LT1942
10
1942fb
OPERATIO
U
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 switch-
ing 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 excel-
lent line and load regulation.
TFT Supply
The three switching regulators for the TFT supply are
named AV
DD
, V
OFF
and V
ON
. Both the AV
DD
and V
ON
switchers are boost type, while the V
OFF
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 AV
DD
switcher is enabled. After the output of the AV
DD
switcher
reaches 97% of its final value, the PGOOD pin is driven low
and both the V
OFF
and V
ON
switchers are enabled. This
event is latched rejecting any later drops on AV
DD
below
97%. The output-disconnect circuitry (Q5 in the Block
Diagram) for the V
ON
switcher is not activated until the
programmable delay set by the CT pin has elapsed. Once
the output-disconnect circuitry is enabled, the V
OUT3
pin is
driven to nearly the same voltage as D3. The V
OUT3
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 AV
DD
switch-
ing 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 refer-
ence 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.
