Datasheet

7
LTC1515 Series
APPLICATIONS INFORMATION
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ing out of shutdown mode. Whenever large V
IN
(or boosted
V
IN
) to V
OUT
voltage differentials are present, 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, especially if the input
supply is a low power DC/DC converter or linear regulator.
The LTC1515 family minimizes inrush currents both at
start-up and under high V
IN
to V
OUT
operation.
Internal soft start circuitry controls the rate at which V
OUT
may be charged from 0V to its final regulated value. The
typical start-up time from V
OUT
= 0V to 5V is 4ms. This
corresponds to an effective V
OUT
charging current of only
12.5mA for a 10µF output capacitor (27.5mA for 22µF,
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 cur-
rent both at initial power-up and when coming out of
shutdown.
As the V
IN
(or boosted V
IN
) to V
OUT
voltage differential
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
IN
whenever the charge pump is enabled and helps
to reduce both input and output ripple.
Power-On Reset
The POR pin is an open-drain output that pulls low when
the output voltage is out of regulation. When the V
OUT
rises to within 6.5% of regulation, an internal timer is
started which releases POR after 200ms (typ). In shut-
down, the POR output is pulled low. In normal operation,
an external pull-up resistor is generally used between the
POR pin and V
OUT
.
Protection Features
All of the parts 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
OUT
< 100mV) exists. The parts can survive an
indefinite short to GND.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
(equivalent series resistance) in the output capacitor.
Typical output ripple (V
IN
< 8V) under maximum load is
100mV peak-to-peak with a low ESR, 10µF output capaci-
tor. For applications requiring V
IN
to exceed 8V, a 22µF or
larger C
OUT
capacitor is recommended to maintain maxi-
mum ripple in the 100mV range.
The magnitude of the ripple voltage depends on several
factors. High input voltages increase the output ripple
since more charge is delivered to C
OUT
per charging cycle.
A large C1 flying capacitor (> 0.22µF) also increases ripple
in step-up mode for the same reason. Large output
current load and/or a small output capacitor (<10µF)
results in higher ripple due to higher output voltage dV/dt.
High ESR capacitors (ESR > 0.5Ω) on the output pin cause
high frequency voltage spikes on V
OUT
with every clock
cycle.
There are several ways to reduce the output voltage ripple.
A large C
OUT
capacitor (22µF or greater) will reduce both
the low and high frequency ripple due to the lower C
OUT
charging and discharging dV/dt and the lower ESR typi-
cally found with higher value (larger case size) capacitors.
A low ESR (<0.5Ω) ceramic output capacitor will mini-
mize the high frequency ripple, but will not reduce the low
frequency ripple unless a high capacitance value is cho-
sen. A reasonable compromise is to use a 10µF to 22µF
tantalum capacitor in parallel with a 1µF to 3.3µF ceramic
capacitor on V
OUT
to reduce both the low and high
frequency ripple. An RC or LC filter may also be used to
reduce high frequency voltage spikes (see Figure 4).
Inrush Currents
A common problem with switched capacitor regulators is
inrush current—particularly during power-up and com-
15µF
TANTALUM
LT1515 • F04
V
OUT
V
OUT
8
1µF
CERAMIC
10µF
TANTALUM
V
OUT
V
OUT
8
10µF
TANTALUM
2Ω
LTC1515/
LTC1515-X
LTC1515/
LTC1515-X
+
+ +
Figure 4. Output Ripple Reduction Techniques