LTC3418 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3418' PDF : 20 Pages View PDF
LTC3418
APPLICATIONS INFORMATION
The output voltage during tracking can be calculated with
the following equation:
VOUT = VTRACK 1+ RR21,VTRACK < 0.8V
To implement the coincident tracking in Figure 2a, con-
nect an extra resistor divider to the output of VOUT2 and
connect its midpoint to the TRACK pin of the LTC3418
as shown in Figure 3. The ratio of this divider should be
selected the same as that of VOUT1’s resistor divider. To
implement the ratiometric sequencing in Figure 2b, the extra
resistor divider’s ratio should be set so that the TRACK pin
voltage exceeds 1.05V by the end of the start-up period.
The LTC3418 utilizes a method in which the TRACK pin’s
control over the output voltage is gradually released as
the TRACK pin voltage approaches 0.8V. With this tech-
nique, some overdrive will be required on the TRACK pin
to ensure that the tracking function is completely disabled
at the end of the start-up period.
For coincident tracking, the following condition should
be satisfied to ensure that tracking is disabled at the end
of start-up.
VOUT2 ≥ 1.32 VOUT1
For ratiometric tracking, the following equation can be
used to calculate the resistor values:
R4
=
R3
VOUT2
VTRACK
– 1
VTRACK 1.05V
VOUT2
(MASTER)
TRACK
PIN
R4 R2
VFB(MASTER)
PIN
R3 R1
3418 F03
Figure 3
Frequency Synchronization
The LTC3418’s internal oscillator can be synchronized
to an external clock signal. During synchronization, the
12
top MOSFET turn-on is locked to the falling edge of the
external frequency source. The synchronization frequency
range is 300kHz to 4MHz. Synchronization only occurs if
the external frequency is greater than the frequency set
by the external resistor. Because slope compensation
is generated by the oscillator’s RC circuit, the external
frequency should be set 25% higher than the frequency
set by the external resistor to ensure that adequate slope
compensation is present.
Soft-Start
The RUN/SS pin provides a means to shut down the
LTC3418 as well as a timer for soft-start. Pulling the RUN/
SS pin below 0.5V places the LTC3418 in a low quiescent
current shutdown state (IQ < 1.5μA).
The LTC3418 contains a soft-start clamp that can be set
externally with a resistor and capacitor on the RUN/SS
pin as shown in Typical Application on the front page of
this data sheet. The soft-start duration can be calculated
by using the following formula:
tSS
=RSS
•
CSS
• In
VIN
VIN
– 1.8V
⎡⎣Seconds⎤⎦
When the voltage on the RUN/SS pin is raised above 2V,
the full current range becomes available on ITH.
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses: VIN quiescent current and I2R losses.
TheVIN quiescentcurrentlossdominatestheefficiencyloss
at very low load currents whereas the I2R loss dominates
the efficiency loss at medium to high load currents. In a
typical efficiency plot, the efficiency curve at very low
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