LTC1522 Linear Technology, LTC1522 Datasheet - Page 5

• Manufacturer Part Number: LTC1522
• Description: Micropower/ Regulated 5V Charge Pump DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIONS
A ceramic capacitor is recommended for the flying capaci-
tor with a value in the range of 0.1 F to 0.22 F. Note that
a large value flying cap (> 0.22 F) will increase output
ripple unless C
applications, C
This will reduce output ripple at the expense of efficiency
and maximum output current.
Output Ripple
Normal LTC1522 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 50mV
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. A large flying capacitor (> 0.22 F) also increases
ripple 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 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 1).
OUT
OUT
pin. Output voltage ripple is required for the LTC1522
to reduce both the low and high frequency ripple.
OUT
capacitor (22 F or greater) will reduce both
FLY
OUT
may be reduced to 0.01 F to 0.047 F.
U
OUT
is also increased. For very low load
with every clock cycle.
INFORMATION
U
IN
> 3.3V) increase the output
P-P
W
with a low ESR 10 F
OUT
U
per clock
OUT
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
Inrush Currents
During normal operation, V
sients in the 50mA to 100mA range whenever the charge
pump is enabled. During start-up, these inrush currents
may approach 250mA. For this reason, it is important to
minimize the source resistance between the input supply
and the V
regulation problems or even prevent start-up.
Ultralow Quiescent Current (I
Regulated Supply
The LTC1522 contains an internal resistor divider (refer to
the Block Diagram) that draws only 1 A (typ) from V
During no-load conditions, the internal load causes a
droop rate of only 100mV per second on V
C
cycle signal to the SHDN pin ensures that the circuit of
Figure 2 comes out of shutdown frequently enough to
maintain regulation during no-load or low-load condi-
tions. Since the part spends nearly all of its time in
shutdown, the no-load quiescent current (see Figure 3a) is
approximately equal to (V
FLY
OUT
may be used to reduce output ripple. A smaller flying
= 10 F. Applying a 2Hz to 100Hz, 95% to 98% duty
Figure 1. Output Ripple Reduction Techniques
IN
LTC1522
LTC1522
pin. Too much source resistance may result in
V
V
OUT
OUT
3
3
+
+
IN
OUT
15 F
TANTALUM
10 F
TANTALUM
applications, smaller values for
OUT
3.9
capability as well as efficiency.
IN
will experience current tran-
)(1 A)/(V
Q
= 2.1 A)
+
1 F
CERAMIC
10 F
TANTALUM
IN
)(Efficiency).
LTC1522
1522 F01
V
5V
V
5V
OUT
OUT
OUT
OUT
with
5
.