LT1940/LT1940L
U
W U U
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.
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)
There is one special consideration regarding the two-
phase circuit. When the difference between the input
voltageandoutputvoltageislessthan2.5V,thentheboost
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
switchesenoughcurrenttogetitsboostcapacitorcharged.
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.
R5 prevents the current through R3 and R4 from pulling
OUT2 high when there is no load current.
R5 < (R3 + R4) VOUT2/(VOUT1 – VOUT2).
V
If VOUT1 is out of regulation (during start-up or if it is
overloadedorshorted)thenVOUT2 willregulatetoahigher
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 = 243ΩandR5 =4.7k.
Generating an Output Under 1.25V
Other Linear Technology Publications
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.
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
noteDN100showshowtogenerateadual(+and–)output
supply using a buck regulator.
R3
SW1
FB2
V
OUT1
R4
LT1940
SW2
R2
V
OUT2
FB1
V
C2
R5
PG1 GND
R1
1940 F09
Figure 9. This circuit can be used when VOUT1 is greater than 1.25V and VOUT2 is less than 1.25V.
1940fa
16
Linear [ Linear ]
