7
DEMO MANUAL DC194
MICROPOWER BOOST REGULATOR
OPERATIO
U
passes through the finite impedance of the output capaci-
tor. The capacitor should have low impedance at the
600kHz switching frequency of the LT1317. The imped-
ance at this frequency is usually dominated by the
capacitor’s equivalent series resistance (ESR). Choosing
a capacitor with lower ESR will result in lower output
ripple. Note also that the AC current contains fast edges,
so that you need low impedance at the switching regulator’s
harmonics. This can be obtained by adding a small ceramic
capacitor in parallel with the main output capacitor.
The DC194 uses a surface mount tantalum capacitor from
AVX. Other companies, including Kemet and Sprague,
make similar products. Some tantalum capacitor manu-
factures recommend doubling the voltage rating for power
supply applications; for highest reliability in 5V applica-
tions, the output capacitor of the DC194 should be
replaced with a 10V version. The ESR of tantalum capaci-
tors designed for DC/DC converters is specified by the
manufacturers and you have some choice in trading ripple
performance for cost and size.
Newer technologies also offer low ESR capacitors.
Panasonic’s SP series and Sanyo’s POSCAP series of
surface mount capacitors use an organic electrolyte to
achieve a lower ESR than tantalum capacitors of the same
size.
Loop Compensation Components
The components connected to the V
C
pin of the LT1317
(C3, R3 and C5) compensate the control loop of the
DC194. The values chosen here are conservative and
provide stable operation for a wide range of input voltage,
output voltage and output capacitor types. However, the
loop response can be optimized further once the power
components have been chosen. Figure 4 shows the tran-
sient response of the DC194; the upper trace in each photo
is the output voltage and the lower trace is the load current.
The lower photo shows the improvement in dynamic
response after changing the compensation components.
All Ceramic, Low Profile Design
Large value ceramic capacitors are now available that are
suitable for use as the main output capacitor of an LT1317
boost regulator. These capacitors have very low ESR and
200
µ
s/DIV
DC194 F04a
I
LOAD
100mA/DIV
V
OUT
AC COUPLED
50mV/DIV
Figure 4. The Dynamic Response of the Circuit Can Be
Improved by Optimizing the Compensation Network. The
Upper Photo Shows the Response to a Load Current Step from
20mA to 120mA. With R3 = 68k and C3 = 1.5nF, the Circuit
Responds Faster to Changing Loads (Lower Photo). When the
Load Current Is High, the 25mV
P-P
Output Ripple Makes the
V
OUT
Trace Appear as Two Traces.
V
IN
= 2.5V, V
OUT
= 3.3V, LT1317B
200
µ
s/DIV
DC194 F04b
I
LOAD
100mA/DIV
V
OUT
AC COUPLED
50mV/DIV
therefore offer very low output ripple in a small package.
However you should approach their use with some
caution.
Ceramic capacitors are manufactured using a number of
dielectrics, each with different behavior across tempera-
ture and applied voltage. Y5V is a common dielectric used
for high value capacitors, but you can lose more than 80%
of the original capacitance with applied voltage and
extreme temperatures. The transient behavior and loop
stability of the switching regulator depend on the value of
the output capacitor, so you may not be able to afford this
loss. Other dielectrics (X7R and X5R) result in more stable
characteristics and are suitable for use as the output
