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LT1942 Quad DC/DC Converter for Triple Output TFT Supply Plus LED Driver
FEATURES

DESCRIPTIO
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 x 4mm)
The LT(R)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. 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. The LT1942 is available in a low profile (0.75mm) 24-lead QFN (4mm x 4mm) package.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

Poly-Silicon TFT Displays Amorphous Silicon TFT Displays
TFT Bias and White LED Backlight Power Supply
VIN 3V TO 4.2V 0.22F 698k VON 10V 2mA 10k 0.22F VOFF -10V 2mA 665k 47H 47H 0.1F 0.22F 100k PGND23 VOUT3 NFB2 D2 SW2 SHDN SGND/ CTRL4 AGND SS1 SS4 0.1F 0.1F LT1942 D3 FB3 SW3 22H 22H VCC SW1 FB1 PGND14 PGOOD SW4 D4 LED1 LED2 FB4 CT 0.1F 4.99
1942 TA01
CMDSH-3
301k
4.7pF 4.7F
100k
33H
VIN
VIN SHUTDOWN LED CONTROL
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Si2301BDS 1M AVDD 5V 40mA 4.7F 20mA 20mA
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LT1942
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
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 ......................... 125C Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................ - 65C to 125C
PGND14
SW2
SW1
SW4
VCC
ORDER PART NUMBER LT1942EUF
18 LED1 17 LED2
24 23 22 21 20 19 PGND23 1 SW3 2 D3 3 VOUT3 4 FB3 5 NFB2 6 7 8 9 10 11 12 25
D2
16 D4 15 FB4 14 FB1 13 PGOOD
UF PART MARKING 1942
AGND
SS1
SHDN
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 37C/W, JC = 4.3C/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
PARAMETER Quiescent Current Input Voltage Range AVDD Feedback Voltage AVDD Feedback Pin Bias Current (FB1) AVDD Feedback Voltage for Power Good Signal VOFF Feedback Voltage VOFF Feedback Pin Bias Current (NFB2) VON Feedback Voltage VON Feedback Pin Bias Current (FB3) LED Feedback Voltage LED Feedback Pin Bias Current (FB4) LED Feedback Voltage with Dimming
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
CONDITIONS VSHDN = 3.3V, VCTRL4 = 0V, Not Switching VSHDN = VCTRL4 = 0V, In Shutdown VCC Pin FB1 Pin FB1 Pin (Note 3) FB1 Pin Voltage as a Percent of Nominal Voltage NFB2 Pin NFB2 Pin (Note 3) FB3 Pin FB3 Pin (Note 3) FB4 Pin, VCTRL4 = 3.3V FB4 Pin (Note 3) FB4 Pin, VCTRL4 = 1V 85

CTRL4
SS4
CT
MIN
TYP 7 0
MAX 10 1 16 1.275 60 100 -140 60 1.275 60 210 60 105
UNITS mA A V V nA % mV nA V nA mV A mV
2.6 1.225 95 -160 1.225 190 1.25 15 97 -150 25 1.25 25 200 20 95
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LT1942
ELECTRICAL CHARACTERISTICS
PARAMETER LED1-LED2 Current Matching LED1 Voltage LED2 Voltage LED1, LED2 Maximum Current LED1, LED2 Leakage Current AVDD Maximum Duty Cycle VOFF Maximum Duty Cycle VON Maximum Duty Cycle LED Maximum Duty Cycle AVDD Switch Current Limit at Minimum DC (SW1) AVDD Switch Current Limit at Maximum DC (SW1) VOFF Switch Current Limit at Minimum DC (SW2) VOFF Switch Current Limit at Maximum DC (SW2) VON Switch Current Limit at Minimum DC (SW3) VON Switch Current Limit at Maximum DC (SW3) LED Switch Current Limit at Minimum DC (SW4) LED Switch Current Limit at Maximum DC (SW4) VOFF Schottky Diode Forward Drop VOFF Schottky Diode Leakage Current VON Schottky Diode Forward Drop VON Schottky Diode Leakage Current LED Schottky Diode Forward Drop LED Schottky Diode Leakage Current Switching Frequency SHDN Pin Current CTRL4 Pin Current SW1-SW4 leakage Current SW1 VCESAT SW2 VCESAT SW3 VCESAT SW4 VCESAT SS1 Charging Current SS4 Charging Current SHDN Input Voltage High SHDN Input Voltage Low CTRL4 Input Voltage High CTRL4 Input Voltage Low
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
CONDITIONS Current Into Each Pin = 20mA, VLED1 = VLED2 Current Into Each Pin = 20mA, |VLED1-VLED2| = 1V 20mA Into Pin, VLED2 = 2V, FB4 = 0 20mA Into Pin, VLED1 = 2V, FB4 = 0 Into Either Pin, VLEDX < 1.5V, FB4 = 0 VSHDN = 0V, VLEDX = 45V SW1 Pin SW2 Pin SW3 Pin SW4 Pin SW1 Pin (Note 4) SW1 Pin (Note 4) SW2 Pin (Note 4) SW2 Pin (Note 4) SW3 Pin (Note 4) SW3 Pin (Note 4) SW4 Pin (Note 4) SW4 Pin (Note 4) D2 Pin, I = 60mA D2 Pin, VD2 = -36V D3 Pin, I = 60mA VD3 = 36V, SW3 = 0V, Output Disconnect PNP Off I = 250mA VD4 = 36V, SW4 = 0V VSHDN = 3V VSHDN = 0V VCTRL4 = 3V VCTRL4 = 0V VSWX = 36V ISW1 = 100mA (Note 5) ISW2 = 40mA (Note 5) ISW3 = 40mA (Note 5) ISW4 = 400mA (Note 5) VSS = 0.5V VSS = 0.5V 1 1 1 0.4 0.25 0.1

MIN
TYP 0 1 1 1
MAX 1.5 2.5 1.2 1.2 1
UNITS % % V V mA A % % % %
35 88 75 75 88 150 80 50 30 50 30 550 450 93 86 86 93 200 140 80 60 80 60 750 600 0.65 1 0.65 1 0.75 1 0.8

250 200 110 90 110 90 900 850
mA mA mA mA mA mA mA mA V A V A V A MHz A nA A nA A mV mV mV mV A A V V V V
1 70 20 30 -60 0.01 200 150 150 280 3 3
1.2 100 30 60 -150 1 300 250 250 400 5 5
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ELECTRICAL CHARACTERISTICS
PARAMETER PGOOD Leakage Current PGOOD Sink Current CT Charging Current CT Reference Voltage VOUT3 Leakage Current VOUT3 Source Current Minimum D3/VOUT3 Operating Voltage D4 LED Open-Circuit Voltage
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VSHDN = VIN, unless otherwise noted. (Note 2)
CONDITIONS PGOOD NPN Off, VPGOOD = 5V PGOOD NPN On, VPGOOD = 1V 0.5 MIN TYP 0 0.7 12 0.8 Output Disconnect PNP Off, D3 = 36V, VOUT = 0V Output Disconnect PNP On, D3 = 3V, VOUT = 2.5V Minimum D3 Voltage for Proper Output Disconnect PNP Operation 38 10 0 15 3 42 44 1 MAX 0.1 UNITS A mA A V A mA V V
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 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical controls.
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.
TYPICAL PERFOR A CE CHARACTERISTICS
VFB1 Voltage
1.28 1.27
VOLTAGE (mV)
VOLTAGE (V)
-149 -150 -151 -152 -153 -154
1.25 1.24 1.23 1.22 -40 -20
VOLTAGE (V)
1.26
40 20 60 0 TEMPERATURE (C)
4
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80
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NFB2 Voltage
-145 -146 -147 -148
1.27 1.26 1.25 1.24 1.23 1.28
VFB3 Voltage
100
-155 -40
-20
40 20 0 60 TEMPERATURE (C)
80
100
1.22 -40 -20
40 20 60 0 TEMPERATURE (C)
80
100
1942 G02
1942 G01
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LT1942 TYPICAL PERFOR A CE CHARACTERISTICS
FB4 Voltage
210 208 206 204
200 VFB4 (mV) TA = -40C 150 TA = 85C 100 50 0
-20 40 20 0 60 TEMPERATURE (C) 80 100
OSCILLATOR FREQUENCY (kHz)
VFB4 (mV)
202 200 198 196 194 192 190 -40
Quiescent Current
10.0 9.5
QUIESCENT CURRENT (mA)
SW1 SATURATION VOLTAGE (mV)
9.0
DELAY TIME (ms)
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 -40 -20 40 20 0 60 TEMPERATURE (C) 80 100
SW2 Saturation Voltage
400
SW2 SATURATION VOLTAGE (mV)
TA = 25C
SW3 SATURATION VOLTAGE (mV)
300 250 200 150 100 50 0 0 10 40 30 SW2 CURRENT (mA) 20 50 60
1942 G10
300 250 200 150 100 50 0 0 10 40 30 SW3 CURRENT (mA) 20 50 60
1942 G10
SW4 SATURATION VOLTAGE (mV)
350
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VFB4 vs CTRL4
300 250
1200 1100 1000 900 800 700
Oscillator Frequency
TA = 25C
0
0.5
1
1.5 2 CTRL4 (V)
2.5
3
1942 G05
600 -40 -20
40 20 60 0 TEMPERATURE (C)
80
100
1942 G06
CT Timer Delay Time
10 9 8 7 6 5 4 3 2 1 0 -40 -20 40 20 0 60 TEMPERATURE (C) 80 100 CT = 0.1F
600 500 400 300 200 100 0
SW1 Saturation Voltage
TA = 25C
0
25
50 75 100 125 150 175 200 SW1 CURRENT (mA)
1942 G09
1942 G07
1942 G08
SW3 Saturation Voltage
400 350 TA = 25C 800 700 600 500 400 300 200 100 0
SW4 Saturation Voltage
TA = 25C
0
100 200 300 400 500 600 700 800 SW4 CURRENT (mA)
1942 G12
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LT1942 TYPICAL PERFOR A CE CHARACTERISTICS
LED1 to LED2 Current Matching
2.0
LED1 CURRENT MATCHING ERROR (%)
VLED1 = VLED2 + 1V
D3-VOUT3 VOLTAGE DROP (mV)
1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 2 6 10
TA = 85C
TA = 25C TA = -40C
300 250 200 150 100 50 TA = -40C 1 IOUT3 CURRENT (mA) 0.1 10
1942 G14
VOUT4 VOLTAGE (V)
14 18 22 26 LED2 CURRENT (mA)
D4 Forward Voltage
1400 1200
D4 FORWARD VOLTAGE (mV)
TA = 25C
VOFF DIODE FORWARD VOLTAGE (mV)
1200 1000 800 600 400 200 0 0.1
1000 800 600 400 200 0 0.1
VON DIODE FORWARD VOLTAGE (mV)
1
10 100 D4 CURRENT (mA)
AVDD Switcher Current Limit
300 250
700
TA = 25C
200
ILIM (mA)
ILIM (mA)
150 100 50
0 20 30 40 60 50 70 DUTY CYCLE (%) 80 90
6
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30 34
1942 G13
VOUT3 vs IOUT3
500 450 400 350
44.0 43.8 43.6 43.4 43.2 43.0 42.8 42.6 42.4 42.2
LED Open-Circuit Protection Voltage
TA = 25C TA = 85C
0 0.01
42.0 -40
-20
40 20 0 60 TEMPERATURE (C)
80
100
1942 G08
VOFF Diode Forward Voltage
1400 TA = 25C 1400 1200 1000 800 600 400 200
VON Diode Forward Voltage
TA = 25C
1000
1942 G16
10 100 VOFF DIODE CURRENT (mA) 1
1000
1942 G17
0 0.1
10 100 1 VON DIODE CURRENT (mA)
1000
1942 G19
LED Switcher Current Limit
900 800 TA = 25C
600 500 400 300 200 100 0 20 30 40 50 60 70 DUTY CYCLE (%) 80 90
1942 G18
1942 G20
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LT1942
PI FU CTIO S
PGND23 (Pin 1): Power Ground for VOFF and VON Switcher. 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. 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 0.1F. 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). 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. 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. 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. AGND (Pin 10): Analog Ground. Tie directly to local ground plane. SS1 (Pin 11): Soft-Start Pin for AVDD Switcher. Place a soft-start capacitor from this pin to ground. Upon start-up, 3A of current charges the capacitor to 1.25V. Use larger capacitor for slower start-up. Leave floating if not in use. SS4 (Pin 12): Soft-Start for LED Switcher. Place a softstart capacitor from this pin to ground. Upon start-up, 3A of current charges the capacitor to 1.25V. Use larger capacitor for slower start-up. Leave floating if not in use. 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. 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 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. D4 (Pin 16): Output of LED Switcher. Place output decoupling capacitor from this pin to local ground plane. 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.
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LT1942
PI FU CTIO S
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. 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. 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. PGND14 (Pin 21): Power Ground for LED Switcher and AVDD Switcher. Tie directly to local ground plane. VCC (Pin 22): Input Supply. Must be locally bypassed. D2 (Pin 23): Anode Connection of Internal Schottky Diode for VOFF Switcher. 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. 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.
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LT1942
BLOCK DIAGRA W
SS1 11 SOFT-START ENABLE PGOOD 13 FB1 14 1.250V REFERENCE COMPARATOR AVDD BOOST CONVERTER SW1 20
+ - + -
28mV
-
R S Q
DRIVER Q1
+ +
RAMP-PULSE GENERATOR VOFF INVERTING CONVERTER COMPARATOR
7
ONE SHOT
VCC 22
ENABLE
-150mV REFERENCE NFB2 6
RAMP-PULSE GENERATOR VON BOOST CONVERTER ENABLE 1.250V REFERENCE FB3 5
+ - - +
RAMP-PULSE GENERATOR
COMPARATOR DRIVER R S Q Q3
LED BOOST CONVERTER SS4 CTRL4 8 12 ENABLE SOFT-START 90k COMPARATOR 40V SW4 19 D4 DRIVER R S Q Q4 LED1 18 LED2 17 D4 16
- + + -
RAMP-PULSE GENERATOR
AGND 10 200mV
10k FB4
21 PGND14
1942BD
-
-
+
1MHz OSCILLATOR
+
-
+
- - +
SHDN
Q
R
S
-
R S Q
DRIVER Q2
+
D2 D2 23 SW3 2 D3 D3 3
-
PGND14 SW2 24 Q5 VOUT3 4 PNP DRIVE CT 1 PGND23 DELAY GENERATOR 9 Q6 Q7 15 FB4
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LT1942
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.
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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.
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LT1942
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
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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 0.1F. 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 = 10A * 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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LT1942
APPLICATIO S I FOR ATIO
SHUTDOWN IQ < 1A START
VSHDN > 1V
AVDD ENABLED VOFF DISABLED VON DISABLED
VSHDN < 0.4V
TFT SUPPLY SEQUENCE COMPLETE
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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FB1 > (97% OF 1.25V) PGOOD GOES LOW VOFF ENABLED VON ENABLED CT TIMER STARTS VCT VBE OUTPUT DISCONNECT PNP TURNS ON
1942 F01
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LT1942
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.
VOFF OUTPUT
C2
L5 L2 L3 C3
VON OUTPUT
Figure 3: Power Path Component Recommended Layout (Refer to Figure 4 Component Names)
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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.
AVDD OUTPUT VIN L1 C1 L4 C6 C5 D1 VIN 1 LED1 STRING LED2 STRING LED OUTPUT C4 LT1942 GND PLANE
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LT1942
TYPICAL APPLICATIO S
VIN 3V TO 4.2V L1 22H C5 22F C3 0.22F 16V L3 22H R5 698k R6 100k PGND23 C2 0.22F 16V VOFF -10V 2mA R4 10k R3 665k VIN SHUTDOWN LED CONTROL LED CONTROL VOLTAGE LED CURRENT >2.5V 20mA 0.25V <0.1V 2.5mA DISABLED C5 0.1F 16V L5 47H C6 L2 47H VOUT3 NFB2 D2 SW2 SHDN SGND/ CTRL4 AGND SS1 SS4 LT1942 D3 FB3 SW3 VCC SW1 FB1 PGND14 PGOOD SW4 D4 LED1 LED2 FB4 CT C9 0.1F R7 4.99
1942 F04a
VON 10V 2mA
Figure 4. TFT Bias (5V, 10V, -10V) and White LED Backlight Supply from Single Li-Ion Cell
EFFICIENCY (%)
TFT Power Supply Sequencing
VAVDD 5V/DIV VOFF 10V/DIV
VON 10V/DIV
VSHDN 1V/DIV 2ms/DIV
1942 F04c
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U
D1 R1 301k R2 100k 4.7pF C1 4.7F 6.3V R8 1M
M1 PMOS
AVDD 5V 40mA
C4 4.7F 25V X5R
20mA
20mA
L4 33H
VIN
C7 0.1F C8 0.1F
C1 TO C9: X5R OR X7R D1: CMDSH-3 CENTRAL SEMICONDUCTOR L1: 22H MURATA LQH32CN220K53 L2, L5: 47H TAIYO YUDEN LB2012B470 L3: 22H TAIYO YUDEN LB2012B220 L4: 33H SUMIDA CDPH4D19-330MC M1: Si2301BDS SILICONIX
Efficiency
90 AVDD LOAD = 40mA VOFF LOAD = 2mA VON LOAD = 2mA 85 80 75 VIN = 3.6V 70 65 60 5 10 25 35 40 20 30 15 TOTAL LED CURRENT (mA) 1942 F04b VIN = 3V VIN = 4.2V
AVDD Transient Response, ILOAD Stepped from 30mA40mA30mA
VAVDD 20mV/DIV AC-COUPLED
ILI 50mA/DIV
100s/DIV
1942 F04d
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LT1942
PACKAGE DESCRIPTIO U
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW--EXPOSED PAD 0.23 TYP R = 0.115 (4 SIDES) TYP 23 24 0.38 0.10 1 2 2.45 0.10 (4-SIDES)
(UF24) QFN 1103
4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES)
4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
0.200 REF 0.00 - 0.05 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
0.25 0.05 0.50 BSC
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.
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LT1942 RELATED PARTS
PART NUMBER LT1613 LT1615/LT1615-1 LT1930/LT1930A LT1932 LT1943 (Quad) LT1944/LT1944-1 (Dual) LT1945 LT1947 LTC3450 LT3461/LT3461A LT3464 LT3465/LT3465A LT3466 LT3467/LT3467A DESCRIPTION 550mA (ISW), 1.4MHz High Efficiency Step-Up DC/DC Converter 300mA/80mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter 1A (ISW), 1.2MHz/2.2MHz High Efficiency Step-Up DC/DC Converter Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Quad Output, 2.6A Buck, 2.6A Boost, 0.3A Boost, 0.4A Inverter, 1.2MHz TFT DC/DC Converter Dual Output 350mA/100mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter Dual Output, Pos/Neg, 350mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter Adjustable Output TFT-LCD Triple Switching Regulator Triple Output Power Supply for Small TFT-LCDs 0.3A (ISW), 1.3MHz/3MHz, High Efficiency Step-Up DC/DC Converter with Integrated Schottky 0.08A (ISW), High Efficiency Step-Up DC/DC Converter with Integrated Schottky, Output Disconnect Constant Current, 1.2MHz/2.7MHz High Efficiency White LED Boost Regulator with Integrated Schottky Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky 1.1A (ISW), 1.3MHz/2.1MHz, High Efficiency Step-Up DC/DC Converter with Integrated Soft-Start COMMENTS VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1A, ThinSOT Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, ThinSOT Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD < 1A, ThinSOT Package VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1A, ThinSOT Package VIN: 4.5V to 22V, VOUT(MAX) = 40V, IQ = 10mA, ISD < 35A, TSSOP28E Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, MS10 Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, MS10 Package VIN: 2.7V to 8V, VOUT(MAX) = 30V, IQ = 9.5mA, ISD < 1A, MS10 Package VIN: 1.5V to 4.6V, VOUT(MAX) = 15V, IQ = 75A, ISD < 1A, DFN Package VIN: 2.5V to 16V, VOUT(MAX) = 38V, IQ = 2.8mA, ISD < 1A, SC70, ThinSOT Packages VIN: 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25A, ISD < 1A, ThinSOT Package VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT Package VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16A, DFN Package VIN: 2.4V to 16V, VOUT(MAX) = 40V, IQ = 1.2mA, ISD < 1A, ThinSOT Package
1942fa
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT/LT 0605 REV A * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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