Transcript
LT1942 Quad DC/DC Converter for Triple Output TFT Supply Plus LED Driver FEATURES ■ ■ ■
■ ■ ■ ■
■ ■ ■ ■
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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.
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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Poly-Silicon TFT Displays Amorphous Silicon TFT Displays
The LT1942 is available in a low profile (0.75mm) 24-lead QFN (4mm × 4mm) package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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TYPICAL APPLICATIO
TFT Bias and White LED Backlight Power Supply 22µH
VIN 3V TO 4.2V
22µH
2.2µF 698k 0.22µF
VON 10V 2mA
D3 FB3
VCC
SW3
SW1 FB1
665k
VOUT3
VIN SHUTDOWN LED CONTROL
NFB2 47µH 47µH
0.1µF
4.7pF 4.7µF
100k
1M
4.7µF
AVDD 5V 40mA
20mA
20mA
PGND14 PGND23
0.22µF
Si2301BDS 301k
100k 10k
VOFF –10V 2mA
CMDSH-3
PGOOD SW4
LT1942
D4
D2
LED1
SW2
LED2
SHDN SGND/ CTRL4 AGND
SS1
SS4 0.1µF
33µH
VIN
FB4 CT 0.1µF
4.99Ω
1942 TA01
0.1µF
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LT1942 AXI U
RATI GS U
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ABSOLUTE
U U W 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
●
●
TYP
MAX
UNITS
7 0
10 1
mA µA
16
V
1.25
1.275
V
15
60
nA
95
97
100
%
–160
–150
–140
mV
25
60
nA
1.225
1.25
1.275
V
25
60
nA
190
200
210
mV
20
60
µA
95
105
mV
85
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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: 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 LT1942 is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating
1
µA
15
mA
3
V
42
44
V
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.
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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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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
200 198
TA = –40°C
OSCILLATOR FREQUENCY (kHz)
208
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)
300 200 100
50
60 1942 G10
0
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
400
SW3 Saturation Voltage SW3 SATURATION VOLTAGE (mV)
SW2 SATURATION VOLTAGE (mV)
400
500
0
100
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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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
1
10 100 VOFF DIODE CURRENT (mA)
1942 G16
1000
800 600 400 200 0 0.1
1 10 100 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 0
100
20
30
40
60 50 70 DUTY CYCLE (%)
80
90
1942 G18
0
20
30
50 40 60 70 DUTY CYCLE (%)
80
90
1942 G20
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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. 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.
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. 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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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.
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.
PGND14 (Pin 21): Power Ground for LED Switcher and AVDD Switcher. Tie directly to local ground plane.
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BLOCK DIAGRA
SS1
SW1
11
20
AVDD BOOST CONVERTER SOFT-START COMPARATOR
ENABLE 1.250V REFERENCE
13
R
+
–
FB1 14
ENABLE
VCC
+ –
S
–
7
Q1
Q
Σ
+ R
ONE SHOT
S
–
Q
SHDN
DRIVER
–
+
+
PGOOD
28mV RAMP-PULSE GENERATOR
PGND14
VOFF INVERTING CONVERTER –150mV REFERENCE
22
–
24 DRIVER
–
NFB2
+
6
SW2
COMPARATOR
R
+
1MHz OSCILLATOR
S
Q2
Q
+ Σ
D2
D2
– RAMP-PULSE GENERATOR ENABLE
+ 5
2
D3
COMPARATOR
3
DRIVER
–
FB3
D3
SW3 VON BOOST CONVERTER
1.250V REFERENCE
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
+
10k
S
Q
LED1
LED2
18
17
Q4
– Σ
Q6
–
FB4
DRIVER
–
90k 200mV
D4
COMPARATOR
+
10
16
19 SOFT-START
AGND
D4
SW4
ENABLE
8
RAMP-PULSE GENERATOR
21 PGND14 1942BD
Q7
15 FB4
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LT1942 U 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.
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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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LT1942 U
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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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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)
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LT1942 U
TYPICAL APPLICATIO S L1 22µH
VIN 3V TO 4.2V
C5 2.2µF C3 0.22µF 16V
VON 10V 2mA
L3 22µH R5 698k
D3 FB3
SW3
R1 301k SW1 FB1
VCC
R6 100k
VOFF –10V 2mA
R4 10k R3 665k VIN
LED1
SW2
LED2 SS1
LED CONTROL VOLTAGE LED CURRENT >2.5V 20mA
AVDD 5V 40mA
C4 4.7µF 25V X5R
20mA
20mA
L4 33µH
VIN
FB4 CT
SS4
C7 0.1µF C8 0.1µF
LED CONTROL
0.25V <0.1V
D4
D2 SHDN SGND/ CTRL4 AGND
SHUTDOWN
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
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LT1942 U
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
PIN 1 TOP MARK (NOTE 6)
0.38 ± 0.10 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
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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
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Linear Technology Corporation
LT/LT 1106 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507
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© LINEAR TECHNOLOGY CORPORATION 2004
