8
LT1610
APPLICATIONS INFORMATION
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COMPONENT SELECTION
Inductors
Inductors used with the LT1610 should have a saturation
current rating (–30% of zero current inductance) of ap-
proximately 0.5A or greater. DCR should be 0.5Ω or less.
The value of the inductor should be matched to the power
requirements and operating voltages of the application. In
most cases a value of 4.7µH or 10µH is suitable. The Murata
LQH3C inductors specified throughout the data sheet are
small and inexpensive, and are a good fit for the LT1610.
Alternatives are the CD43 series from Sumida and the
DO1608 series from Coilcraft. These inductors are slightly
larger but will result in slightly higher circuit efficiency.
Chip inductors, although tempting to use because of their
small size and low cost, generally do not have enough
energy storage capacity or low enough DCR to be used
successfully with the LT1610.
Diodes
The Motorola MBR0520 is a 0.5 amp, 20V Schottky diode.
This is a good choice for nearly any LT1610 application,
unless the output voltage or the circuit topology require a
diode rated for higher reverse voltages. Motorola also
offers the MBR0530 (30V) and MBR0540 (40V) versions.
Most one-half amp and one amp Schottky diodes are
suitable; these are available from many manufacturers. If
you use a silicon diode, it must be an ultrafast recovery
type. Efficiency will be lower due to the silicon diode’s
higher forward voltage drop.
Capacitors
The input capacitor must be placed physically close to the
LT1610. ESR is not critical for the input. In most cases
inexpensive tantalum can be used.
The choice of output capacitor is far more important. The
quality of this capacitor is the greatest determinant of the
output voltage ripple. The output capacitor performs two
major functions. It must have enough capacitance to
satisfy the load under transient conditions and it must
shunt the AC component of the current coming through
the diode from the inductor. The ripple on the output
results when this AC current passes through the finite
impedance of the output capacitor. The capacitor should
have low impedance at the 1.7MHz switching frequency of
the LT1610. At this frequency, the impedance is usually
dominated by the capacitor’s equivalent series resistance
(ESR). Choosing a capacitor with lower ESR will result in
lower output ripple.
Perhaps the best way to decrease ripple is to add a 1µF
ceramic capacitor in parallel with the bulk output capaci-
tor. Ceramic capacitors have very low ESR and 1µF is
enough capacitance to result in low impedance at the
switching frequency. The low impedance can have a
dramatic effect on output ripple voltage. To illustrate,
examine Figure 6’s circuit, a 4-cell to 5V/100mA SEPIC
DC/DC converter. This design uses inexpensive aluminum
electrolytic capacitors at input and output to keep cost
down. Figure 7 details converter operation at a 100mA
load, without ceramic capacitor C5. Note the 400mV
spikes on V
OUT
.
After C5 is installed, output ripple decreases by a factor of
8 to about 50mV
P-P
. The addition of C5 also improves
efficiency by 1 to 2 percent.
Low ESR and the required bulk output capacitance can be
obtained using a single larger output capacitor. Larger
tantalum capacitors, newer capacitor technologies (for
example the POSCAP from Sanyo and SPCAP from
Panasonic) or large value ceramic capacitors will reduce
the output ripple. Note, however, that the stability of the
circuit depends on both the value of the output capacitor
and its ESR. When using low value capacitors or capaci-
tors with very low ESR, circuit stability should be evalu-
ated carefully, as described below.
Loop Compensation
The LT1610 is a current mode PWM switching regulator
that achieves regulation with a linear control loop. The
LT1610 provides the designer with two methods of com-
pensating this loop. First, you can use an internal compen-
sation network by tying the COMP pin to the V
C
pin. This
results in a very small solution and reduces the circuit’s
total part count. The second option is to tie a resistor R
C
and a capacitor C
C
in series from the V
C
pin to ground. This
allows optimization of the transient response for a wide
variety of operating conditions and power components.
