Datasheet

9
LT1610
APPLICATIONS INFORMATION
WUU
U
V
OUT
200mV/DIV
I
DIODE
500mA/DIV
SWITCH
VOLTAGE
10V/DIV
100ns/DIV
1610 F07
Figure 7. Switching Waveforms Without Ceramic Capacitor C5
V
OUT
50mV/DIV
I
DIODE
500mA/DIV
SWITCH
VOLTAGE
10V/DIV
V
IN
= 4.1V 100ns/DIV 1610 F08
LOAD = 100mA
Figure 8. Switching Waveforms with Ceramic Capacitor C5.
Note the 50mV/DIV Scale for V
OUT
Figure 6. 4-Cell Alkaline to 5V/120mA SEPIC DC/DC Converter
It is best to choose the compensation components empiri-
cally. Once the power components have been chosen
(based on size, efficiency, cost and space requirements),
a working circuit is built using conservative (or merely
guessed) values of R
C
and C
C
. Then the response of the
circuit is observed under a transient load, and the compen-
sation network is modified to achieve stable operation.
Linear Technology’s Application Note 19 contains a de-
tailed description of the method. A good starting point for
the LT1610 is C
C
~ 220pF and R
C
~ 220k.
All Ceramic, Low Profile Design
Large value ceramic capacitors that are suitable for use as
the main output capacitor of an LT1610 regulator are now
available. These capacitors have very low ESR and there-
fore 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 it 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 character-
istics and are suitable for use as the output capacitor. The
X7R type has better stability across temperature, whereas
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
SHUTDOWN
C2
22µF
6.3V
C1
22µF
6.3V
L1
22µH
L2
22µH
C3
1µF
CERAMIC
D1
V
OUT
5V
120mA
1610 F06
+
+
C4
1µF
CERAMIC
1M
324k
C1, C2: ALUMINUM ELECTROLYTIC
C3 TO C5: CERAMIC X7R OR X5R
D1: MBR0520
L1, L2: MURATA LQH3C220 OR SUMIDA CLS62-220
C5
1µF
CERAMIC
65
•
•
2
3
4
7
8
1
V
IN
SW
PGND
FB
SHDN
GND
COMP
LT1610
V
C
4 CELLS