 and VIN. This is the collector
of the internal NPN power switch. Connect an external
Schottky diode between SW2 and VLED. Keep PCB trace
lengths as short and wide as possible to minimize EMI
and voltage overshoot.
ELED2 (Pin 22): Enable and PWM Dimming Control Input
for the LED2 String. The LED2 string is disabled when this
pin is grounded. Digital dimming can be implemented
by driving the ELED2 pin between 0V and >1.2V at low
⎛ 2 × 10 6 ⎞
ILED1 = ILED2 = ⎜
⎟ mA
⎝ RPROG ⎠
A 100K resistor programs 20mA in each string. Analog
dimming can be implemented by connecting a second
resistor between PROG and a control voltage.
ELED1 (Pin 24): Enable and Pulse Dimming Control Input
for the LED1 String. For applications with five or fewer
LEDs, better efficiency is achieved by operating a single
LED string. For example, ELED1 = 1, ELED2 = 0, LED2 left
open circuit and the LED string connected to LED1.
3524f
7
LTC3524
BLOCK DIAGRAM
2.5V TO 6V
10μH
5
2
SW1
4.7μH
21
VIN
SW2
VLED
20
VBEST
+5V
4
STRING ENABLE
LED CURRENT
SHARING
OVP
VOUT
SHDN
R1
1M
R2
324k
3
7
6
SYNCHRONOUS
PWM BOOST
CONVERTER
HIGH
VOLTAGE
PWM BOOST
CONVERTER
LED2
18
C2+
IN
PROG
23
VOUT
CHARGE
PUMP
DOUBLER
V2x
+10V
LED1
19
FBVO 1.225V
C2–
OUT
8
SHDN
CHARGE
PUMP
SEQUENCER
17
+12.5V
15
CH+
CH –
VH
SHUTDOWN WHEN
ELED1
ELCD = ELED1 = ELED2 = 0V
24
ENABLE/PULSE DIM LED1 STRING
ELCD
OUT
10μF
1
LCD BIAS ENABLE
9
V2x
VNIN
REGULATED
CHARGE
PUMP
INVERTER
OUT
ANALOG DIMMING
ENABLE/PULSE DIM LED2 STRING
IN
IN
REGULATED
CHARGE
PUMP
QUADRUPLER
RPROG
ELED2
22
OSCILLATOR
CONTROL
16
10μF
CN+
10
NC
11
VN
12
–7.5V
SHDN
R3
2M
SHDN
R4
220k
14
FBH
1.225V
GND,
EXPOSED PAD
25
1V
R6
1M
FBN
13
R5
470k
VOUT
3524 BD
3524f
8
LTC3524
OPERATION
The LTC3524 is a highly integrated power converter
intended for small to medium-sized TFT LCD display
modules. The part generates the required bias voltages
for the LCD panel as well as regulated current for one or
two white LED backlight strings. The LCD bias and white
LED boost converters are powered from a common input
voltage between 2.5V and 6V and share a 1.5MHz oscillator, allowing tiny inductors and capacitors to be used.
The LCD bias supply and each white LED string can be
independently enabled and a low current shutdown mode
(<2μA) is activated when all outputs are disabled.
The LCD bias includes a synchronous PWM boost converter that can be programmed between 3.0V and 6.0V.
This output (VOUT) is used as the main LCD supply and to
power three charge pump converters. The charge pump
circuits operate at one-sixteenth the boost frequency
(about 94kHz). The generated output voltages are internally
sequenced to insure proper initialization of the LCD panel.
A digital shutdown input (ELCD) rapidly discharges each
generated output voltage to provide a near instantaneous
turn-off of the LCD display.
The white LED driver circuitry consists of a PWM boost
converter with an internal low loss NPN power switch and
external Schottky diode. The LED boost output (VLED) can
power as many as ten white LEDs at up to 25mA. LED current is programmable and current in each string matched
with an internal loop. PWM dimming can be implemented
through the enable pins (ELED1 and ELED2) to extend the
dimming range of the application.
LCD Bias Boost Converter
A synchronous boost converter is used to generate the
main analog LCD bias supply for the TFT display. The
converter utilizes current mode control and includes
internally set control loop and slope compensation for
optimized performance and a simple design. Only an
inductor, output capacitor and VOUT programming resistors at FBVO are required to complete the design of the
25mA boost. The 1.5MHz operating frequency produces
very low output ripple and allows the use of small low
profile inductors and tiny external ceramic capacitors.
The boost converter also disconnects its output from VIN
during shutdown to avoid loading the input power source.
Soft-start produces a controlled ramp of the converter
input current during start-up, greatly reducing the burden
on the input power source. Very low operating quiescent
current and synchronous operation allow for greater than
90% conversion efficiency.
VIN
RSS
1M
ELCD
CSS
6.8nF
3524 F01
Figure 1. 1ms Soft-Start with 3.6V VIN
Soft-start operation provides a gradual increase in the
current drawn from the input power source during initial
start-up of the LCD bias boost converter. The rate at which
the input current will increase is set by two external components (RSS and CSS) connected to ELCD (refer to Figure
2). Upon initial application of power the voltage on ELCD
will increase relative to the time constant RSS × CSS. After
one time constant, ELCD will rise to approximately 63.2%
of the voltage on VIN. From 0V to approximately 0.65V on
ELCD, no switching will occur because the threshold is
0.65V (typ). From 0.65V to 1V the maximum switch pin
current capability of the LTC3524 will gradually increase
from near 0A to the maximum current limit.
LCD Bias Charge Pumps
The LTC3524 uses three internal charge pump circuits to
generate low current, high voltage outputs typically used
to bias the LCD gate drive. The three charge pumps include
a doubler, quadrupler, and inverting configuration. Each
charge pump requires two small external capacitors, one
to transfer charge, and one for filtering. The charge pumps
feature fixed frequency operation for high efficiency and
lowest noise performance. The charge pump converters
operate at one-sixteenth the boost converter frequency.
3524f
9
LTC3524
OPERATION
The doubler is internally connected to VOUT and generates
a voltage of approximately 2X VOUT at V2x. The quadrupler
has its input connected to V2x and output to VH. The regulated VH voltage is programmed at FBH and can be set to
produce a voltage up to 4X VOUT. 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 VNIN and
output at VN. Regulated VN voltage is set at FBN and can
be programmed to a minimum negative voltage of VNIN
minus diode drops. VNIN can be connected to VOUT, V2x,
or VH depending on the negative voltage value required
for the application. Efficiency is improved by using the
lowest voltage possible on VNIN. 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 programmed value. When the boost converter output (VOUT)
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 VOUT.
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 VNIN is connected to VH, a diode-OR
circuit is needed between V2x, VH, and VNIN (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, followed by VOUT. VOUT 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
V2x
VOUT
VOUT
ELCD
ELCD
TIME
VN
VN
3524 F02
Figure 2. LCD Power-Up and Power-Down Timing Diagram
3524f
10
LTC3524
OPERATION
output voltage is not set to a fixed voltage, but rather
controlled to produce the programmed current in the
LED strings. The output (VLED) is rated for a maximum
of 21V which will support two strings of up to five series
LED in most cases.
The boost output is used to power one or two white LED
strings with a common ground. If only one string is enabled (ELED1 or ELED2) the voltage on that string (LED1
or LED2) will be controlled to regulate the LED current
set at the PROG pin. The voltage on VLED will be slightly
greater due to the overhead needed for the internal sense
element and share circuitry. For example, a single string
application with four white LEDs programmed at 20mA
would require 14.4V on LED1 if the forward drop on each
LED is 3.6V. The voltage on VLED may need to be 15V to
support the drops on the internal share circuitry. For applications with five or fewer LED elements, a single-string
operation will provide better efficiency.
If both strings are enabled, the boost output (VLED)
will generate the voltage required to regulate current in
the higher voltage string. Voltage on the lower string is
controlled by the internal share circuit to provide the programmed current. The LTC3524 achieves current matching
between the strings while minimizing the voltage drop
between VLED and the higher voltage string (to maintain
high efficiency). For example, an application with four
LEDs on LED1 and five LEDs on LED2 is programmed for
20mA (RPROG = 100k). In this instance, assuming a 3.6V
forward drop, LED1 is 14.4V, LED2 is 18V, and VLED is
18.6V. The drop between VLED and LED1 is 4V at 20mA,
resulting in lower efficiency. For this reason, it is recommended when possible to keep the number of LEDs in
each string matched.
Analog Dimming:
The LTC3524’s white LED driver allows both analog and
PWM dimming to be implemented. Analog dimming
provides a lower noise solution but a reduced dynamic
range. Analog dimming can be implemented by resistively summing a current into the PROG pin. The LED
string currents with RPROG, VSUM, and RSUM will be:
⎛ 1 . 225V 1 . 225V − VSUM ⎞
ILED = 1625 • ⎜
+
⎟⎠
R SUM
⎝ RPROG
A 0V to 3V VSUM with RSUM = 300k and RPROG = 150k will
produce LED currents between 3mA and 20mA.
VSUM
0V – 3V
RSUM
300k
PROG
RPROG
150k
3524 F03
Figure 3. Analog Dimming Circuit Using VSUM
True Color PWM Dimming:
PWM dimming can be implemented by enabling and disabling the LED strings with ELED1 and ELED2. A PWM
frequency between 100Hz and 500Hz is generally recommended to get wide dimming range while operating at a
frequency faster than the eye can detect. For best results,
the LCD bias portion of the device should be enabled (to
keep the device out of shutdown) and ELED1 and ELED2
should be driven with a common low frequency PWM
signal. PWM dimming waveforms are shown in the Typical
Performance Characteristics section of this datasheet.
The achievable dimming range is dependant on the PWM
dimming frequency (FPWM) and the settling time of the
LED strings when enabled (TSETTLE). The minimum duty
cycle (or light output) that the strings can be controlled
to is given by:
MinDuty = FPWM • TSETTLE
For example, if the settling time is 50μS and the PWM
frequency is 100Hz, the minimum duty cycle is 0.5%
which corresponds to a 200:1 dimming range.
Open LED:
The LTC3524 has internal over voltage protection in the
event that one of the white LED strings becomes open
circuited. If VLED reaches 24V (nominal) due to an open
circuit on either string, the boost converter will regulate
at 24V while current in the remaining string (if enabled)
is controlled to the programmed value.
3524f
11
LTC3524
APPLICATIONS INFORMATION
Inductor Selection
3.3μH to 15μH inductors are recommended for use with
the LTC3524’s two boost converters. The synchronous
LCD bias boost inductor should have a saturation current
(ISAT) rating of at least 150mA, where the nonsynchronous
white LED boost inductor should have a rating of at least
600mA. In most applications, the inductor value for the
LCD bias will be larger (10μH to 15μH) to prevent operation in deep discontinuous mode. The inductor value for
the white LED can be smaller (3.3μH to 6.8μH), since it
operates at higher currents. Ferrite core materials are
strongly recommended for their superior high frequency
performance characteristics. Inductors meeting these
requirements are listed in Table 1. The maximum current
and DCR ranges in the table correspond to the respective Inductance range (for example, the 3.3μH inductor
will have the highest maximum current and lowest DCR).
Shielded inductor series parts are in bold text.
The VIN input capacitor should be an X5R type of at least
2.2μF using a low impedance connection to the battery.
The VLED output capacitor should be X5R type and at least
1μF for analog dimming and 4.7μF for PWM dimming.
The VOUT capacitor should also be an X5R type between
2.2μF and 10μF. A larger capacitor (10μF) should be used
if lower output ripple is desired or the output load required
is close to the 25mA maximum.
The charge pumps require flying capacitors (C2+ to C2–,
CN+, and CH+ to CH–) that should be at least 0.1μF to obtain
specified performance. Ceramic X5R types are strongly
recommended for their low ESR and ESL and capacitance
vs bias voltage stability. The filter capacitors on V2x, VN,
Table 1. Recommended Inductors
L
(μH)
MAXIMUM
CURRENT
(mA)
DCR (Ω)
DIMENSIONS
(mm)
(L × W × H)
ME3220
LP03010
MSS4020
3.3-15
3.3-10
3.3-15
1300-700
950-570
1100-440
0.14-0.52
0.2-0.52
0.09-0.33
3.2 × 2.5 × 2.0
3.0 × 3.0 × 1.0
4.0 × 4.0 × 2.0
Coil Craft
www.coilcraft.com
SD3112
3.3-15
970-405
0.16-0.65
3.1 × 3.1 × 1.2
Cooper
www.cooperet.com
MIP3226D
3-10
1000-200
0.1-0.16
3.2 × 2.6 × 1.0
FDK
www.fdk.com
LQH32CN
LQH2MC
4.7-15
4.7-15
650-300
300-200
0.15-0.58
0.8-1.6
3.2 × 2.5 × 1.5
2 × 1.6 × 0.9
Murata
www.murata.com
CDRH3D16
CDRH2D14
3.3-15
3.3-12
1100-520
820-420
0.09-0.41
0.12-0.32
3.8 × 3.8 × 1.8
3.2 × 3.2 × 1.5
Sumida
www.sumida.com
NR3010
NR3015
3.3-15
3.3-15
750-400
1200-560
0.16-0.74
0.1-0.36
3.0 × 3.0 × 1.0
3.0 × 3.0 × 1.5
Taiyo Yuden
www.t-yuden.com
PART
MANUFACTURER
3524f
12
LTC3524
APPLICATIONS INFORMATION
and VH should be at least 0.47μF. Please be certain that
the capacitors used are rated for the maximum voltage
with adequate safety margin. Refer to Table 2 for a listing
of capacitor vendors.
Printed Circuit Board Layout Guidelines
High-speed operation of the LTC3524 demands careful attention to PCB layout. You will not get advertised
performance with a careless layout. Figure 4 shows the
recommended component placement for a double layer
PCB. The bottom layer is used as a common ground plane
except for the VN trace.
Table 2. Capacitor Vendor Information
Supplier
Phone
Website
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
Samsung
(408) 544-5200
www.sem.samsung.com
Taiyo Yuden
(800) 368-2496
www.t-yuden.com
TDK
(847) 803-6100
www.component.tdk.com
ELCD ELED1 ELED2
SCHOTTKY DIODE
L2
LAYOUT NOTES:
LIGHT GREY TOP LAYER
WHITE LEDs
VIA TO BOTTOM GROUND PLANE.
GROUND PLANE FILLS BOTTOM
*
*KEEP RPROG AWAY FROM SW2 TRACES
ELED1
24
ELCD
1
VIN
VIN
2
R1
VLED
20
LED1
19
–
CH
17
GND
VOUT
4
+
CH
16
VH
15
GND
SW1
5
WHITE LEDs
LED2
18
COMPONENT AND IC SIZES
NOT TO SCALE
GND
L1
SW2
21
TOP VIEW
FBVO
3
VOUT
ELED2
22
GND
GND
PROG
23
VH
FBH
14
C2–
6
FBN
13
C2+
7
V2x
8
VNIN
9
CN+
10
NC
11
VN
12
SCHOTTKY
DIODE
VN
3524 F04
Figure 4. Suggested Layout Two Layer Board (Not to Scale)
3524f
13
LTC3524
TYPICAL APPLICATIONS
Li-Ion to +5V, 25mA, +16V, 1mA, –13V, 1mA TFT LCD Power Supply + 10 White LEDs
+
–
2.2μF
Li-Ion
10μH
SW1
+5V, 25mA
10μF
4.7μH
VIN
VOUT
LED2
1M
324k
LED1
FBVO
100k FOR 20mA
+10V
V2x
0.47μF
PROG
ELED2
ELED1
ELCD
LTC3524
C2+
0.1μF
C2–
+16V, 1mA
0.47μF
10μF
SW2
VLED
0.47μF
VIN
VNIN
V2x
VH
VOUT
1M
FBH
CH+
REQUIRED
FOR LCD BIAS
SEQUENCING
WHEN |VN| > V2X
287k
FBN
2M
165k
VH
CH– GND CN+
0.1μF
–13V, 1mA
0.47μF
VN
0.1μF
3524 TA02a
100
LCD (VIN )
EFFICIENCY (%)
90
4.2
80
3.6
LED (VIN )
3.1
70 4.2
3.6
3.1
60
5
10
15
20
VOUT OR LED STRING CURRENT (mA)
25
3524 TA02b
3524f
14
LTC3524
PACKAGE DESCRIPTION
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 ±0.05
4.50 ± 0.05
2.45 ± 0.05
3.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
0.75 ± 0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
23 24
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3524f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3524
TYPICAL APPLICATION
3NiMH or NiCD to +3.3V, 25mA, +10V, 1mA, –5V, 1mA TFT LCD Power Supply + 6 White LEDs
COILCRAFT
MSS4020 SERIES
+
10μH
2.2μF
3 NiMH
OR NiCD
SW1
–
4.7μH
VIN
100
10μF
SW2
VLED
90
EFFICIENCY (%)
VOUT
LED2
510k
10μF
301k
LED1
FBVO
+6.6V
100k FOR 20mA
LTC3524
V2x
0.47μF
ELED2
ELED1
C2+
165k
50
V2x
5
VOUT
FBN
FBH
CH+
3.6V 3.6V LCD (VIN )
232k
2M
VN
10
15
20
VOUT OR LED STRING CURRENT (mA)
25
3524 TA03b
0.47μF
604k
– 5V, 1mA
CH– GND CN+
0.1μF
2.5V
VIN
VNIN
VH
LED (VIN )
70
60
ELCD
C2–
+10V, 1mA
80 2.5V
3.1V
PROG
0.1μF
0.47μF
LCD (VIN )
3.1V
+3.3V, 25mA
PHILIPS
PMEG3005
0.1μF
3524 TA03a
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1942
Quad DC/DC Converter for Triple Output TFT Supply Plus
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VIN : 2.6V to 16V, VOUT(MAX) = 36V, IQ = 7mA, ISD = < 1μA,
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LT1947
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Triple Switching Regulator
VIN : 1.5V to 4.6V, VOUT(MAX) = 15V, IQ = 75μA, ISD = < 1μA,
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LT3465/LT3465A
Constant-Current, 1.2MHz/2.7MHz High Efficiency White
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LT3471
Dual Output, Boost/Inverter, 1.3A ISW, 1.2MHZ, High
Efficiency Boost-Inverting DC/DC Converter
VIN : 2.4V to 16V, VOUT(MAX) = ±40V, IQ = 2.5mA, ISD = < 1μA,
3mm × 3mm DFN-10 Package
LT3491
Constant-Current, 2.3MHz, High Efficiency White LED Boost
Regulator with Integrated Schottky Diode
VIN : 2.5V to 12V, VOUT(MAX) = 27V, IQ = 2.6mA, ISD = < 8μA,
2mm × 2mm DFN-6 SC70 Package
LT3494/LT3494A
40V, 180mA/350mA Micropower Low Noise Boost Converter
with Output Disconnect
VIN : 2.3V to 16V, VOUT(MAX) = 40V, IQ = 65μA, ISD = < 1μA,
3mm × 2mm DFN-8 Package
LT3497
Constant-Current, 2.3MHz, Dual High Efficiency White LED
Boost Regulator with Integrated Schottky Diode for 12 LEDs
VIN : 2.5V to 10V, VOUT(MAX) = 32V, IQ = 6mA, ISD = < 12μA,
3mm × 2mm DFN-10 Package
LT3591
Constant-Current, 1MHz, High Efficiency White LED Boost
Regulator with Integrated Schottky Diode
VIN : 2.5V to 12V, VOUT(MAX) = 40V, IQ = 4mA, ISD = < 9μA,
3mm × 2mm DFN-8 Package
ThinSOT is a trademark of Linear Technology Corporation.
3524f
16 Linear Technology Corporation
LT 0208 • PRINTED IN USA
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