LTC1514-5 Linear Technology, LTC1514-5 Datasheet - Page 5

• Manufacturer Part Number: LTC1514-5
• Description: Step-Up/Step-Down Switched Capacitor DC/DC Converters with Low-Battery Comparator
• Manufacturer: Linear Technology

APPLICATIONS
battery comparator and 1.145V reference are kept alive in
shutdown.
GND (Pin 4): Ground. Should be tied to a ground plane for
best performance.
C1
Terminal.
C1
Terminal.
Regulator Operation
The regulator section of the LTC1514-3.3/LTC1514-5
consists of a charge pump, reference, comparator and
some logic. The divided down output voltage is com-
pared to the internal reference voltage. When the divided
output drops below the reference voltage, the charge
pump is enabled, which boosts the output back into
regulation. Hysteresis in the comparator forces the regu-
lator to burst on and off and causes approximately
100mV of peak-to-peak ripple to appear at the output. By
enabling the charge pump only when needed, the
LTC1514-3.3 and LTC1514-5 are able to achieve high
efficiencies with low output load currents.
Each part’s charge pump has a unique architecture that
allows the input voltage to be either stepped up or
stepped down to produce a regulated output. Internal
circuitry senses the V
controls the charge pump operating mode. In addition,
the effective output impedance of the charge pump is
internally adjusted to prevent large inrush currents and
allow for a wide input voltage range. When the input
voltage is lower than the output voltage, the charge pump
operates as a step-up voltage doubler. When the input
voltage is greater than the output, the charge pump
operates as a step-down gated switch.
Capacitor Selection
For best performance, low ESR capacitors are recom-
mended for both C
The C
PIN
–
+
U
(Pin 5): Charge Pump Flying Capacitor, Negative
(Pin 6): Charge Pump Flying Capacitor, Positive
IN
FUNCTIONS
and C
U
OUT
capacitors should be either ceramic or
IN
U
and C
U
IN
INFORMATION
to V
U
OUT
OUT
to reduce noise and ripple.
differential voltage and
W
U
V
2V and 8V (LTC1514-3.3) or between 2.7V and 10V
(LTC1514-5). V
ESR capacitor as close as possible to the pin for best
performance.
V
age is internally set to either 3.3V (LTC1514-3.3) or to 5V
(LTC1514-5) using an internal resistor divider. V
be bypassed with a 10 F low ESR capacitor as close as
possible to the pin for best performance.
tantalum and should be 10 F or greater. If the input
source impedance is very low (< 0.5 ), C
needed. Increasing the size of C
reduce output voltage ripple—particularly with high V
voltages (8V or greater). A ceramic capacitor is recom-
mended for the flying capacitor C1 with a value of 0.1 F
or 0.22 F. Smaller value flying capacitors may be used in
low output current applications.
Output Ripple
Normal LTC1514-3.3/LTC1514-5 operation produces
voltage ripple on the V
required for the parts to regulate. Low frequency ripple
exists due to the hysteresis in the sense comparator and
propagation delays in the charge pump enable/disable
circuits. High frequency ripple is also present mainly
from the ESR (equivalent series resistance) in the output
capacitor. Typical output ripple (V
mum load is 100mV peak-to-peak with a low ESR (< 0.5 )
10 F output capacitor. For applications requiring V
exceed 8V, a 22 F or larger C
mended to maintain max 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
cycle. A large C1 flying capacitor (> 0.22 F) also
increases ripple in step-up mode for the same reason.
Large output current load and/or a small output capacitor
(< 10 F) results in higher ripple due to higher output
voltage dV/dt. High ESR capacitors (ESR > 0.5 ) on the
IN
OUT
(Pin 7): Charge Pump Input Voltage. May be between
(Pin 8): Regulated Output Voltage. The output volt-
LTC1514-3.3/LTC1514-5
IN
should be bypassed with a 10 F low
OUT
pin. Output voltage ripple is
OUT
OUT
IN
to 22 F or greater will
capacitor is recom-
< 8V) under maxi-
OUT
IN
per charging
may not be
OUT
should
IN
5
to
IN