5

LTC1522

A ceramic capacitor is recommended for the flying capaci-

tor with a value in the range of 0.1

µF to 0.22µF. Note that

a large value flying cap (> 0.22

µF) will increase output

This will reduce output ripple at the expense of efficiency

and maximum output current.

Output Ripple

Normal LTC1522 operation produces voltage ripple on the

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

output capacitor.

The magnitude of the ripple voltage depends on several

cycle. A large flying capacitor (> 0.22

µF) also increases

ripple for the same reason. Large output current load and/

or a small output capacitor (< 10

µF) results in higher ripple

due to higher output voltage dV/dt. High ESR capacitors

(ESR > 0.5

Ω) on the output pin cause high frequency

There are several ways to reduce the output voltage ripple.

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

µF to 22µF tantalum

capacitor in parallel with a 1

µF to 3.3µF ceramic capacitor

An RC filter may also be used to reduce high frequency

voltage spikes (see Figure 1).

5V

LTC1522

3

15

µF

TANTALUM

1

µF

CERAMIC

5V

+

LTC1522

3

1522 F01

3.9

Ω

10

µF

TANTALUM

10

µF

TANTALUM

+

+

Figure 1. Output Ripple Reduction Techniques

capacitor (0.01

µF to 0.047µF) delivers less charge per

clock cycle to the output capacitor resulting in lower

output ripple. However, the smaller value flying caps also

Inrush Currents

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

regulation problems or even prevent start-up.

Regulated Supply

The LTC1522 contains an internal resistor divider (refer to

the Block Diagram) that draws only 1

During no-load conditions, the internal load causes a

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