Datasheet

LT3506/LT3506A

3506afc

Single, Low-Ripple 3.2A Output

The LT3506 can generate a single, low-ripple 3.2A 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

pins together, the

two 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 volt-

age 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 circuits may

prevent the two channels from properly sharing current.

If, for example, channel 1 gets started ﬁ rst, 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 generated from

either SW pin that will service both BOOST pins. The low

proﬁ le, single output 5V to 3.3V converter shown in the

Typical Applications section shows how to do this.

Other Linear Technology Publications

Application Notes 19, 35 and 44 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 100

shows how to generate a dual (+ and –) output supply

using a buck regulator.

APPLICATIONS INFORMATION