LT1943EFE Linear Technology, LT1943EFE Datasheet - Page 17

• 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

OPERATIO
should be 5Ω of resistance between the V
connection to the load, as shown on Figure 3. The resis-
tance will damp resonant tank circuit created by the output
short. As the transient on the V
condition will be highly dependent on the layout and the
type of short, be sure to test the short condition and
examine the voltage at the V
swing below ground.
Printed Circuit Board Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 4
shows the high-current paths in the step down regulator
circuit. Note that in the step-down regulators, large,
switched currents flow in the power switch, the catch
diode, and the input capacitor. In the step-up regulators,
large, switched currents flow through the power switch,
the switching diode, and the output capacitor. In SEPIC
Figure 4. Subtracting the Current when the Switch is ON (a) From the Current when the Switch is OFF (b) Reveals the Path of
Switched; Keep these Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
the High Frequency Switching Current (c) Keep this Loop Small. The Voltage on the SW and BOOST Nodes will also be
Figure 3. Transient Short Protection for V
0.47µF
C1
V
ON
U
5Ω
R1
R1 IS ONLY NECESSARY
IF LOAD MAY HAVE TRANSIENT
SHORT CONDITION. OTHERWISE,
CONNECT V
V
IN
GND
ON
ON
TO LOAD
SW
I
C1
ON
PIN DIRECTLY TO LOAD
(a)
pin to check that it does not
pin during a short-circuit
1943 F03
ON
ON
C1
pin and its
Pin
V
IN
GND
SW
(c)
V
SW
and inverting regulators, the switched currents flow through
the power switch, the switching diode, and the tank
capacitor. The loop formed by the components in the
switched current path should be as small as possible.
Place these components, along with the inductor and
output capacitor, on the same side of the circuit board, and
connect them on that layer. Place a local, unbroken ground
plane below these components, and tie this ground plane
to system ground at one location, ideally at the ground
terminal of the output capacitor C2. Additionally, keep the
SW and BOOST nodes as small as possible.
Thermal Considerations
The PCB must provide heat sinking to keep the LT1943
cool. The exposed pad on the bottom of the package must
be soldered to a ground plane. This ground should be tied
to other copper layers below with thermal vias; these
layers will spread the heat dissipated by the LT1943. Place
additional vias near the catch diodes. Adding more copper
to the top and bottom layers and tying this copper to the
internal planes with vias can reduce thermal resistance
further. With these steps, the thermal resistance from die
(or junction) to ambient can be reduced to θ
less. With 100LFPM airflow, this resistance can fall by
another 25%. Further increases in airflow will lead to lower
thermal resistance.
D1
L1
V
IN
C2
GND
1943 F04
SW
(b)
LT1943
JA
= 25°C or
17
1943fa