lt1776 Linear Technology Corporation, lt1776 Datasheet - Page 13

• Manufacturer Part Number: lt1776
• Description: Wide Input Range, High Efficiency, Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS
The high speed switching current path is shown schemati-
cally in Figure 3. Minimum lead length in these paths is
essential to ensure clean switching and minimal EMI. The
paths containing the input capacitor, output switch and
output diode are the only ones containing nanosecond rise
and fall times. Keep these paths as short as possible.
Additionally, it is possible for the LT1776 to cause EMI
problems by “coupling to itself”. Specifically, this can
occur if the V
uncontrolled manner to the part’s high impedance nodes,
i.e., SHDN, SYNC, V
operation such as odd/even cycle behavior, pulse width
“nervousness”, improper output voltage and/or prema-
ture current limit action.
V
IN
Figure 3. High Speed Current Switching Paths
+
C1
SW
pin is allowed to capacitively couple in an
LT1776
U
C
V
and FB. This can cause erratic
IN
INFORMATION
U
V
SW
W
D1
+
U
C2
1776 F03
V
OUT
As an example, assume that the capacitance between the
V
further assume that the high impedance node in question
exhibits a capacitance of 1pF to ground. Due to the high
dV/dt, large excursion behavior of the V
couple a nearly 4V transient to the high impedance pin,
causing abnormal operation. (This assumes the “typical”
40V
added to the node will reduce the amplitude of the distur-
bance to more like 50mV (although settling time will
increase).
Specific pin recommendations are as follows:
SW
SHDN: If unused, add a 100pF capacitor to ground.
SYNC: Ground if unused.
V
explicit compensation network. A value of one-tenth of
the main compensation capacitor is recommended, up
to a maximum of 100pF.
FB: Assuming the V
usually requires no explicit capacitor of its own, but
keep this node physically small to minimize stray
capacitance.
IN
C
node and a high impedance pin node is 0.1pF, and
: Add a capacitor directly to ground in addition to the
to 5V
OUT
application.) An explicit 100pF capacitor
C
pin is handled properly, this pin
SW
node, this will
LT1776
13