5
DEMO MANUAL DC242
DC/DC CONVERTER
OPERATIO
U
How to Measure Output Voltage Ripple
In order to measure output voltage ripple, care must be
taken to avoid a long ground lead on the oscilloscope
probe. A sturdy wire should be soldered on the output GND
terminal. The other end of the wire is looped around the
ground connection of the probe and should be kept as
short as possible. The tip of the probe is touched directly
to V
OUT
(see Figure 4). Bandwidth is generally limited to
20MHz for ripple measurements. Also, if multiple pieces of
line-powered test equipment are used, be sure to use
isolation transformers on their power lines to prevent
ground loops, which can cause erroneous results. Figure
5 shows the output voltage ripple with a steady state load
of 500mA.
Heat Dissipation Issues
Since the LT1777 includes a 500mA onboard power
switch, care must be taken not to exceed the maximum
junction temperature for the part. A simple technique is to
use the PC board as a heat sink. The four corner pins on
the LT1777 IC are connected to the ground plane on both
sides of the PC board. The two sides are connected
through vias to better handle the power dissipation. If the
LT1777 is placed on a multilayer board, it is desirable to
have metal on the inner layers directly underneath the
LT1777. This helps in spreading heat and improves the
power dissipation capability of the PCB.
Layout Guidelines
Since the LT1777 is a switching regulator, attention to
proper layout is essential for good load regulation and to
minimize radiated/conducted noise. Be sure to follow
these layout guidelines:
1. The LT1777’s V
SW
pin, sense inductor L2 and Schottky
diode D1 should be placed as close as possible to each
other.
2. The anode of Schottky diode D1 should be tied directly
to the ground plane.
3. Keep the trace from the FB pin to the junction of R3 and
R2 short and use a long trace from the top of resistor R3
to the output terminal, rather than vice versa.
4. Output capacitor C2’s grounds should be tied directly to
the ground plane.
5. The ground of the feedback resistors and the loop
compensation resistor/capacitor (connected to the V
C
pin) should be referenced to the chip SGND pin, which,
in turn, is directly tied to the ground plane.
6. C5 should be as close as possible to Pin 10.
Figure 5. Output Voltage Ripple for the LT1777, I
L
= 500mA
DC242 F05
Figure 4. Measuring Output Voltage Ripple
V
OUT
GND
PROBE
DC242 F04
DC242
LT1777
