LT1940/LT1940L
16
1940fa
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.
V
OUT1
, which must be greater than 1.25V, is used as a
reference voltage for the feedback divider from V
OUT2
to
the FB2 pin (R3 and R4). Calculate the resistor values with
these equations:
R2/R1 = V
OUT1
/1.25V - 1
R4/R3 = (1.25V – V
OUT2
)/(V
OUT1
– 1.25V)
R5 prevents the current through R3 and R4 from pulling
V
OUT2
high when there is no load current.
R5 < (R3 + R4) V
OUT2
/(V
OUT1
– V
OUT2
).
If V
OUT1
is out of regulation (during start-up or if it is
overloaded or shorted) then V
OUT2
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 (V
C2
) of the channel 2. This disables channel 2
until V
OUT1
is in regulation. Accuracy is good, especially
when R4/R3 is small.
For example, for V
OUT1
= 3.3V and V
OUT2
= 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.
Figure 9. This circuit can be used when V
OUT1
is greater than 1.25V and V
OUT2
is less than 1.25V.
APPLICATIO S I FOR ATIO
WUU
U
SW1
SW2
GND
PG1
V
C2
FB1
FB2
LT1940
V
OUT2
V
OUT1
R2
R4
R3
R1
R5
1940 F09
