lt1940l Linear Technology Corporation, lt1940l Datasheet - Page 15

• Manufacturer Part Number: lt1940l
• Description: Lt1940l - Dual Monolithic 1.4a, 1.1mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Figure 8. A Good PCB Layout Ensures Proper Low EMI Operation
V
OUT1
VIA TO LOCAL GROUND PLANE
VIA TO V
Figure 7. Subtracting the Current when the Switch is ON (a) From the Current when the Switch is OFF (b) Reveals the Path
of the High Frequency Switching Current (c) Keep This Loop Small. The Voltage on the SW and BOOST Nodes will also be
Switched; Keep these Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane.
IN
U
V
IN
U
GND
GND
SW
I
C1
(7a)
W
C1
U
V
OUT2
V
IN
1940 F08
GND
SW
(7c)
V
SW
Thermal Considerations
The PCB must also provide heat sinking to keep the
LT1940 cool. The exposed metal 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
LT1940. 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 re-
duced to
The power dissipation in the other power components—
catch diodes, boost diodes and inductors, cause addi-
tional copper heating and can further increase what the IC
sees as ambient temperature. See the LT1767 data sheet’s
Thermal Considerations section.
Single, Low-Ripple 2.8A Output
The LT1940 can generate a single, low-ripple 2.8A output
if the outputs of the two switching regulators are tied
together and share a single output capacitor. By tying the
two FB pins together and the two V
channels will share the load current. There are several
advantages to this two-phase buck regulator. Ripple cur-
rents at the input and output are reduced, reducing voltage
ripple and allowing the use of smaller, less expensive
D1
JA
L1
= 45 C/W.
V
IN
1940 F07
C2
GND
SW
LT1940/LT1940L
(7b)
C
pins together, the two
15
1940fa