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LT1940

Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator

Linear Technology 

LT1940/LT1940L

APPLICATIO S I FOR ATIO

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

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.

Generating an Output Under 1.25V

The LT1940 regulates its feedback pins to 1.25V. Two

resistors can be used to program an output that is higher

than 1.25V. Generating an output voltage that is less than

the internal reference is generally more difficult, but the

LT1940 can easily generate an output voltage less than

1.25V if the other output is greater than 1.25V. Figure 9

shows how.

VOUT1, which must be greater than 1.25V, is used as a

reference voltage for the feedback divider from VOUT2 to

the FB2 pin (R3 and R4). Calculate the resistor values with

these equations:

R2/R1 = VOUT1/1.25V - 1

R4/R3 = (1.25V – VOUT2)/(VOUT1 – 1.25V)

R5 prevents the current through R3 and R4 from pulling

VOUT2 high when there is no load current.

R5 < (R3 + R4) VOUT2/(VOUT1 – VOUT2).

If VOUT1 is out of regulation (during start-up or if it is

overloaded or shorted) then VOUT2 will regulate to a higher

voltage than intended. To avoid this, the power good

output from the channel 1 (PG1) is tied to the compensa-

tion pin (VC2) of the channel 2. This disables channel 2

until VOUT1 is in regulation. Accuracy is good, especially

when R4/R3 is small.

For example, for VOUT1 = 3.3V and VOUT2 = 1.2V, choose

R1 = 10k, R2 = 16.5k, R3 = 10k, R4 = 243Ω and R5 = 4.7k.

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.

VOUT1

R2

R1

SW1

FB2

LT1940

FB1

SW2

PG1 GND VC2

R3

R4

VOUT2

R5

1940 F09

Figure 9. This circuit can be used when VOUT1 is greater than 1.25V and VOUT2 is less than 1.25V.

16

1940fa

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