LTC1697
High Efficiency Low Power
1W CCFL Switching Regulator
FEATURES
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DESCRIPTIO
Operates from Single Li-Ion Battery
2.8V to 5.5V Input Voltage Range
Very Low Shutdown Current: <2µA
Synchronous Buck Architecture for High Efficiency
PWM Dimming Frequency Adjustable with a Single
Capacitor
Accurate Lamp Current Maximizes Lamp Lifetime
Fixed Frequency Operation at 300kHz
Internal or External PWM Dimming
Small 10-Pin MSOP Package
The LTC
®
1697 is designed to control a single 1W cold
cathode fluorescent lamp (CCFL). An internal PWM dim-
ming system maximizes efficiency and dimming range.
Accurate lamp currents can be set with a single external
resistor.
The LTC1697 includes a synchronous current mode PWM
controller with internal 1A MOSFET switches. It contains
a 300kHz oscillator, 0.8V reference, and internal current
sense. It operates from a 2.8V to 5.5V input voltage. The
LTC1697 also has a thermal limit and a shutdown that
reduces supply current to <2µA.
The LTC1697 is available in the MSOP-10 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Protected by U.S. Patent 6522116.
APPLICATIO S
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PDAs
Handheld Computers
Portable Instruments
Handheld GPS with Map Display
Handheld TV/Video Monitor
TYPICAL APPLICATIO
6
1
10
2
3
5
4
27pF
1kV
V
IN
2.8V TO 5.5V
330Ω
0.15µF
CCFL
LAMP
10
9
8
7
6
I
CCFL
(mA)
33µH
200k
5
4
3
Li-Ion
CELL
+
OFF ON
DIMMING INPUT
1V(0%) – 2V(100%)
V
IN
SW 0V
SEN
SHDN
LTC1697
V
DIM
LAMP
C
DIM
R
PROG
GND
8.25k
V
C
0.1µF
2
4k
10µF
0.022µF
1697 TA01
U
I
CCFL
vs R
PROG
6k
8k 10k
R
PROG
(Ω)
20k
1967 TA01b
U
U
1697f
1
LTC1697
ABSOLUTE
(Notes 1, 2)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
C
DIM
V
DIM
0V
SEN
SW
GND
1
2
3
4
5
10
9
8
7
6
SHDN
R
PROG
V
C
V
IN
LAMP
V
IN
Voltage .................................................. –0.5V to 6V
V
C
, OV
SEN
, C
DIM
, R
PROG
, SW
Voltages ................................... –0.5V to (V
IN
+ 0.3V)
SHDN, V
DIM
Pins ......................................... –0.5V to 6V
LAMP Pin ................................................. –0.5V to 0.5V
Operating Temperature Range (Note 5) ...–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1697EMS
MS PART MARKING
LTZR
MS PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 160°C/W 1 LAYER BOARD
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. 2.8V < V
IN
≤
5.5V unless otherwise specified.
SYMBOL
V
IN
V
UVLO
I
Q-SHDN
I
Q-Active
I
LEAK
R
ON
I
MAX
% DC
f
SWITCH
f
DIM
V
DIM
I
DIM
R
LAMP
I
L(ERROR)
R
P(RANGE)
V
SHDN-H
V
SHDN-L
I
SHDN
I
OVSEN
V
OVSEN
V
DIM(SD-I)
V
DIM(SD-H)
PARAMETER
Operating Supply Voltage Range
Undervoltage Lockout
Quiescent Current - SHDN
Quiescent Current - Active
Switch Leakage
Switch On Resistance
Switch Current Limit
Duty Cycle
Switching Frequency
Dimming PWM Frequency
V
DIM
Input Voltage
V
DIM
Input Bias Current
Internal R
LAMP
Resistance
Lamp Current Accuracy
Programming Resistor Range
SHDN Input High
SHDN Input Low (Note 9)
SHDN Input Current
Overvoltage Sense Protect Current (Note 7)
Overvoltage Sense Pin Voltage
Passive Shutdown Voltage (Note 8)
Not in Passive Shutdown
V
SHDN
= V
IN
I
LAMP
= 5mA, R
PROG
= 6.4k
●
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
(Note 2)
SHDN = 0V; V
DIM
= 0V
V
C
= 0V, SHDN = V
IN
= V
DIM
= 4.2V (Note 4)
V
IN
= 4V
●
●
●
●
●
MIN
2.8
TYP
MAX
5.5
2.77
UNITS
V
V
µA
mA
µA
Ω
A
1
0.9
0.1
0.18
0.9
0
240
190
300
250
1.0
2.0
1.6
2
1.5
1
95
370
310
C
DIM
= 0.022µF (Note 3)
Dimming PWM Duty Cycle = 0%
Dimming PWM Duty Cycle = 100%
V
DIM
= 2V
LAMP Pin to GND
|1– I
LAMP(AVG)
/(32/6.4k)| • 100% (Note 6)
I
LAMP
= 5mA
(Note 10)
●
±1
50
●
●
60
6
16
0.4
2
6.4
1.2
0.1
16.65
0.95
1.05
21.5
1.2
1
26.35
1.5
0.4
U
%
kHz
Hz
V
V
µA
Ω
%
kΩ
V
V
µA
µA
V
V
V
1697f
W
U
U
W W
W
LTC1697
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
All voltages specified with respect to GND pin.
Note 3:
The dimming PWM frequency is set by the equation 5Hz/C
DIM
(µF).
Note 4:
Actual operating current will be higher due to lamp operating
current.
Note 5:
The LTC1697 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 6:
I
LAMP(AVG)
= the average of the magnitude (absolute value) of the
positive and negative lamp current flowing into and out of the LAMP pin.
Note 7:
For currents at or above I
OVSEN(ON)
, the switch duty cycle will be
0%.
Note 8:
At V
DIM
voltages below V
DIM(SHDNON)
the LTC1697 behaves as if
the SHDN pin was pulled low.
Note 9:
To minimize I
Q
shutdown, pull the SHDN pin below 0.1V.
Note 10:
2mA
≤
I
LAMP
≤
5mA.
PI FU CTIO S
C
DIM
(Pin 1):
Dimming Capacitor. Connect the pin to GND
with a 0.022µF capacitor (nominal). The value of capaci-
tance on the C
DIM
pin determines the dimming PWM
frequency. The transfer function of capacitance to fre-
quency is 5Hz/C
DIM
(µF).
V
DIM
(Pin 2):
Dimming Control Pin. The V
DIM
pin controls
the duty cycle of the dimming PWM. It ranges from 0% at
V
DIM
= 1V to 100% at V
DIM
= 2V. If the V
DIM
voltage is
<0.4V the LTC1697 will enter shutdown mode after
≈50ms.
0V
SEN
(Pin 3):
Overvoltage Sense Pin. Protects the high-
voltage transformer from the overvoltage condition that
occurs when the lamp is open or not present. This pin is
connected through a resistor to the emitters of the drive
transistors of the Royer oscillator.
SW (Pin 4):
Switch Pin. Connect the inductor and optional
Schottky diode here. Minimize trace length to keep EMI
and high frequency ringing down.
GND (Pin 5):
Signal and Power Ground for the LTC1697.
LAMP (Pin 6):
Lamp Current Feedback Pin. Connect this
pin to the CCFL lamp.
V
IN
(Pin 7):
Input Supply Pin.
V
C
(PIN 8):
Compensation Node. Connect this pin to GND
through a 0.1µF capacitor. See Application Information
section. A frequency compensation network is connected
to this pin to compensate the loop. See the section “V
C
Compensation” for guidelines.
R
PROG
(Pin 9):
Lamp Current Programming. Connect this
pin to GND with a 6.4k 1% resistor (nominal). See Appli-
cation Information section for resistor selection.
SHDN (Pin 10):
Shutdown. Grounding this pin shuts down
the LTC1697. Tie to >1V to enable.
U
U
U
1697f
3
LTC1697
BLOCK DIAGRA
SHDN 10
SHUTDOWN
PWM
OSC
SLOPE
COMP
GND
–
5
DIMMING
OSC
2V
1V
+
I
LAMP
LAMP
FB
R
PROG
50Ω
+
–
0.7V
1
C
DIM
2
V
DIM
–
+
+
PWM
LOGIC
CURRENT
COMP
–
W
TO ROYER
OSCILLATOR
SW
4
V
IN
7 2.8V
TO 5.5V
200k
ANTI
SHOOTTHRU
PROTECTION
N
I
SENSE
AMP
3
0V
SEN
+
CURRENT
LIMIT
–
1.6A
TYP
TRANSFORMER
VOLTAGE
PROTECTION
8
V
C
0.8V
+
Σ
–
9
R
PROG
R
P
6 LAMP
1697 BD
1697f
LTC1697
APPLICATIO S I FOR ATIO
Background
Current generation handheld computers and instruments
typically use backlit liquid crystal displays (LCDs). Cold
cathode fluorescent lamps (CCFLs) provide the highest
available efficiency for backlighting the display, where
providing the most light out for the least amount of input
power is the most important goal. These lamps require
high voltage AC to operate, mandating an efficient high
voltage DC/AC converter. The lamps operate from DC, but
migration effects damage the lamp and shorten its life-
time. Lamp drive should ideally contain zero DC compo-
nent. In addition to good efficiency, the converter should
deliver the lamp drive in the form of a sine wave. This
minimizes EMI and RF emissions, which can interfere with
other devices and degrade overall operating efficiency.
Sinusoidal CCFL drive also maximizes current-to-light
conversion in the lamp. The circuit also permits lamp
intensity control from zero to full brightness with no
hysteresis or “pop-on.”
The small size and battery-powered operation associated
with LCD-equipped apparatus dictate low component
count and high efficiency for these circuits. Size con-
straints place severe limitations on circuit architecture and
long battery life is usually a priority. Handheld portable
computers offer an excellent example. The CCFL and its
power supply can be responsible for almost 50% of the
total battery drain.
The CCFL regulator drives an inductor that acts as a
switch-mode current source for a current-driven Royer-
class converter with efficiencies as high as 90%. The
control loop forces the CCFL PWM to modulate the aver-
age inductor current to maintain constant current in the
lamp. This constant current and the resulting lamp inten-
sity is programmable. Lamp intensity is further controlled
by modulating the current to the Royer converter at 150Hz
to 500Hz.
U
Operation
The LTC1697 is a fixed frequency, current mode regulator.
Such a switcher controls switch duty cycle directly by
switch current rather than by output voltage. Referring to
the block diagram for the LTC1697, the NMOS switch
turns ON at the start of each oscillator cycle. The NMOS
switch turns back OFF when switch current reaches a
predetermined level.
Current Sensing
Lossless current sensing converts the peak current signal
to a voltage which is summed with the internal slope
compensation. This summed signal is compared to V
C
to
provide a peak current control command for the PWM.
Current Limit
The current limit amplifier will shut the NMOS switch off
once the current exceeds the current limit threshold. The
current amplifier delay to the output is typically 50ns.
Synchronous Rectifier
The LTC1697 operates as a synchronous converter. When
the NMOS switch turns OFF as mentioned above, the
PMOS switch turns ON. This gives a low resistance current
path for the inductor current back to V
IN
.
Dimming PWM
An on-chip PWM dimming circuit enables and disables the
current mode regulator for each dimming cycle. It also
disconnects the feedback network from the compensation
node (V
C
) to reduce slew time at the next enable time. The
oscillator for the dimming PWM produces a triangle wave
whose frequency is determined by an external capacitor
on the C
DIM
pin. The dimming PWM frequency is equal to
5Hz/C
DIM
(µF) with its duty cycle set by the voltage on the
1697f
W
U U
5
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