Datasheet

8
LT1616
in combination with a 0.1µF ceramic capacitor. However,
input voltage ripple will be higher, and you may want to
include an additional 0.1µF ceramic a short distance away
from the LT1616 circuit in order to filter the high frequency
ripple. The input capacitor should be rated for the maxi-
mum input voltage.
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT1616 to produce the DC output. In this role it
determines the output ripple. The second function is to
store energy in order to satisfy transient loads and stabi-
lize the LT1616’s control loop.
In most switching regulators the output ripple is deter-
mined by the equivalent series resistance (ESR) of the
output capacitor. Because the LT1616’s control loop doesn’t
depend on the output capacitor’s ESR for stable operation,
you are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. You can estimate
output ripple with the following equations:
V
RIPPLE
= ∆I
L
• ESR for electrolytic capacitors (tantalum
and aluminum)
V
RIPPLE
= ∆I
L
/(2π • f • C
OUT
) for ceramic capacitors
APPLICATIO S I FOR ATIO
WUUU
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this require-
ment becomes
C
OUT
> 10 • L(I
LIM
/V
OUT
)
2
Finally, there must be enough capacitance for good tran-
sient performance. The last equation gives a good starting
point. Alternatively, you can start with one of the designs
in this data sheet and experiment to get the desired
performance. Figure 3 illustrates some of the trade-off
between different output capacitors. Figure 4 shows the
test circuit. The lowest trace shows total output current,
which jumps from 100mA to 250mA. The other traces
show the output voltage ripple and transient response
with different output capacitors. The capacitor value, size
and type are listed. Note that the time scale at 50µs per
divison is much larger than the switching period, so you
can’t see the output ripple at the switching frequency. The
output ripple appears as vertical broadening of the trace.
The first trace (C
OUT
= 4.7µF) has peak-to-peak output
ripple of ~6mV, while the third trace shows peak-to-peak
ripple of ~15mV.
Table 1. Inductor Vendors
Vendor Phone URL Part Series Comments
Murata [phone] www.murata.com LQH3C Small, Low Cost, 2mm Height
Sumida [phone] www.sumida.com CR43
CLS62 1:1 Coupled
CLQ61 1.5mm Height
Coilcraft [phone] www.coilcraft.com DO1607C
DO1608C
DT1608C
Coiltronics [phone] www.coiltronics.com CTXxx-1 1:1 Coupled Toroid
TP1 1.8mm Height
Toko www.tokoam.com 3DF
D52LC
Table 2. Capacitor Vendors
Vendor Phone URL Part Series Comments
Taiyo-Yuden [phone] www.t-yuden.com Ceramic Caps X5R Dielectric
AVX [phone] www.avxcorp.com Ceramic Caps
Tantalum Caps
Murata [phone] www.murata.com Ceramic Caps
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