LT1943EFE Linear Technology, LT1943EFE Datasheet - Page 16

• Manufacturer Part Number: LT1943EFE
• Description: IC REG SW QUAD TFT LCD 28-TSSOP
• Manufacturer: Linear Technology

Specifications of LT1943EFE

• Applications: Converter, TFT, LCD
• Voltage - Input: 4.5 ~ 22 V
• Number Of Outputs: 4
• Voltage - Output: 1.25 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LT1943
OPERATIO
must have low impedance at the switching frequency to do
this effectively and it must have an adequate ripple current
rating. The input capacitor RMS current can be calculated
from the step-down output voltage and current, and the
input voltage:
and is largest when V
ripple current contribution from the other channels will be
minimal. Considering that the maximum load current from
switcher 1 is ~2.8A, RMS ripple current will always be less
than 1.4A.
The high frequency of the LT1943 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 10µF. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for ex-
ample a 1µF ceramic capacitor in parallel with a low ESR
tantalum capacitor. For the electrolytic capacitor, a value
larger than 10µF will be required to meet the ESR and
ripple current requirements. Because the input capacitor
is likely to see high surge currents when the input source
is applied, only consider a tantalum capacitor if it has the
appropriate surge current rating. The manufacturer may
also recommend operation below the rated voltage of the
capacitor. Be sure to place the 1µF ceramic as close as
possible to the V
noise immunity.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plug-
ging the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT1943. The
16
C
INRMS
=
I
OUT
IN
U
•
and GND pins on the IC for optimal
V
IN
OUT IN
= 2 V
(
V
V
IN
OUT
–
V
(50% duty cycle). The
0
UT
)
<
I
OUT
2
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor (an electrolytic) in
parallel with the ceramic capacitor. For details, see Appli-
cation Note 88.
Soft-Start and Shutdown
The RUN/SS (Run/Soft-Start) pin is used to place the
switching regulators and the internal bias circuits in shut-
down mode. It also provides a soft-start function, along
with SS-234. If the RUN/SS is pulled to ground, the
LT1943 enters its shutdown mode with all regulators off
and quiescent current reduced to ~35µA. An internal
1.7µA current source pulls up on the RUN/SS and SS-234
pins. If the RUN/SS pin reaches ~0.8V, the internal bias
circuits start and the quiescent currents increase to their
nominal levels.
If a capacitor is tied from the RUN/SS or SS-234 pins to
ground, then the internal pull-up current will generate a
voltage ramp on these pins. This voltage clamps the V
pin, limiting the peak switch current and therefore input
current during start-up. The RUN/SS pin clamps V
the SS-234 pin clamps the V
value for the soft-start capacitors is C
C
To shut down SW2, SW3, and SW4, pull the SS-234 pin to
ground with an open drain or collector.
If the shutdown and soft-start features are not used, leave
the RUN/SS and SS-234 pins floating.
V
The V
3. When the C
PNP turns on, connecting V
limited, and will protect the LT1943 and input source from
a shorted output. However, if the V
high output voltage, and then shorted to ground through
a long wire, unpredictable results can occur. The resonant
tank circuit created by the inductance of the long wire and
the capacitance at the V
volts below ground. This can lead to large and potentially
damaging currents internal to the LT1943. If the V
output may be shorted after being fully charged, there
OUT
ON
Pin Considerations
is the value of the largest output capacitor.
ON
pin is the delayed output for switching regulator
T
pin reaches 1.1V, the output disconnect
ON
pin can ring the V
ON
C2
to E3. The V
, V
C3
, and V
ON
OUT
pin is charged to a
ON
/10,000, where
C4
ON
pin is current
pins. A good
pin several
C1
, and
1943fa
ON
C