LTC1983-3/LTC1983-5

7

sn1983 1983fs

There are many aspects of the capacitors that must be

taken into account. First, the temperature stability of the

dielectric is a main concern. For ceramic capacitors, a

three character code specifies the temperature stability

(e.g. X7R, Y5V, etc.). The first two characters represent

the temperature range that the capacitor is specified and

the third represents the absolute tolerance that the ca-

pacitor is specified to over that temperature range. The

ceramic capacitor used for the flying and output capaci-

tors should be X5R or better. Second, the voltage coef-

ficient of capacitance for the capacitor must be checked

and the actual value usually needs to be derated for the

operating voltage (the actual value has to be larger than

the value needed to take into account the loss of capaci-

tance due to voltage bias across the capacitor). Third, the

frequency characteristics need to be taken into account

because capacitance goes down as the frequency of

oscillation goes up. Typically, the manufacturers have

capacitance vs frequency curves for their products. This

curve must be referenced to be sure the capacitance will

not be too small for the application. Finally, the capacitor

ESR and ESL must be low for reasons mentioned in the

following section.

Output Ripple

Normal LTC1983 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 of the output capacitor.

with a low ESR 10

µF output capacitor. The magnitude of

the ripple voltage depends on several factors. High input

(>1

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 > 0.1

Ω) on the output pin cause

cycle.

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 4.7µF ceramic capacitor

However, the best solution is to use 10

µF to 22µF, X5R

ceramic capacitors which are available in 1206 package

sizes. An RC filter may also be used to reduce high

frequency voltage spikes (see Figure 1).

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

Figure 1. Output Ripple Reduction Techniques

LTC1983-X

10

µF

TANTALUM

10

µF

TANTALUM

LTC1983-X

15

µF

TANTALUM

1

µF

CERAMIC

3.9

Ω

1983 F01

OPERATIO

(Refer to Block Diagram)