LINER LT1942EUF Quad dcdc converter for triple outputs tft supply plus led driver Datasheet

LT1942
Quad DC/DC Converter
for Triple Output TFT Supply
Plus LED Driver
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FEATURES
DESCRIPTIO
■
The LT®1942 is a highly integrated quad switching regulator designed to provide all necessary power supply
functions for TFT displays, including the white LED
backlight driver. The TFT supply incorporates two boost
and one inverting DC/DC converters. The TFT supply
output voltages are independently set, unlike charge
pump solutions which have many limitations. Power
sequencing for the TFT supply is built into the part and is
user programmable.
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■
■
■
■
■
■
■
■
■
■
Triple Output TFT Supply with Built-In
Power Sequencing
Integrated White LED Backlight Driver
Integrated Schottky Diodes
Active Ballast Circuitry Ensures Precise Current
Matching in White LEDs
Low Noise 1MHz Fixed Frequency Operation
2.6V to 16V Input Voltage Range
Soft-Start Limits Inrush Current
TFT Supply Output Voltages Independently Set
(Not Charge Pump Derived)
Power Good and Output Disconnect for TFT Supply
Built-In LED Dimming Capability
Open LED Protection for LED Driver
24-Lead QFN Package (4mm × 4mm)
The LED driver is a boost DC/DC converter that can be
independently controlled. The LED driver has built-in
dimming control for precise control of LED current. An 8:1
dimming range is achieved by adjusting the CTRL4
voltage.The user can elect to drive a single string or two
strings of LEDs. An LED ballast circuit is included to
precisely match the LED currents if two strings of LEDs are
used. Soft-start is built into the LED driver as well as the
primary TFT supply.
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APPLICATIO S
■
■
Poly-Silicon TFT Displays
Amorphous Silicon TFT Displays
The LT1942 is available in a low profile (0.75mm) 24-lead
QFN (4mm × 4mm) package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TFT Bias and White LED Backlight Power Supply
22µH
VIN
3V TO 4.2V
22µH
0.22µF
698k
0.22µF
VON
10V
2mA
D3
FB3
VCC
SW3
SW1
FB1
665k
VOUT3
VIN
SHUTDOWN
LED CONTROL
47µH
0.1µF
4.7µF
100k
1M
4.7µF
20mA
20mA
PGOOD
SW4
LT1942
D2
LED1
SW2
LED2
SHDN SGND/
CTRL4 AGND
33µH
VIN
D4
NFB2
47µH
4.7pF
AVDD
5V
40mA
PGND14
PGND23
0.22µF
Si2301BDS
301k
100k
10k
VOFF
–10V
2mA
CMDSH-3
SS1
SS4
0.1µF
FB4
CT
0.1µF
4.99Ω
1942 TA01
0.1µF
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LT1942
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AXI U
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
ORDER PART
NUMBER
SW4
SW1
PGND14
VCC
D2
SW2
TOP VIEW
LT1942EUF
24 23 22 21 20 19
PGND23 1
18 LED1
SW3 2
17 LED2
D3 3
16 D4
25
VOUT3 4
15 FB4
FB3 5
14 FB1
NFB2 6
UF PART
MARKING
13 PGOOD
1942
SS4
SS1
9 10 11 12
AGND
8
CT
SHDN
7
CTRL4
VCC Voltage............................................................. 16V
SW1, SW2, SW3 Voltage .......................... –0.4V to 36V
SW4 .......................................................... –0.4V to 45V
FB1, FB3 Voltage ................................................... 2.5V
FB4 Voltage ....................................................... 400mV
NFB2 Voltage ................................................... –200mV
SHDN Voltage ......................................................... 10V
CTRL4 Voltage ........................................................ 16V
SS1, SS4 Voltage .................................................. 1.5V
Current Into D2 ......................................................... 1A
D2 Voltage ............................................................ –36V
Current Out of D3 ..................................................... 1A
D3 Voltage .............................................................. 36V
Current Out of D4 ..................................................... 4A
D4 Voltage .............................................................. 45V
LED1, LED2 Voltage ............................................... 45V
Current Into LED1, LED2 ..................................... 35mA
PGOOD Voltage ...................................................... 16V
VOUT3 Voltage ......................................................... 36V
CT Current ........................................................... ±1mA
Maximum Junction Temperature ......................... 125°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 125°C
UF PACKAGE
24-LEAD (4mm × 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W, θJC = 4.3°C/W
EXPOSED PAD (PIN 25) IS SGND
(MUST BE SOLDERED TO PCB)
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
Quiescent Current
VSHDN = 3.3V, VCTRL4 = 0V, Not Switching
VSHDN = VCTRL4 = 0V, In Shutdown
●
●
Input Voltage Range
VCC Pin
●
2.6
AVDD Feedback Voltage
FB1 Pin
●
1.225
AVDD Feedback Pin Bias Current (FB1)
FB1 Pin (Note 3)
AVDD Feedback Voltage for Power Good Signal
FB1 Pin Voltage as a Percent of Nominal Voltage
VOFF Feedback Voltage
NFB2 Pin
VOFF Feedback Pin Bias Current (NFB2)
NFB2 Pin (Note 3)
VON Feedback Voltage
FB3 Pin
VON Feedback Pin Bias Current (FB3)
FB3 Pin (Note 3)
LED Feedback Voltage
FB4 Pin, VCTRL4 = 3.3V
LED Feedback Pin Bias Current (FB4)
FB4 Pin (Note 3)
LED Feedback Voltage with Dimming
FB4 Pin, VCTRL4 = 1V
MIN
●
MAX
7
0
10
1
UNITS
mA
µA
16
V
1.25
1.275
V
15
60
nA
95
97
100
%
–160
–150
–140
mV
25
60
nA
1.25
1.275
V
25
60
nA
190
200
210
mV
20
60
µA
85
95
105
mV
1.225
●
TYP
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LT1942
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
LED1-LED2 Current Matching
TYP
MAX
UNITS
Current Into Each Pin = 20mA, VLED1 = VLED2
Current Into Each Pin = 20mA, |VLED1-VLED2| = 1V
0
1
1.5
2.5
%
%
LED1 Voltage
20mA Into Pin, VLED2 = 2V, FB4 = 0
1
1.2
V
LED2 Voltage
20mA Into Pin, VLED1 = 2V, FB4 = 0
1
1.2
V
LED1, LED2 Maximum Current
Into Either Pin, VLEDX < 1.5V, FB4 = 0
LED1, LED2 Leakage Current
VSHDN = 0V, VLEDX = 45V
AVDD Maximum Duty Cycle
SW1 Pin
MIN
35
mA
●
1
88
93
µA
%
VOFF Maximum Duty Cycle
SW2 Pin
75
86
%
VON Maximum Duty Cycle
SW3 Pin
75
86
%
LED Maximum Duty Cycle
SW4 Pin
88
93
%
AVDD Switch Current Limit at Minimum DC (SW1)
SW1 Pin (Note 4)
150
200
250
mA
AVDD Switch Current Limit at Maximum DC (SW1)
SW1 Pin (Note 4)
80
140
200
mA
VOFF Switch Current Limit at Minimum DC (SW2)
SW2 Pin (Note 4)
50
80
110
mA
VOFF Switch Current Limit at Maximum DC (SW2)
SW2 Pin (Note 4)
30
60
90
mA
VON Switch Current Limit at Minimum DC (SW3)
SW3 Pin (Note 4)
50
80
110
mA
VON Switch Current Limit at Maximum DC (SW3)
SW3 Pin (Note 4)
30
60
90
mA
LED Switch Current Limit at Minimum DC (SW4)
SW4 Pin (Note 4)
550
750
900
mA
LED Switch Current Limit at Maximum DC (SW4)
SW4 Pin (Note 4)
450
600
850
mA
VOFF Schottky Diode Forward Drop
D2 Pin, I = 60mA
VOFF Schottky Diode Leakage Current
D2 Pin, VD2 = –36V
VON Schottky Diode Forward Drop
D3 Pin, I = 60mA
VON Schottky Diode Leakage Current
VD3 = 36V, SW3 = 0V, Output Disconnect PNP Off
LED Schottky Diode Forward Drop
I = 250mA
LED Schottky Diode Leakage Current
VD4 = 36V, SW4 = 0V
0.65
●
V
1
0.65
●
V
1
µA
1
µA
0.75
●
Switching Frequency
0.8
µA
V
1
1.2
MHz
SHDN Pin Current
VSHDN = 3V
VSHDN = 0V
●
70
20
100
30
µA
nA
CTRL4 Pin Current
VCTRL4 = 3V
VCTRL4 = 0V
●
30
–60
60
–150
µA
nA
SW1-SW4 leakage Current
VSWX = 36V
●
0.01
1
µA
SW1 VCESAT
ISW1 = 100mA (Note 5)
200
300
mV
SW2 VCESAT
ISW2 = 40mA (Note 5)
150
250
mV
SW3 VCESAT
ISW3 = 40mA (Note 5)
150
250
mV
SW4 VCESAT
ISW4 = 400mA (Note 5)
280
400
mV
SS1 Charging Current
VSS = 0.5V
1
3
5
µA
SS4 Charging Current
VSS = 0.5V
1
3
5
µA
SHDN Input Voltage High
1
SHDN Input Voltage Low
CTRL4 Input Voltage High
CTRL4 Input Voltage Low
V
0.4
0.25
V
V
0.1
V
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LT1942
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
PGOOD Leakage Current
PGOOD NPN Off, VPGOOD = 5V
PGOOD Sink Current
PGOOD NPN On, VPGOOD = 1V
0.5
TYP
MAX
UNITS
0
0.1
µA
0.7
mA
CT Charging Current
12
µA
CT Reference Voltage
0.8
V
VOUT3 Leakage Current
Output Disconnect PNP Off, D3 = 36V, VOUT = 0V
VOUT3 Source Current
Output Disconnect PNP On, D3 = 3V, VOUT = 2.5V
0
Minimum D3/VOUT3 Operating Voltage
Minimum D3 Voltage for Proper Output
Disconnect PNP Operation
10
D4 LED Open-Circuit Voltage
38
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1942 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical controls.
µA
1
15
mA
3
V
42
44
V
Note 3: Current flows out of the pin.
Note 4: Current limit guaranteed by design and/or correlation to static test.
Note 5: VCESAT 100% tested at wafer level.
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TYPICAL PERFOR A CE CHARACTERISTICS
VFB1 Voltage
NFB2 Voltage
VFB3 Voltage
–145
1.28
1.28
–146
1.27
–147
1.25
1.24
VOLTAGE (V)
–148
1.26
VOLTAGE (mV)
VOLTAGE (V)
1.27
–149
–150
–151
1.25
1.24
–152
–153
1.23
1.26
1.23
–154
1.22
–40 –20
40
20
60
0
TEMPERATURE (°C)
80
100
1942 G01
–155
–40
–20
40
20
0
60
TEMPERATURE (°C)
80
100
1942 G02
1.22
–40 –20
40
20
60
0
TEMPERATURE (°C)
80
100
1942 G01
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LT1942
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TYPICAL PERFOR A CE CHARACTERISTICS
FB4 Voltage
VFB4 vs CTRL4
210
Oscillator Frequency
300
1200
250
1100
206
200
202
VFB4 (mV)
VFB4 (mV)
204
OSCILLATOR FREQUENCY (kHz)
208
200
198
TA = –40°C
TA = 25°C
150
TA = 85°C
100
196
194
50
1000
900
800
700
192
190
–40
0
–20
40
20
0
60
TEMPERATURE (°C)
80
100
0
0.5
1
1.5
2
CTRL4 (V)
2.5
9
9.0
8
8.5
7
7.5
7.0
6.5
5
4
3
2
5.5
1
–20
40
20
0
60
TEMPERATURE (°C)
80
0
–40
100
600
CT = 0.1µF
6
6.0
–20
40
20
0
60
TEMPERATURE (°C)
80
1942 G07
400
300
250
200
150
100
50
0
10
20
40
30
SW2 CURRENT (mA)
400
300
200
100
0
100
0
50
60
1942 G10
25
50 75 100 125 150 175 200
SW1 CURRENT (mA)
1942 G09
SW4 Saturation Voltage
800
TA = 25°C
350
SW4 SATURATION VOLTAGE (mV)
TA = 25°C
350
0
500
SW3 Saturation Voltage
SW3 SATURATION VOLTAGE (mV)
SW2 SATURATION VOLTAGE (mV)
400
TA = 25°C
1942 G08
SW2 Saturation Voltage
300
250
200
150
100
50
0
0
10
20
40
30
SW3 CURRENT (mA)
100
SW1 Saturation Voltage
SW1 SATURATION VOLTAGE (mV)
9.5
80
1942 G06
CT Timer Delay Time
10
DELAY TIME (ms)
QUIESCENT CURRENT (mA)
Quiescent Current
10.0
8.0
40
20
60
0
TEMPERATURE (°C)
1942 G05
1942 G04
5.0
–40
600
–40 –20
3
50
60
1942 G10
TA = 25°C
700
600
500
400
300
200
100
0
0
100 200 300 400 500 600 700 800
SW4 CURRENT (mA)
1942 G12
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LT1942
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TYPICAL PERFOR A CE CHARACTERISTICS
LED1 to LED2 Current Matching
TA = 85°C
1.5
1.0
0.5
TA = 25°C
0
TA = –40°C
–0.5
–1.0
44.0
43.8
400
43.6
350
43.4
300
250
200
TA = 25°C
150
TA = 85°C
100
–1.5
–2.0
500
450
6
10
14 18 22 26
LED2 CURRENT (mA)
30
0
0.01
34
1
IOUT3 CURRENT (mA)
1200
D4 FORWARD VOLTAGE (mV)
42.0
–40
10
TA = 25°C
600
400
200
10
100
D4 CURRENT (mA)
TA = 25°C
1000
800
600
400
200
1000
100
1200
1000
800
80
VON Diode Forward Voltage
1400
1200
1000
40
20
0
60
TEMPERATURE (°C)
–20
1942 G08
VON DIODE FORWARD VOLTAGE (mV)
VOFF DIODE FORWARD VOLTAGE (mV)
TA = 25°C
0
0.1
10
100
VOFF DIODE CURRENT (mA)
1000
1
1942 G16
800
600
400
200
0
0.1
10
100
1
VON DIODE CURRENT (mA)
1942 G17
1000
1942 G19
LED Switcher Current Limit
AVDD Switcher Current Limit
300
42.6
VOFF Diode Forward Voltage
1400
1
42.8
1942 G14
D4 Forward Voltage
0
0.1
43.0
42.2
0.1
1942 G13
1400
43.2
42.4
TA = –40°C
50
2
VOUT4 VOLTAGE (V)
VLED1 = VLED2 + 1V
D3-VOUT3 VOLTAGE DROP (mV)
LED1 CURRENT MATCHING ERROR (%)
2.0
LED Open-Circuit Protection
Voltage
VOUT3 vs IOUT3
900
TA = 25°C
TA = 25°C
800
250
700
600
ILIM (mA)
ILIM (mA)
200
150
500
400
300
100
200
50
100
0
0
20
30
40
60
50
70
DUTY CYCLE (%)
80
90
1942 G18
20
30
40
50
60
70
DUTY CYCLE (%)
80
90
1942 G20
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LT1942
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PI FU CTIO S
PGND23 (Pin 1): Power Ground for VOFF and VON Switcher.
Tie directly to local ground plane.
AGND (Pin 10): Analog Ground. Tie directly to local
ground plane.
SW3 (Pin 2): Switch Pin for VON Switcher. This is the
collector of an internal NPN power switch. Minimize the
metal trace area connected to this pin to minimize EMI.
SS1 (Pin 11): Soft-Start Pin for AVDD Switcher. Place a
soft-start capacitor from this pin to ground. Upon start-up,
3µA of current charges the capacitor to 1.25V. Use larger
capacitor for slower start-up. Leave floating if not in use.
D3 (Pin 3): Output of VON Switcher. Place output decoupling capacitor from this pin to local ground plane.
VOUT3 (Pin 4): Collector of Internal Output Disconnect
PNP Transistor for the VON Switcher. Place a capacitor
from this pin to local ground plane of at least 0.1µF.
SS4 (Pin 12): Soft-Start for LED Switcher. Place a softstart capacitor from this pin to ground. Upon start-up, 3µA
of current charges the capacitor to 1.25V. Use larger
capacitor for slower start-up. Leave floating if not in use.
FB3 (Pin 5): Feedback Pin for VON Switcher. Reference
voltage is 1.25V. Connect resistive divider tap here.
Minimize trace area at FB3. Set VOUT3 according to VOUT3
= 1.25(1 + R5/R6).
PGOOD (Pin 13): Power Good Indication for AVDD Switcher.
Open-collector NPN. Stays open until the VFB1 voltage
reaches 97% of target value at which point the pin will pull
down.
NFB2 (Pin 6): Feedback Pin for VOFF Switcher. Reference
voltage is –150mV. Connect resistive divider tap here.
Minimize trace area at NFB2. Set VOUT2 according to
VOUT2 = (– 0.15)(1 + R3/R4).
SHDN (Pin 7): Shutdown. Tie to 1V or greater to enable the
part. Tie to 0.4V or lower to disable. This pin is the global
shutdown pin for all four switching regulators.
FB1 (Pin 14): Feedback Pin for AVDD Switcher. Connect
resistive divider tap here. Minimize trace area at FB1. Set
VOUT1 according to VOUT1 = 1.25(1 + R1/R2).
FB4 (Pin 15): Feedback Pin for LED Switcher. Place a
resistor from this node to ground. Choose R7 to set LED
current (ILED) according to the following formula:
if VCTRL4 ≥ 2.5V, R7 = 0.2/ILED
CTRL4 (Pin 8): Shutdown for the LED Switcher. Tie to
0.25V or greater to enable the LED switcher. Tie to 0.1V or
lower to disable. CTRL4 can also override the default 200mV
reference for the LED switcher. If CTRL4 is tied to 2.5V or
greater, the reference voltage is 200mV. If CTRL4 is less
than 2.5V, the LED reference voltage is approximately
VCTRL4/10. This allows an 8:1 dimming range for the LEDs.
Refer to the Applications Section for more information.
In this case, ILED is the total LED current. If two strings of
LEDs are used, ILED is the sum of the current in the two
strings. If only a single string of LEDs is used, ILED is
simply the single string current. For dimming LED current,
drive VCTRL4 to a lower voltage. See the Applications
Section for more information.
CT (Pin 9): Programmable Delay for the TFT Supplies.
Place a capacitor from this pin to local ground plane. After
the delay, the output disconnect PNP for the VON supply is
turned on. The delay time is initiated when AVDD reaches
97% of its final value.
LED2 (Pin 17): Second Input of Current Mirror for an LED
String. Connect the negative end of second string of LEDs
to this pin. If not in use, leave floating.
D4 (Pin 16): Output of LED Switcher. Place output decoupling capacitor from this pin to local ground plane.
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LT1942
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LED1 (Pin 18): First Input of Current Mirror for an LED
String. Connect the negative end of first string of LEDs to
this pin. If not in use, leave floating.
VCC (Pin 22): Input Supply. Must be locally bypassed.
D2 (Pin 23): Anode Connection of Internal Schottky Diode
for VOFF Switcher.
SW4 (Pin 19): Switch Pin for LED Switcher. This is the
collector of an internal NPN power switch. Minimize the
metal trace area connected to this pin to minimize EMI.
SW2 (Pin 24): Switch Pin for VOFF Switcher. This is the
collector of an internal NPN Power switch. Minimize the
metal trace area connected to this pin to minimize EMI.
SW1 (Pin 20): Switch Pin for AVDD Switcher. This is the
collector of an internal NPN Power switch. Minimize the
metal trace area connected to this pin to minimize EMI.
SGND (Pin 25): Signal Ground. This is the Exposed Pad,
which must be soldered directly to a local ground plane to
achieve optimum thermal performance.
PGND14 (Pin 21): Power Ground for LED Switcher and
AVDD Switcher. Tie directly to local ground plane.
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LT1942
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BLOCK DIAGRA
SS1
SW1
11
20
AVDD BOOST CONVERTER
SOFT-START
COMPARATOR
ENABLE
1.250V
REFERENCE
13
R
Σ
+
R
+
–
S
7
28mV
RAMP-PULSE
GENERATOR
–
ONE
SHOT
Q1
Q
–
Q
SHDN
S
+
–
FB1 14
DRIVER
–
+
+
PGOOD
PGND14
VOFF INVERTING CONVERTER
ENABLE
VCC
–150mV
REFERENCE
22
–
24
DRIVER
–
NFB2
+
6
SW2
COMPARATOR
R
S
Q2
Q
+
1MHz
OSCILLATOR
+
Σ
D2
D2
–
RAMP-PULSE
GENERATOR
1.250V
REFERENCE
+
5
2
D3
COMPARATOR
3
DRIVER
–
FB3
D3
SW3
VON BOOST CONVERTER
ENABLE
23
–
R
S
Q
Q5
Q3
+
VOUT3
4
+
PNP
DRIVE
Σ
–
RAMP-PULSE
GENERATOR
CT
1
PGND23
DELAY
GENERATOR
9
LED BOOST CONVERTER
SS4
40V
12
CTRL4
R
S
Q
LED1
LED2
18
17
Q4
+
10k
+
–
Σ
Q6
–
FB4
DRIVER
–
+
200mV
D4
COMPARATOR
90k
10
16
19
SOFT-START
AGND
D4
SW4
ENABLE
8
RAMP-PULSE
GENERATOR
21 PGND14
1942BD
Q7
15
FB4
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LT1942
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OPERATIO
Refer to the figure on the first page of this data sheet and
the Block Diagram when following the description of the
operation of the LT1942. The LT1942 contains four switching regulators. Three switchers are designed to be used as
the power supply for driving small to medium sized TFT
displays. The fourth switcher is designed to power an LED
backlight. All four switching regulators use a constant
frequency, current mode control scheme to provide excellent line and load regulation.
TFT Supply
The three switching regulators for the TFT supply are
named AVDD, VOFF and VON. Both the AVDD and VON
switchers are boost type, while the VOFF is an inverting
type. Power sequencing circuitry is integrated into the
LT1942 to properly power up the TFT power supply. When
the SHDN pin is driven above 1V initially, only the AVDD
switcher is enabled. After the output of the AVDD switcher
reaches 97% of its final value, the PGOOD pin is driven low
and both the VOFF and VON switchers are enabled. This
event is latched rejecting any later drops on AVDD below
97%. The output-disconnect circuitry (Q5 in the Block
Diagram) for the VON switcher is not activated until the
programmable delay set by the CT pin has elapsed. Once
the output-disconnect circuitry is enabled, the VOUT3 pin is
driven to nearly the same voltage as D3. The VOUT3 pin can
only source current and is limited to 15mA nominally. All
three switching regulators are internally compensated and
operated at a fixed frequency of 1MHz. The AVDD switching regulator has integrated soft-start. Simply place a
capacitor from the SS1 pin to GND to implement soft-start.
LED Supply
The fourth switching regulator in the LT1942 is designed
to drive up to 20 LEDs (2 strings of 10) to power a
backlight. The LED switcher has an integrated current
ballaster (Q6 and Q7 in the Block Diagram) which enables
two strings of LEDs to have active current matching. The
current into LED1 and LED2 will always be actively matched,
regardless of which LED string has the highest voltage
drop. The LED ballast circuitry can be left unused when
driving a single string of LEDs. Simply leave the LED1 and
LED2 inputs floating and return the LED string directly to
the FB4 pin. The LED switcher has an independent control
pin which serves two functions: shutdown and reference
adjustment. The CTRL4 pin needs to be driven above
0.25V to enable the LED switcher. When CTRL4 is between
0.25V and 2.5V, the internal reference for the LED switcher
is attenuated. If CTRL4 is above 2.5V, the internal reference is limited to 200mV no matter how high CTRL4 is.
This feature allows an 8:1 dimming range for the LEDs.
The LED switching regulator is internally compensated
and operates at a fixed frequency of 1MHz. The LED
switching regulator has LED open-circuit protection. If
any LED fails open, D4 is not allowed to exceed 42V
(typical). This prevents damage to the power switch. The
LED switching regulator also has built in soft-start. By
placing a capacitor from the SS4 pin to GND, the user can
program the soft-start time.
1942fa
10
LT1942
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APPLICATIO S I FOR ATIO
Setting the Output Voltages
Unlike charge-pump-based TFT supplies, the LT1942 TFT
outputs can all be independently set. The AVDD, VOFF and
VON outputs are all inductively based DC/DC converters.
As such, the output voltages are set by choosing the right
feedback divider ratio. The AVDD feedback pin is FB1 and
the reference voltage is 1.25V. The VOFF feedback pin is
NFB2 and the reference voltage is –150mV. The VON
feedback pin is FB3 and the reference voltage is 1.25V. The
LED driver is also an inductively based DC/DC converter.
Output Disconnect
The VON switching regulator has built in output disconnect. When the VON supply is enabled, the boost converter
will charge up the D3 node. With the output disconnect
PNP (Q5) turned off, the voltage on VOUT3 will be zero.
Once the power sequencing circuitry turns on the PNP,
VOUT3 will go to nearly the same voltage as D3.
The VOFF switching regulator naturally achieves output
disconnect due to the switching topology (inverting configuration). When the LT1942 is in shutdown, VOFF will go
to zero volts.
The AVDD switching regulator provides a PGOOD pin to be
used for an external output disconnect function. This pin
can drive the gate of a PMOS device. When the LT1942 is
first enabled, the AVDD switching regulator begins to
charge up the output capacitor. Since PGOOD is still an
open collector at this point, the PMOS device is turned off,
leaving the AVDD output at zero volts. Once the output
capacitor charges to 97% of the final value, PGOOD
latches low turning on the PMOS device and bringing the
AVDD output quickly to the desired voltage.
Soft-Start
The higher current regulators, AVDD and LED, have user
programmable soft-start functions built into the part.
Simply place a capacitor from the SS1 pin to GND to
achieve soft-start for the AVDD switcher. For the LED
switcher, place a capacitor from the SS4 pin to GND. A
good value for both soft-start capacitors is 0.1µF. The
soft-start time is dependent on the load characteristics
which will vary depending on the type and size of the TFT
display. It is a good practice to experiment to determine
the best value for the soft-start capacitors.
Power Sequencing
The TFT supply has integrated supply sequencing built-in
to ensure proper initiation of the TFT display. Figure 1
shows a state diagram of the power sequencing circuitry.
The power sequencing circuitry insures that the AVDD
supply comes up first, then the negative VOFF supply and
lastly the VON positive supply. To pick the CT capacitor for
a particular delay, use the following equation:
CT =
10µA • tDELAY
0.7V
Figure 2 shows the power sequencing during start-up of
the TFT power supply for the circuit shown in the figure on
the first page of this data sheet. All bias supplies start up
in a well controlled and well timed manner. The LED
backlight driver is independently controlled via the CTRL4
pin.
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11
LT1942
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APPLICATIO S I FOR ATIO
SHUTDOWN
IQ < 1µA
START
VSHDN > 1V
AVDD ENABLED
VOFF DISABLED
VON DISABLED
FB1 > (97% OF 1.25V)
VSHDN < 0.4V
VOFF ENABLED
VON ENABLED
PGOOD GOES LOW
CT TIMER STARTS
VCT ≥ VBE
TFT SUPPLY
SEQUENCE
COMPLETE
OUTPUT DISCONNECT
PNP TURNS ON
1942 F01
Figure 1. Power Sequencing State Diagram
VC1
5V/DIV
VAVDD
5V/DIV
VOFF
10V/DIV
VD3
10V/DIV
VON
10V/DIV
VSHDN
1V/DIV
2ms/DIV
1942 F02
Figure 2. Power Sequencing During TFT Supply Start-Up
(Refer to Figure 4 Node Names)
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LT1942
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APPLICATIO S I FOR ATIO
Layout Hints
Proper layout techniques must be followed to achieve
advertised performance of the part. Keep all SW and FB
traces short and small. There are several power ground
pins on the LT1942 and it is important to implement the
grounding properly. PGND14 is the power ground pin for
the AVDD and LED switchers. The output capacitor for both
the AVDD and LED switcher must be returned back to this
pin before being allowed to mix into the ground plane.
Similarly, PGND23 is the power ground pin for the VON and
VOFF switcher. Again, both output capacitors for the VON
and VOFF switchers need to return to the PGND23 pin
before mixing with the ground plane. Place the input
decoupling capacitor, which ties from the VCC pin to GND,
as close to the part as possible. Please refer to Figure 3 for
a recommended layout of the power path components.
The component names are those shown in the circuit of
Figure 4.
VOFF OUTPUT
AVDD OUTPUT
VIN
L1
C1
L4
C2
L5
C6
C5
D1
VIN
L2
L3
LED1 STRING
LED2 STRING
LED OUTPUT
1
C3
C4
LT1942
VON OUTPUT
GND
PLANE
1942 F03
Figure 3: Power Path Component Recommended Layout
(Refer to Figure 4 Component Names)
1942fa
13
LT1942
U
TYPICAL APPLICATIO S
L1
22µH
VIN
3V TO 4.2V
C5
22µF
C3
0.22µF
16V
VON
10V
2mA
L3 22µH
R5
698k
D3
FB3
R1
301k
VCC
SW3
SW1
FB1
R6
100k
VOFF
–10V
2mA
R4
10k
R3
665k
VIN
D2
LED1
SW2
LED2
SHDN SGND/
CTRL4 AGND
SHUTDOWN
AVDD
5V
40mA
C4
4.7µF
25V X5R
20mA
20mA
L4 33µH
VIN
D4
SS1
FB4
CT
SS4
C7
0.1µF
C8
0.1µF
LED CONTROL
LED CONTROL VOLTAGE LED CURRENT
>2.5V
20mA
0.25V
<0.1V
SW4
LT1942
NFB2
L2 47µH
C1
4.7µF
6.3V
R2
100k
R8
1M
PGOOD
VOUT3
C5 0.1µF 16V
L5 47µH
C6
4.7pF
PGND14
PGND23
C2
0.22µF
16V
M1
PMOS
D1
C9
0.1µF
R7
4.99Ω
2.5mA
DISABLED
1942 F04a
C1 TO C9: X5R OR X7R
D1: CMDSH-3 CENTRAL SEMICONDUCTOR
L1: 22µH MURATA LQH32CN220K53
L2, L5: 47µH TAIYO YUDEN LB2012B470
L3: 22µH TAIYO YUDEN LB2012B220
L4: 33µH SUMIDA CDPH4D19-330MC
M1: Si2301BDS SILICONIX
Figure 4. TFT Bias (5V, 10V, –10V) and White LED Backlight Supply from Single Li-Ion Cell
Efficiency
90
EFFICIENCY (%)
AVDD LOAD = 40mA
VOFF LOAD = 2mA
V
85
ON LOAD = 2mA
VIN = 4.2V
80
75
VIN = 3.6V
VIN = 3V
70
65
60
5
10
25
35
40
20
30
15
TOTAL LED CURRENT (mA) 1942 F04b
AVDD Transient Response, ILOAD Stepped from
30mA→40mA→30mA
TFT Power Supply Sequencing
VAVDD
5V/DIV
VAVDD
20mV/DIV
AC-COUPLED
VOFF
10V/DIV
VON
10V/DIV
ILI
50mA/DIV
VSHDN
1V/DIV
2ms/DIV
1942 F04c
100µs/DIV
1942 F04d
1942fa
14
LT1942
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PACKAGE DESCRIPTIO
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
0.23 TYP
R = 0.115
(4 SIDES)
TYP
23 24
0.75 ± 0.05
0.38 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF24) QFN 1103
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
1942fa
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
LT1942
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1613
550mA (ISW), 1.4MHz High Efficiency Step-Up
DC/DC Converter
VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1µA, ThinSOT Package
LT1615/LT1615-1
300mA/80mA (ISW), Constant Off-Time, High Efficiency
Step-Up DC/DC Converter
VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD < 1µA,
ThinSOT Package
LT1930/LT1930A
1A (ISW), 1.2MHz/2.2MHz High Efficiency Step-Up
DC/DC Converter
VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD < 1µA,
ThinSOT Package
LT1932
Constant Current, 1.2MHz, High Efficiency White
LED Boost Regulator
VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1µA,
ThinSOT Package
LT1943 (Quad)
Quad Output, 2.6A Buck, 2.6A Boost, 0.3A Boost,
0.4A Inverter, 1.2MHz TFT DC/DC Converter
VIN: 4.5V to 22V, VOUT(MAX) = 40V, IQ = 10mA, ISD < 35µA,
TSSOP28E Package
LT1944/LT1944-1
(Dual)
Dual Output 350mA/100mA (ISW), Constant Off-Time,
High Efficiency Step-Up DC/DC Converter
VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD < 1µA, MS10 Package
LT1945
Dual Output, Pos/Neg, 350mA (ISW), Constant Off-Time,
High Efficiency Step-Up DC/DC Converter
VIN: 1.2V to 15V, VOUT(MAX) = ±34V, IQ = 20µA, ISD < 1µA, MS10 Package
LT1947
Adjustable Output TFT-LCD Triple Switching Regulator
VIN: 2.7V to 8V, VOUT(MAX) = 30V, IQ = 9.5mA, ISD < 1µA, MS10 Package
LTC3450
Triple Output Power Supply for Small TFT-LCDs
VIN: 1.5V to 4.6V, VOUT(MAX) = ±15V, IQ = 75µA, ISD < 1µA, DFN Package
LT3461/LT3461A
0.3A (ISW), 1.3MHz/3MHz, High Efficiency Step-Up
DC/DC Converter with Integrated Schottky
VIN: 2.5V to 16V, VOUT(MAX) = 38V, IQ = 2.8mA, ISD < 1µA,
SC70, ThinSOT Packages
LT3464
0.08A (ISW), High Efficiency Step-Up DC/DC Converter
with Integrated Schottky, Output Disconnect
VIN: 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25µA, ISD < 1µA,
ThinSOT Package
LT3465/LT3465A
Constant Current, 1.2MHz/2.7MHz High Efficiency
White LED Boost Regulator with Integrated Schottky
VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1µA,
ThinSOT Package
LT3466
Dual Constant Current, 2MHz, High Efficiency White LED
Boost Regulator with Integrated Schottky
VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16µA, DFN Package
LT3467/LT3467A
1.1A (ISW), 1.3MHz/2.1MHz, High Efficiency Step-Up
DC/DC Converter with Integrated Soft-Start
VIN: 2.4V to 16V, VOUT(MAX) = 40V, IQ = 1.2mA, ISD < 1µA,
ThinSOT Package
1942fa
16
Linear Technology Corporation
LT/LT 0605 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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