LT3477 LINER [Linear Technology], LT3477 Datasheet - Page 10

• Manufacturer Part Number: LT3477
• Description: 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT3477
is a triangular waveform (not a DC waveform like the
output current) some tweaking of the compensation val-
ues (R
a clean inductor ripple current while the constant-current
loop is in effect. For these applications, the constant-
current loop response can usually be improved by reduc-
ing the R
approximately C
pensation network.
Frequency Compensation
The LT3477 has an external compensation pin (V
allows the loop response to be optimized for each applica-
tion. An external resistor and capacitor (or sometimes just
a capacitor) are placed at the V
a zero (or just a pole) to ensure proper loop compensation.
Several other poles and zeroes are present in the closed-
loop transfer function of a switching regulator, so the V
pin pole and zero are positioned to provide the best loop
response. A thorough analysis of the switching regulator
control loop is not within the scope of this data sheet, and
will not be presented here, but values of 1k and 4.7nF will
be a good choice for many designs. For those wishing to
optimize the compensation, use the 1k and 4.7nF as a
starting point.
Board Layout
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent radiation and high fre-
quency resonance problems, proper layout of the high
frequency switching path is essential. Minimize the length
and area of all traces connected to the SW pin and always
use a ground plane under the switching regulator to
minimize interplane coupling. The signal path including
the switch, output diode D1 and output capacitor C
contains nanosecond rise and fall times and should be
kept as short as possible.
10
C
and C
C
value or by adding a capacitor (with a value of
C
on the V
C
/10) in parallel with the R
U
C
U
pin) may be required to ensure
C
pin to provide a pole and
W
C
and C
U
C
), which
C
com-
OUT
C
,
Table 4. Switching Frequency
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. A typical value for the soft-start capaci-
tor is 10nF.
Switching Frequency
The switching frequency of the LT3477 is set by an
external resistor attached to the R
pin open. A resistor must always be connected for proper
operation. See Table 4 and Figure 4 for resistor values and
corresponding frequencies.
Increasing switching frequency reduces output voltage
ripple but also reduces efficiency. The user should set the
frequency for the maximum tolerable output voltage ripple.
SWITCHING FREQUENCY (MHz)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.1
3.5
2.5
1.5
0.5
0.2
3
2
1
Figure 4. Switch Frequency
R
T
10
(kΩ)
T
pin. Do not leave this
R
T
2.43
3.65
4.87
6.81
10.2
17.4
43.2
3477 F04
107
100
(kΩ)
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