5
DEMO MANUAL DC237/DC238
NO-DESIGN SWITCHER
OPERATIO
U
The output capacitor C5 is an AVX tantalum capacitor. A
ceramic is not recommended as the main output capacitor
since loop stability relies on a resistive characteristic at
higher frequencies to form a zero. The AVX TPS series was
specifically designed for switch-mode power supplies to
have very low ESR. At switching frequencies, ripple volt-
age is more a function of ESR than of absolute capacitance
value. If lower output ripple voltage is required, use the
optional capacitor C7 to reduce ESR rather than increasing
the capacitance of C5. For very low ripple, an additional LC
filter in the output may be a less expensive solution. The
output contains very narrow voltage spikes because of the
parasitic inductance of C5. A small ceramic capacitor, C6,
removes these spikes on the demo board. In application,
trace inductance and local bypass capacitors will perform
this function, negating the need for C6.
Catch Diode D1
Use diodes designed for switching applications, with
adequate current rating and fast turn-on times, such as
Schottky or ultrafast diodes. In selecting a diode, the basic
parameters of interest are forward voltage, maximum
reverse voltage, average operating current and peak cur-
rent. Lower forward voltage yields higher circuit efficiency
and lowers power dissipation in the diode. The MBRD835L
has a maximum forward drop of 0.4V at 3A. The reverse
voltage rating must be greater than the input voltage.
Average diode current is always less than output current,
but under a shorted output condition, diode current can
equal switch current limit. If the application must with-
stand this condition, the diode must be rated for maximum
switch current.
Compensation: C1, C2 and R1
A detailed discussion of frequency compensation can be
found in the LT1506 data sheet. C1, a 1500pF capacitor
from V
C
to ground, gives a stable loop response over a
wide range of input and output conditions. Options R1 and
C2 are included for optimization of the dynamic response
to a specific application.
Boost Voltage: D2, D3 and C4
A boost voltage of at least 2.8V is required throughout
the on-time of the switch to guarantee that it remains
saturated. At output currents greater than 3A and
higher ambient temperatures, diode D2 must be moved
to position D3 to prevent boost from falling below this
minimum. For output voltages above 3.3V, diode D2
provides sufficient boost voltage to C4.
PCB LAYOUT
In many cases, the layout of the demonstration board
may be dropped directly into the application with mini-
mal changes. If not, there are several precautions that
must be taken when laying out high frequency con-
verter circuits. The high frequency switching path runs
from ground, through C3, to the V
IN
pin of the LT1506,
out of the SW pin, through D1 and back to ground. This
loop acts as an antenna and will radiate noise if not kept
as short as possible. Also, at higher switching currents,
the associated trace inductance can cause excessive
voltage spikes across the switch. The use of a ground
plane will reduce many noise problems. The ground pin
of the LT1506 contains some high frequency signal
currents, but more importantly, it is the 0V reference for
the output voltage. Connect the ground pin directly to
the ground plane. The FB and V
C
components should be
kept away from the power components as much as
possible. The ground for these components should be
separated from power grounds. Run a Kelvin sense to
V
OUT
as required but keep the divider network close to
the LT1506 to prevent noise pickup on the FB node.
Noise pickup on the V
C
pin appears as various prob-
lems, including poor load regulation, subharmonic
oscillation and instability. Thermal management must
also be considered. The SO-8 package has a fused
ground pin. Soldering this pin to a large copper area will
significantly reduce its thermal resistance. Solder-filled
feedthroughs close to the ground pin provide a good
thermal path to the ground plane. For the DD package,
the grounded tab should be treated in the same manner.
For more information or advice, contact the LTC Appli-
cations department.
