Datasheet

LT3477
10
3477fc
some tweaking of the compensation values (R
C
and C
C
on the V
C
pin) may be required to ensure 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 reducing the R
C
value or by
adding a capacitor (with a value of approximately C
C
/10)
in parallel with the R
C
and C
C
compensation network.
Frequency Compensation
The LT3477 has an external compensation pin (V
C
), which
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
C
pin to provide a pole and
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
C
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 place-
ment. To maximize effi ciency, switch rise and fall times
are made as short as possible. To prevent radiation and
high frequency 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
OUT
,
contains nanosecond rise and fall times and should be
kept as short as possible.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
signifi cantly reduces the start-up current spike and out-
put voltage overshoot. A typical value for the soft-start
capacitor is 10nF.
Switching Frequency
The switching frequency of the LT3477 is set by an ex-
ternal resistor attached to the R
T
pin. Do not leave this
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 effi ciency. The user should set
the frequency for the maximum tolerable output voltage
ripple.
Figure 4. Switch Frequency
R
T
(kΩ)
0
1.5
1.0
0.5
3.5
3.0
2.5
2.0
3477 F04
SWITCH FREQUENCY (MHz)
0.1
100
10
APPLICATIONS INFORMATION
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
3.5 2.43
3 3.65
2.5 4.87
2 6.81
1.5 10.2
1 17.4
0.5 43.2
0.2 107