lt3471edd-trpbf Linear Technology Corporation, lt3471edd-trpbf Datasheet - Page 5

• Manufacturer Part Number: lt3471edd-trpbf
• Description: Dual 1.3a, 1.2mhz Boost/inverter In 3mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

OPERATIO
The LT3471 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Refer to the Block Diagram. At the start of each
oscillator cycle, the SR latch is set, which turns on the
power switch, Q1 (Q2). A voltage proportional to the
switch current is added to a stabilizing ramp and the
resulting sum is fed into the positive terminal of the PWM
comparator A2 (A4). When this voltage exceeds the level
at the negative input of A2 (A4), the SR latch is reset,
turning off the power switch Q1 (Q2). The level at the
negative input of A2 (A4) is set by the error amplifier A1
(A3) and is simply an amplified version of the difference
between the negative feedback voltage and the positive
feedback voltage, usually tied to the reference voltage
V
peak current level to keep the output in regulation. If the
error amplifier’s output increases, more current is deliv-
ered to the output. Similarly, if the error decreases, less
current is delivered. Each switcher functions indepen-
dently but they share the same oscillator and thus the
switchers are always in phase. Enabling the part is done by
APPLICATIONS
Duty Cycle
The typical maximum duty cycle of the LT3471 is 94%.
The duty cycle for a given application is given by:
Where V
in the worst case 330mV (at 1.3A)
The LT3471 can be used at higher duty cycles, but it must
be operated in the discontinuous conduction mode so that
the actual duty cycle is reduced.
Setting Output Voltage
Setting the output voltage depends on the topology used.
For normal noninverting boost regulator topologies:
REG
DC
V
OUT
. In this manner, the error amplifier sets the correct
=
D
=
|
V
is the diode forward voltage drop and V
V
|
OUT
V
FBP
OUT
| |
⎛
⎝ ⎜
+
U
1
| |
+
+
U
V
R
R
D
V
2
1
D
| – |
⎞
⎠ ⎟
INFORMATION
| – |
U
V
CESAT
V
IN
|
|
W
U
CESAT
is
taking either SHDN/SS pin above 1.8V. Disabling the part
is done by grounding both SHDN/SS pins. The soft-start
feature of the LT3471 allows for clean start-up conditions
by limiting the amount of voltage rise at the output of
comparator A1 and A2, which in turn limits the peak
switching current. The soft-start feature for each switcher
is enabled by slowly ramping that switcher’s SHDN/SS pin,
using an RC network, for example. Typical resistor and
capacitor values are 0.33µF and 4.7kΩ, allowing for a
start-up time on the order of milliseconds. The LT3471 has
a current limit circuit not shown in the Block Diagram. The
switch current is constantly monitored and not allowed to
exceed the maximum switch current (typically 1.6A). If the
switch current reaches this value, the SR latch is reset
regardless of the state of the comparator A2 (A4). Also not
shown in the Block Diagram is the thermal shutdown
circuit. If the temperature of the part exceeds approxi-
mately 160°C, both latches are reset regardless of the state
of comparators A2 and A4. The current limit and thermal
shutdown circuits protect the power switch as well as the
external components connected to the LT3471.
where V
Typical Applications section for examples).
Select values of R1 and R2 according to the following
equation:
A good value for R2 is 15k which sets the current in the
resistor divider chain to 1.00V/15k = 67µA.
V
be tied to a divided down version of V
voltage as long as the absolute maximum ratings for the
feedback pins are not exceeded (see Absolute Maximum
Ratings).
For inverting topologies, V
is connected between R1 and R2. R2 is between V
FBP
R
1
is usually just tied to V
=
R
FBN
2
⎡
⎢
⎣
⎛
⎝ ⎜
is connected between R1 and R2 (see the
V
V
OUT
REF
⎞
⎠ ⎟
–
1
⎤
⎥
⎦
FBN
REF
is tied to ground and V
= 1.00V, but V
REF
or some other
LT3471
FBP
can also
FBP
3471fa
5
and
FBP