8

LTC1261

OUTPUT RIPPLE

Output ripple in the LTC1261 comes from two sources;

voltage droop at the output capacitor between clocks and

frequency response of the regulation loop. Voltage droop

is easy to calculate. With a typical clock frequency of

550kHz, the charge on the output capacitor is refreshed

once every 1.8

µs. With a 15mA load and a 3.3µF output

capacitor, the output will droop by:

= 15mA

×

∆t

)

)

= 8.2mV

1.8

µs

3.3

µF

)

)

This can be a significant ripple component when the

output is heavily loaded, especially if the output capacitor

is small. If absolute minimum output ripple is required, a

10

µF or greater output capacitor should be used.

Regulation loop frequency response is the other major

contributor to output ripple. The LTC1261 regulates the

output voltage by limiting the amount of charge trans-

ferred to the output capacitor on a cycle-by-cycle basis.

The output voltage is sensed at the ADJ pin (COMP for

fixed output versions) through an internal or external

resistor divider from the OUT pin to ground. As the flying

capacitors are first connected to the output, the output

voltage begins to change quite rapidly. As soon as it

exceeds the set point COMP1 trips, switching the state of

the charge pump and stopping the charge transfer. Be-

cause the RC time constant of the capacitors and the

switches is quite short, the ADJ pin must have a wide AC

bandwidth to be able to respond to the output in time.

External parasitic capacitance at the ADJ pin can reduce

the bandwidth to the point where the comparator cannot

respond by the time the clock pulse finishes. When this

happens the comparator will allow a few complete pulses

through, then overcorrect and disable the charge pump

until the output drops below the set point. Under these

conditions the output will remain in regulation but the

output ripple will increase as the comparator “hunts” for

the correct value.

To prevent this from happening, an external capacitor can

be connected from ADJ (or COMP for fixed output parts)

to ground to compensate for external parasitics and in-

crease the regulation loop bandwidth (Figure 3). This

sounds counterintuitive until we remember that the inter-

nal reference is generated with respect to OUT, not ground.

COMP 1

1.24V

R2

ADJ/COMP

RESISTORS ARE

INTERNAL FOR

FIXED OUTPUT PARTS

LTC1261 • F03

R1

100pF

TO CHARGE

PUMP

REF

+

–

Figure 3. Regulator Loop Compensation

The feedback loop actually sees ground as its “output,”

thus the compensation capacitor should be connected

across the “top” of the resistor divider, from ADJ (or

COMP) to ground. By the same token, avoid adding

slow down the feedback loop and increase output ripple.

A 100pF capacitor from ADJ or COMP to ground will

compensate the loop properly under most conditions.

OUTPUT FILTERING

If extremely low output ripple (< 5mV) is required, addi-

tional output filtering is required. Because the LTC1261

uses a high 550kHz switching frequency, fairly low value

RC or LC networks can be used at the output to effectively

filter the output ripple. A 10

Ω series output resistor and a

3.3

µF capacitor will cut output ripple to below 3mV (Figure

4). Further reductions can be obtained with larger filter

capacitors or by using an LC output filter.