LTC1516 Linear Technology, LTC1516 Datasheet - Page 5

• Manufacturer Part Number: LTC1516
• Description: Micropower/ Regulated 5V Charge Pump DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIONS
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1516 will draw
between 200mA and 400mA from V
the junction temperature. On-chip thermal shutdown
circuitry disables the charge pump once the junction
temperature exceeds 135 C, and reenables the charge
pump once the junction temperature falls back to 115 C.
The LTC1516 will cycle in and out of thermal shutdown
indefinitely without latchup or damage until the V
is removed.
Capacitor Selection
For best performance, it is recommended that low ESR
capacitors be used for both C
and ripple. The C
ceramic or tantalum and should be 10 F or greater. If the
input source impedance is very low, C
needed. Increasing the size of C
reduce output voltage ripple.
Ceramic or tantalum capacitors are recommended for the
flying caps C1 and C2 with values in the range of 0.1 F to
1 F. Note that large value flying caps (> 0.22 F) will
increase output ripple unless C
very low load applications, C1 and C2 may be reduced to
0.01 F to 0.047 F. This will reduce output ripple at the
expense of efficiency and maximum output current.
Output Ripple
Normal LTC1516 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 (Equivalent Series
Resistance) in the output capacitor. Typical output ripple
under maximum load is 100mV
output capacitor.
The magnitude of the ripple voltage depends on several
factors. High input voltages (V
ripple since more charge is delivered to C
cycle. Large C1 and C2 flying capacitors (> 0.22 F) also
increase ripple for the same reason. Large output current
load and/or a small output capacitor (< 10 F) results in
OUT
pin. Output voltage ripple is required for the LTC1516
IN
U
and C
INFORMATION
OUT
U
IN
IN
capacitors should be either
> 3.3V) increase the output
OUT
OUT
and C
P-P
W
to 22 F or greater will
is also increased. For
with a low ESR 10 F
IN
OUT
causing a rise in
to reduce noise
IN
OUT
may not be
U
per clock
OUT
short
higher ripple due to higher output voltage dV/dt. High ESR
capacitors (ESR > 0.5 ) on the output pin cause high
frequency voltage spikes on V
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 3.3 F ceramic capacitor
on V
An RC filter may also be used to reduce high frequency
voltage spikes (see Figure 2).
In low load or high V
and C2 may be used to reduce output ripple. The smaller
C1 and C2 flying capacitors (0.022 F to 0.1 F) deliver 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
efficiency.
Inrush Currents
During normal operation, V
sients in the 100mA to 200mA range whenever the charge
pump is enabled. During start-up, these inrush currents
may approach 500mA. For this reason, it is important to
minimize the source resistance between the input supply
and the V
input voltage transients.
OUT
Figure 2. Output Ripple Reduction Techniques
to reduce both the low and high frequency ripple.
OUT
IN
LTC1516
LTC1516
pin to prevent start-up problems and large
V
V
capacitor (22 F or greater) will reduce both
OUT
OUT
3
3
+
+
IN
applications, smaller values for C1
15 F
TANTALUM
10 F
IN
2
will experience current tran-
OUT
OUT
+
with every clock cycle.
capability as well as
1 F
CERAMIC
10 F
LTC1516
1516 F02
V
5V
V
5V
OUT
OUT
5
OUT