lt3580ems8e-trpbf Linear Technology Corporation, lt3580ems8e-trpbf Datasheet - Page 7

• Manufacturer Part Number: lt3580ems8e-trpbf
• Description: Boost/inverting Dc/dc Converter With 2a Switch, Soft-start, And Synchronization
• Manufacturer: Linear Technology Corporation

OPERATION
The LT3580 uses a constant-frequency, current mode con-
trol scheme to provide excellent line and load regulation.
Refer to the Block Diagram which shows the LT3580 in a
boost confi guration. At the start of each oscillator cycle,
the SR latch (SR1) is set, which turns on the power switch,
Q1. The switch current fl ows through the internal current
sense resistor generating a voltage proportional to the
switch current. This voltage (amplifi ed by A4) is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A3. When this
voltage exceeds the level at the negative input of A3, the SR
latch is reset, turning off the power switch. The level at the
negative input of A3 (VC pin) is set by the error amplifi er A1
(or A2) and is simply an amplifi ed version of the difference
between the feedback voltage (FB pin) and the reference
voltage (1.215V or 5mV depending on the confi guration).
In this manner, the error amplifi er sets the correct peak
current level to keep the output in regulation.
The LT3580 has a novel FB pin architecture that can be
used for either boost or inverting confi gurations. When
confi gured as a boost converter, the FB pin is pulled up
to the internal bias voltage of 1.215V by the R
connected from V
inactive and comparator A1 performs the inverting ampli-
fi cation from FB to VC. When the LT3580 is in an inverting
SHUTDOWN
V
V
V
IN
IN
IN
> V
= V
< V
Figure 1. SEPIC Topology Allows for the Input to Span
the Output Voltage. Coupled or uncoupled inductors
can be used. Follow noted phasing if coupled.
OUT
OUT
OUT
OR
OR
+
C1
RT
OUT
SHDN
RT
V
SYNC
IN
to FB. Comparator A2 becomes
LT3580
L1
•
SS
SW
GND
VC
FB
C
SS
C2
RC
CC
•
L2
D1
R1
FB
3580 F01
+
resistor
C3
V
OUT
confi guration, the FB pin is pulled down to 5mV by the
R
A1 becomes inactive and comparator A2 performs the
noninverting amplifi cation from FB to VC.
SEPIC Topology
The LT3580 can be confi gured as a SEPIC (single-ended
primary inductance converter). This topology allows for
the input to be higher, equal, or lower then the desired
output voltage. Output disconnect is inherently built into
the SEPIC topology, meaning no DC path exists between
the input and output. This is useful for applications requir-
ing the output to be disconnected from the input source
when the circuit is in shutdown.
Inverting Topology
The LT3580 can also work in a dual inductor inverting
topology. The part’s unique feedback pin allows for the
inverting topology to be built by simply changing the
connection of external components. This solution results
in very low output voltage ripple due to inductor L2 in
series with the output. Abrupt changes in output capacitor
current are eliminated because the output inductor deliv-
ers current to the output during both the off-time and the
on-time of the LT3580 switch.
SHUTDOWN
FB
resistor connected from V
Figure 2. Dual Inductor Inverting Topology Results in
Low Output Ripple. Coupled or uncoupled inductors
can be used. Follow noted phasing if coupled.
V
IN
+
C1
RT
SHDN
RT
V
SYNC
IN
LT3580
L1
•
SS
SW
GND
VC
FB
C
SS
OUT
C2
to FB. Comparator
RC
CC
D1
•
LT3580
L2
R1
3580 F02
+
C3
3580fa
7
V
OUT