LTC7138
13
7138f
For more information www.linear.com/LTC7138
applicaTions inForMaTion
at light load can be approximated by:
∆V
OUT
≈
I
PEAK
2
–I
LOAD
•
4•10
–6
C
OUT
+
V
OUT
160
The output ripple is a maximum at no load and approaches
lower limit of V
OUT
/160 at full load. Choose the output
capacitor C
OUT
to limit the output voltage ripple ∆V
OUT
using the following equation:
C
OUT
≥
I
PEAK
•2•10
–6
∆V
OUT
–
V
OUT
The value of the output capacitor must also be large enough
to accept the energy stored in the inductor without a large
change in output voltage during a single switching cycle.
Setting this voltage step equal to 1% of the output voltage,
the output capacitor must be:
C
OUT
>
L
2
•
I
PEAK
V
OUT
2
•
100%
1%
Typically, a capacitor that satisfies the voltage ripple re-
quirement is adequate to filter the inductor ripple. To avoid
overheating, the output capacitor must also be sized to
handle the ripple current generated by the inductor. The
worst-case ripple current
in the output capacitor is given
by I
RMS
= I
PEAK
/2. Multiple capacitors placed in parallel
may be needed to meet the ESR and RMS current handling
requirements.
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant input voltage ringing.
Input Voltage Steps
If the input voltage falls below the regulated output voltage,
the body diode of the internal MOSFET will conduct current
from the output supply to the input supply. If the input
voltage falls rapidly, the voltage across the inductor will be
significant and may saturate the inductor. A large current
will then flow through the MOSFET body diode, resulting
in excessive power dissipation that may damage the part.
If rapid voltage steps are expected on the input supply, put
a small silicon or Schottky diode in series with the V
IN
pin
to prevent reverse current and inductor saturation, shown
below as D1 in Figure 4. The diode should be sized for a
reverse voltage of greater than the regulated output volt
-
age, and to withstand repetitive currents higher than the
maximum peak current of the LTC7138.
Figure 4. Preventing Current Flow to the Input
SW
INPUT
LTC7138
C
OUT
C
IN
V
V
IN
L
D1
Ceramic Capacitors and Audible Noise
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. Their high ripple
current, high voltage rating, and low ESR make them ideal
for switching regulator applications. However
, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
