LT1940EFE#TRPBF Linear Technology, LT1940EFE#TRPBF Datasheet - Page 16

• Manufacturer Part Number: LT1940EFE#TRPBF
• Description: IC REG SW DUAL 1.4A STDN 16TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1940EFE#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 1.25 ~ 22 V
• Current - Output: 1.4A
• Frequency - Switching: 1.1MHz
• Voltage - Input: 3.6 ~ 25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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.
16
Figure 9. This circuit can be used when V
U
U
V
OUT1
R1
R2
W
U
OUT1
SW1
FB1
PG1
LT1940
is greater than 1.25V and V
GND
V
reference voltage for the feedback divider from V
the FB2 pin (R3 and R4). Calculate the resistor values with
these equations:
R5 prevents the current through R3 and R4 from pulling
V
If V
overloaded or shorted) then V
voltage than intended. To avoid this, the power good
output from the channel 1 (PG1) is tied to the compensa-
tion pin (V
until V
when R4/R3 is small.
For example, for V
R1 = 10k, R2 = 16.5k, R3 = 10k, R4 = 243 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.
SW2
V
FB2
OUT1
OUT2
C2
R2/R1 = V
R4/R3 = (1.25V – V
R5 < (R3 + R4) V
OUT1
, which must be greater than 1.25V, is used as a
OUT1
high when there is no load current.
is out of regulation (during start-up or if it is
C2
is in regulation. Accuracy is good, especially
) of the channel 2. This disables channel 2
OUT1
/1.25V - 1
OUT1
OUT2
OUT2
R3
R4
1940 F09
OUT2
R5
= 3.3V and V
V
is less than 1.25V.
OUT2
/(V
)/(V
OUT1
OUT2
OUT1
– V
will regulate to a higher
– 1.25V)
OUT2
OUT2
).
= 1.2V, choose
OUT2
1940fa
to