Contact Linear Technology for Potential Replacement
FOR INFORMATION PURPOSES ONLY
OBSOLETE:
LT1948
DC/DC Converter for
TFT-LCD Panels
August 2000
Final Electrical Specifications
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Complete Solution Under 1.2mm
Develops Three Outputs from a 3.3V or 5V Supply
Externally Programmable V
ON
Delay
Fixed Frequency Low Noise Outputs
All Ceramic Capacitors
Operates at 3MHz Switching Frequency
Fast Transient Response
Few External Components Required
2.6V to 6V Input Range
Tiny 8-Lead MSOP Package
APPLICATIO S
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TFT-LCD Notebook Display Panels
TFT-LCD Desktop Monitor Display Panels
Digital Cameras
Handheld Computers
The LT
®
1948 is a highly integrated multiple output DC/DC
converter designed for use in TFT-LCD panels. The device
contains two independent switching regulators: the main
regulator has an adjustable output voltage with an internal
1.1A switch that can generate a boosted voltage as high as
30V while the second regulator generates 23V at up to
10mA for positive bias. A simple level-shift charge pump
off the main switch node generates the negative bias
voltage. An external capacitor sets the delay time from
AV
DD
reaching final value to 23V appearing at the V
ON
pin.
The 3MHz switching frequency allows the use of tiny low
profile chip inductors and capacitors throughout, provid-
ing a low noise, low cost total solution with all components
under 1.25mm in height. The device operates from an
input range of 2.6V to 6V and is available in an 8-lead
MSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
D3A
BAT54S
L1, 2.2µH
V
IN
2.5V TO
6V
C1
2.2µF
L2, 2.2µH
L3
2.2µH
D2
CMDSH-3
C3
0.22µF
V
IN
SW2
LT1948
VO2
C
T
C6
22nF
C1, C2: TAIYO YUDEN LMK212BJ225
C3: TAIYO YUDEN TMK316BJ224
C4: TAIYO YUDEN EMK212BJ224
C5: TAIYO YUDEN LMK212BJ105
L1–L3: TAIYO YUDEN LB2012-2R2
Figure 1. 3.3V Powered TFT-LCD Bias Generator
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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TYPICAL APPLICATIO
D3B
C5
1µF
V
OFF
–8V
15mA
C4
0.22µF
D1
MBRM120
SW1
FB1
R1
53.6k
AV
DD
8V
260mA
C2
2.2µF
V
ON
23V
10mA
V
IN
5V/DIV
V
ON
20V/DIV
R2
10k
GND
V
ON
AV
DD
10V/DIV
V
OFF
10V/DIV
2ms/DIV
1948 TA01a
1948 F01
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Start-Up Waveforms
FEATURES
DESCRIPTIO
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1
LT1948
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
FB1
C
T
SW1
GND
1
2
3
4
8
7
6
5
V
ON
VO2
SW2
V
IN
V
IN
Voltage ................................................................ 8V
C
T
Voltage.................................................................. 6V
SW1, SW2 Voltage .................................................. 36V
FB Voltage ................................................................. 3V
V
ON
, VO2 Voltage..................................................... 30V
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1948EMS8
MS8 PART
MARKING
LTNR
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 200°C/W
Consult factory for Industrial and Military grade parts.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.3V unless otherwise specified.
SYMBOL
Supply Current
Reference Voltage
Reference Line Reg
C
T
Source Current
C
T
Voltage to Turn On Q3
FB1 Voltage to Begin C
T
Charge
SW1 Current Limit
SW2 Current Limit
SW1 Saturation Voltage
SW2 Saturation Voltage
Oscillator Frequency
Maximum Duty Cycle
0°C
≤
T
A
≤
85°C
–40°C
≤
T
A
≤
0°C
VO2 Pin Resistance
SW1, SW2 Error Amp Gain
SW1, SW2 Error Amp Gm
FB1 Regulation Voltage
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
Not Switching
2.7V < V
IN
< 8V
V
FB1
= 1.3V
MIN
TYP
7
1.26
0.01
MAX
13
UNITS
mA
V
%/V
4.5
1.25
1.17
5.5
1.28
1.20
1.5
0.8
350
250
6.5
1.30
1.23
(Note 3)
(Note 3)
I
SW1
= 800mA
I
SW2
= 300mA
q
1.2
0.5
410
300
3.6
90
2.4
70
69
67
3.2
75
Measured to Ground
400
100
50
1.240
1.230
22
1.260
0.01
23
200
0.01
0.01
1.280
1.285
0.05
24
260
5
2
µA/V
V
V
V
mV
µA
µA
FB1 Line Regulation
VO2 Regulation Voltage
V
ON
Switch Drop
SW1 Leakage Current
SW2 Leakage Current
2.7V < V
IN
< 8V
VO2 = 25V, 7mA Load from V
ON
, C
T
Voltage >1.30V
Switch Off, SW1 Voltage = 3.3V
Switch Off, SW2 Voltage = 3.3V
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1948 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 process controls.
Note 3:
Current limit guaranteed by design and/or correlation to static test.
2
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µA
V
V
A
A
mV
mV
MHz
%
%
%
kΩ
V/V
W
U
U
W W
W
LT1948
PI FU CTIO S
FB1 (Pin 1):
Feedback Pin for First Switcher. Connect
resistor divider tap here. Set AV
DD
according to AV
DD
=
1.26V(1 + R1/R2).
C
T
(Pin 2):
Timing Capacitor Pin. Connect a 22nF capacitor
from C
T
to ground to program a 3ms delay from FB1
reaching 1.26V to V
ON
turning on.
SW1 (Pin 3):
AV
DD
Switch Node. Connect inductor and D1
here (see Figure 1). Minimize trace area at this pin to keep
EMI down.
GND (Pin 4):
Ground. Connect directly to local ground
plane.
V
IN
(Pin 5):
Input Supply Pin. Must be bypassed with a
ceramic capacitor close to the pin.
SW2 (Pin 6):
VO2 Switch Node. Connect inductor and D2
here. Minimize trace area at this pin to keep EMI down.
VO2 (Pin 7):
Sense Pin for 23V Output. Connect to VO2
output capacitor. This node is also internally connected to
the emitter of Q3 (see Block Diagram), the high side switch
between VO2 and V
ON
.
V
ON
(Pin 8):
This is the Delayed 23V Output. V
ON
becomes
23V after the internal timer times out.
OPERATIO
To best understand operation of the LT1948, please refer
to the LT1948 Block Diagram. The device contains two
switching regulators, a timer and a high side switch. Three
outputs can be generated: an adjustable AV
DD
output, a
charge-pumped inversion of the AV
DD
output, called V
OFF
,
and a 23V/15mA output, called V
ON
. Q3 keeps V
ON
off for
an externally set time interval, set by a capacitor connected
to the C
T
pin.
The switching frequency of both switchers is 3MHz, set
internally. The switchers are current mode and are inter-
nally compensated. The main AV
DD
switcher is current
limited at 1.5A, while the second V
ON
switcher is limited to
800mA. They share the same 1.26V reference voltage.
AV
DD
R1
1
R2
D1
3
C2
4
2
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When the input voltage is below approximately 2.4V, an
undervoltage lockout circuit disables switching.
When AV
DD
is less than its final voltage, Q4 is turned on,
holding the C
T
pin at ground. When AV
DD
reaches final
value, Q4 lets go of the C
T
pin, allowing the 5.5µA current
source to charge the external capacitor, C
T
. When the
voltage on the C
T
pin reaches 1.25V, Q3 turns on, connect-
ing VO2 to V
ON
. Capacitor value can be calculated using
the following formula:
C = (5.5µA • t
DELAY
)/1.25V
A 22nF capacitor results in approximately 3ms of delay.
V
ON
8
7
6
5
L1–L2
V
IN
C1
L3
D2
C3
GND
C6
D3A
D4B
V
OFF
C4
1948 F02
Figure 2. Recommended Component Placement
3
LT1948
BLOCK DIAGRA
W
V
IN
GND
V
IN
FB1
–
–
+
+
REF
R
OSC
S
Q
SW1
Q1
∑
+
+
0.01Ω
SLOPE 1
8µA
28mV
–+
C
T
Q4
REF
REF
1.26V REFERENCE
AND
UNDERVOLTAGE
LOCKOUT
V
ON
3MHz
OSCILLATOR
Q3
REF
VO2
OSC
SLOPE 1
SLOPE 2
–
–
+
+
REF
R
OSC
S
Q
SW2
Q2
∑
+
+
0.03Ω
SLOPE 2
1948BD
RELATED PARTS
PART NUMBER
LT1949
LT1317
LT1308B
LT1615
LT1930
DESCRIPTION
600kHz, 1A Switch PWM DC/DC Converter
2-Cell Micropower DC/DC with Low-Battery Detect
600kHz Single Cell Step-Up Regulator
Micropower Step-Up Regulator in SOT-23
1.2MHz, Step-Up Regulator in SOT-23
COMMENTS
10V at 175mA from 3.3V Input
3.3V at 200mA from 2-Cell Input
5V at 1A from a 1-Cell Li-Ion Battery
20V at 12mA from 2.5V Input, 5-Lead SOT-23 Package
5V at 480mA from 3.3V, 5-Lead SOT-23 Package
1948i LT/TP 0800 4K • PRINTED IN USA
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900
q
FAX: (408) 434-0507
q
www.linear-tech.com
©
LINEAR TECHNOLOGY CORPORATION 2000
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