LTC1983-3 LINER [Linear Technology], LTC1983-3 Datasheet - Page 7

• Manufacturer Part Number: LTC1983-3
• Description: 100mA REGULATED CHARGE-PUMP INVERTERS IN THINSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
There are many aspects of the capacitors that must be
taken into account. First, the temperature stability of the
dielectric is a main concern. For ceramic capacitors, a
three character code specifies the temperature stability
(e.g. X7R, Y5V, etc.). The first two characters represent
the temperature range that the capacitor is specified and
the third represents the absolute tolerance that the ca-
pacitor is specified to over that temperature range. The
ceramic capacitor used for the flying and output capaci-
tors should be X5R or better. Second, the voltage coef-
ficient of capacitance for the capacitor must be checked
and the actual value usually needs to be derated for the
operating voltage (the actual value has to be larger than
the value needed to take into account the loss of capaci-
tance due to voltage bias across the capacitor). Third, the
frequency characteristics need to be taken into account
because capacitance goes down as the frequency of
oscillation goes up. Typically, the manufacturers have
capacitance vs frequency curves for their products. This
curve must be referenced to be sure the capacitance will
not be too small for the application. Finally, the capacitor
ESR and ESL must be low for reasons mentioned in the
following section.
Output Ripple
Normal LTC1983 operation produces voltage ripple on the
V
to regulate. Low frequency ripple exists due to the hyster-
esis in the sense comparator and propagation delays in the
charge pump enable/disable circuits. High frequency ripple
is also present mainly due to ESR of the output capacitor.
Typical output ripple under maximum load is 60mV
with a low ESR 10 F output capacitor. The magnitude of
the ripple voltage depends on several factors. High input
voltage to negative output voltage differentials [(V
V
is delivered to C
(>1 F) also increases ripple for the same reason. Large
output current load and/or a small output capacitor (<10 F)
OUT
OUT
) >1V] increase the output ripple since more charge
pin. Output voltage ripple is required for the LTC1983
OUT
U
per clock cycle. A large flying capacitor
(Refer to Block Diagram)
IN
P-P
+
results in higher ripple due to higher output voltage dV/dt.
High ESR capacitors (ESR > 0.1 ) on the output pin cause
high frequency voltage spikes on V
cycle.
There are several ways to reduce the output voltage ripple.
A larger C
the low and high frequency ripple due to the lower C
charging and discharging dV/dt and the lower ESR typi-
cally found with higher value (larger case size) capacitors.
A low ESR ceramic output capacitor will minimize the high
frequency ripple, but will not reduce the low frequency
ripple unless a high capacitance value is chosen. A reason-
able compromise is to use a 10 F to 22 F tantalum
capacitor in parallel with a 1 F to 4.7 F ceramic capacitor
on V
However, the best solution is to use 10 F to 22 F, X5R
ceramic capacitors which are available in 1206 package
sizes. An RC filter may also be used to reduce high
frequency voltage spikes (see Figure 1).
In low load or high V
C
capacitor (0.01 F to 0.047 F) delivers less charge per
clock cycle to the output capacitor resulting in lower
output ripple. However, the smaller value flying caps also
reduce the maximum I
FLY
OUT
may be used to reduce output ripple. A smaller flying
Figure 1. Output Ripple Reduction Techniques
to reduce both the low and high frequency ripple.
OUT
LTC1983-X
LTC1983-X
LTC1983-3/LTC1983-5
capacitor (22 F or greater) will reduce both
V
V
OUT
OUT
IN
OUT
applications, smaller values for
capability as well as efficiency.
10 F
TANTALUM
15 F
TANTALUM
3.9
OUT
10 F
TANTALUM
1 F
CERAMIC
with every clock
1983 F01
V
V
OUT
OUT
sn1983 1983fs
7
OUT