8
DEMO MANUAL DC194
MICROPOWER BOOST REGULATOR
capacitor. The X7R type has better stability across tem-
perature, while the X5R is less expensive and is available
in higher values.
The second concern in using ceramic capacitors is that
many switching regulators benefit from the ESR of the
output capacitor because it introduces a zero in the
regulator’s loop gain. This zero may not be effective
because the ceramic capacitor’s ESR is very low. Most
current mode switching regulators (including the LT1317)
can be easily compensated without this zero. Any design
should be tested for stability at the extreme operating
temperatures; this is particularly true of circuits that use
ceramic output capacitors.
Figure 5 shows a design that uses ceramic capacitors at
both input and output. It is intended to convert 3.3V to 5V
at 250mA and is a good circuit for use in PCMCIA cards.
The ceramic capacitors result in both low output ripple and
low height. The inductors listed result in a circuit height
under 1.8mm. The Sumida inductor requires a hole in the
circuit board for mounting; however, it requires less board
area than the Coiltronics part. Figure 6 shows the ripple
and transient response. Note that transient response
generally suffers with reduced output capacitance. This is
especially true with the LT1317 in Burst Mode operation,
when the load changes from a very low current (<100
µ
A)
to a higher current.
Low-Battery Detector
The LT1317’s low-battery detector is a comparator whose
open collector output appears at the LBO pin of the
LT1317. The inverting input is internally tied to a 200mV
reference and the noninverting input appears at the LBI
pin. The LBI and LBO pins appear at the edge of the DC194.
There are also pads to add a resistor divider (R5 and R6)
from V
IN
to the LBI pin and a pull-up resistor (R4) from the
LBO to V
IN
. Figure 7 shows two applications of the low-
battery detector. The first shows its intended use, as an
input voltage monitor; the second shows how to use it as
an undervoltage lockout.
OPERATIO
U
Figure 6. Above Is the Transient Response of the All-Ceramic
Design to a 50mA Load Step. The Low Impedance of the
Ceramic Output Capacitor Results in Low Output Ripple.
V
IN
= 3.3V, V
OUT
= 5V
200
µ
s/DIV
DC194 F06
V
OUT
AC COUPLED
100mV/DIV
I
LOAD
100mA/DIV
Figure 5. Ceramic Capacitors Result in Low Output Ripple
and Minimum Circuit Size in This Low Profile Design
SW
D1
LT1317/
LT1317B
L1
C1
2.2
µ
F
6.3V
FB
V
C
6.8nF
15k
GND
332k
1.00M
C2
10
µ
F
6.3V
C1: TAIYO-YUDEN JMK316BJ106ML
C2: TAIYO-YUDEN LMK212BJ225MG
D1: MOTOROLA MBR0520
DC194 F05
V
OUT
5V
(250mA
AT 3.3V
IN
)
V
IN
SHDN
V
IN
1.5V
TO 5V
L1: SUMIDA CLQ61B-8R2 OR
COILTRONICS TP1-100
