LT1940 Linear Technology, LT1940 Datasheet - Page 15

• Manufacturer Part Number: LT1940
• Description: Dual Monolithic 1.4A/ 1.1MHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
capacitors. Although two inductors are required, each will
be smaller than the inductor required for a single-phase
regulator. This may be important when there are tight
height restrictions on the circuit. The Typical Applications
section shows circuits with maximum heights of 1.4mm,
1.8mm and 2.1mm.
There is one special consideration regarding the two-
phase circuit. When the difference between the input
voltage and output voltage is less than 2.5V, then the boost
U
U
Figure 9. A Good PCB Layout Ensures Proper Low EMI Operation
V
OUT1
C
pins together, the two
W
VIA TO LOCAL GROUND PLANE
VIA TO V
IN
U
GND
circuits may prevent the two channels from properly
sharing current. If, for example, channel 1 gets started
first, it can supply the load current, while channel 2 never
switches enough current to get its boost capacitor charged.
In this case, channel 1 will supply the load until it reaches
current limit, the output voltage drops, and channel 2 gets
started. The solution is to generate a boost supply gener-
ated from either SW pin that will service both BOOST pins.
The low profile, single output 5V to 3.3V converter shown
in the Typical Applications section shows how to do this.
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
note DN100 shows how to generate a dual (+ and –) output
supply using a buck regulator.
V
OUT2
1940 F09
LT1940
15
1940i