LTC1514-3.3 LINER [Linear Technology], LTC1514-3.3 Datasheet - Page 6

• Manufacturer Part Number: LTC1514-3.3
• Description: Step-Up/Step-Down Switched Capacitor DC/DC Converters with Low-Battery Comparator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1514-3.3/LTC1514-5
output pin cause high frequency voltage spikes on V
with every clock cycle.
There are several ways to reduce the output voltage
ripple. A larger C
reduce both the low and high frequency ripple due to the
lower C
ESR typically 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 reasonable compromise is to use a 10 F to
22 F tantalum capacitor in parallel with a 1 F to 3.3 F
ceramic capacitor on V
high frequency ripple. An RC or LC filter may also be used
to reduce high frequency voltage spikes (see Figure 1).
Inrush Currents
A common problem with switched capacitor regulators
is inrush current — particularly during power-up and
coming out of shutdown mode. Whenever large V
boosted V
most charge pumps will pull large current spikes from
the input supply. Only the effective charge pump output
impedance limits the current while the charge pump is
enabled. This may disrupt input supply regulation, espe-
cially if the input supply is a low power DC/DC converter
or linear regulator. The LTC1514-3.3/LTC1514-5 mini-
mize inrush currents both at start-up and under normal
high V
6
IN
OUT
Figure 1. Output Ripple Reduction Techniques
to V
LTC1514-X
LTC1514-X
IN
charging and discharging dV/dt and the lower
) to V
OUT
V
V
OUT
OUT
operation.
OUT
OUT
8
8
U
+
+
voltage differentials are present,
capacitor (22 F or greater) will
OUT
15 F
TANTALUM
10 F
TANTALUM
INFORMATION
U
to reduce both the low and
2
W
+
1 F
CERAMIC
10 F
TANTALUM
1514 F01
V
V
U
OUT
OUT
IN
OUT
(or
Internal soft start circuitry controls the rate at which V
may be charged from 0V to its final regulated value. The
typical start-up time from V
corresponds to an effective V
12.5mA for a 10 F output capacitor (27.5mA for 22 F,
etc). Note that any output current load present during
start-up will add directly to the charging currents men-
tioned above. The soft start circuitry limits start-up
current both at initial power-up and when coming out of
shutdown.
As the V
grows, the effective output impedance of the charge
pump is automatically increased by internal voltage
sensing circuitry. This feature minimizes the current
spikes pulled from V
enabled and helps to reduce both input and output ripple.
Protection Features
The LTC1514-X contain thermal shutdown and short-
circuit protection features. The parts will shut down when
the junction temperature reaches approximately 150 C
and will resume operation once the junction temperature
has dropped back to approximately 140 C. The parts will
limit output current to 12mA (typ) when a short-circuit
condition (V
an indefinite short to GND. The LTC1514-X devices use
a low thermal resistance SO-8 package (110 C/W vs
150 C/W for standard SO-8). This permits full output
current, even at high input supply voltages.
Low-Battery Comparator
The internal low-battery comparator trips at 1.145 3%
(LBI ramping negative). Programming the comparator to
trip at a higher voltage can easily be done with an external
Figure 2. Programming the Low-Battery Comparator Trip Voltage
V
BAT
IN
R1
R2
(or boosted V
OUT
1
2
3
4
< 100mV) exists. The parts can survive
SHDN
LBO
LBI
GND
LTC1514-X
IN
V
OUT
C1
C1
1514 F02
whenever the charge pump is
V
IN
IN
+
–
) to V
OUT
8
7
6
5
OUT
= 0V to 5V is 4ms. This
charging current of only
V
(LBI RAMPING NEGATIVE)
OUT
TRIP
= 1.145V(1 + R1/R2)
voltage differential
OUT