LTC1261CS8-4.5(RevA) Ver la hoja de datos (PDF) - Linear Technology

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LTC1261CS8-4.5
(Rev.:RevA)

Switched Capacitor Regulated Voltage Inverter

Linear Technology 

LTC1261

APPLICATIONS INFORMATION

5V

1µF

0.1µF

VCC

2 C1+

6

OUT

LTC1261CS8-4

3 C1–

5

COMP

GND

4

10Ω

VOUT = –4V

+ 3.3µF + 3.3µF

100pF

LTC1261 • F04

Figure 4. Output Filter Cuts Ripple Below 3mV

CAPACITOR SELECTION

Capacitor Sizing

The performance of the LTC1261 can be affected by the

capacitors it is connected to. The LTC1261 requires by-

pass capacitors to ground for both the VCC and OUT pins.

The input capacitor provides most of LTC1261’s supply

current while it is charging the flying capacitors. This

capacitor should be mounted as close to the package as

possible and its value should be at least five times larger

than the flying capacitor. Ceramic capacitors generally

provide adequate performance but avoid using a tantalum

capacitor as the input bypass unless there is at least a

0.1µF ceramic capacitor in parallel with it. The charge

pump capacitors are somewhat less critical since their

peak currents are limited by the switches inside the

LTC1261. Most applications should use 0.1µF as the

flying capacitor value. Conveniently, ceramic capacitors

are the most common type of 0.1µF capacitor and they

work well here. Usually the easiest solution is to use the

same capacitor type for both the input bypass and the

flying capacitors.

In applications where the maximum load current is well-

defined and output ripple is critical or input peak currents

need to be minimized, the flying capacitor values can be

tailored to the application. Reducing the value of the

flying capacitors reduces the amount of charge trans-

ferred with each clock cycle. This limits maximum output

current, but also cuts the size of the voltage step at the

output with each clock cycle. The smaller capacitors

draw smaller pulses of current out of VCC as well, limiting

peak currents and reducing the demands on the input

supply. Table 1 shows recommended values of flying

capacitor vs maximum load capacity.

Table 1. Typical Max Load (mA) vs Flying Capacitor Value at

TA = 25°C, VOUT = – 4V

FLYING

CAPACITOR

VALUE (µF)

MAX LOAD (mA)

MAX LOAD (mA)

VCC = 5V DOUBLER MODE VCC = 3.3V TRIPLER MODE

0.1

22

20

0.047

16

15

0.033

8

11

0.022

4

5

0.01

1

3

The output capacitor performs two functions: it provides

output current to the load during half of the charge pump

cycle and its value helps to set the output ripple voltage.

For applications that are insensitive to output ripple, the

output bypass capacitor can be as small as 1µF. To achieve

specified output ripple with 0.1µF flying capacitors, the

output capacitor should be at least 3.3µF. Larger output

capacitors will reduce output ripple further at the expense

of turn-on time.

Capacitor ESR

Output capacitor Equivalent Series Resistance (ESR) is

another factor to consider. Excessive ESR in the output

capacitor can fool the regulation loop into keeping the

output artificially low by prematurely terminating the charg-

ing cycle. As the charge pump switches to recharge the

output a brief surge of current flows from the flying

capacitors to the output capacitor. This current surge can

be as high as 100mA under full load conditions. A typical

3.3µF tantalum capacitor has 1Ω or 2Ω of ESR; 100mA ×

2Ω = 200mV. If the output is within 200mV of the set point

this additional 200mV surge will trip the feedback com-

parator and terminate the charging cycle. The pulse dissi-

pates quickly and the comparator returns to the correct

state, but the RS latch will not allow the charge pump to

respond until the next clock edge. This prevents the charge

9

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