Capacitor Selection

Choosing the Flying Capacitors

Proper choice of the flying capacitors is dependent pri-

marily upon the desired output current. For flying capaci-

tors in the 0.1µF to 0.33µF range, the maximum output

current can be approximated by the following equation:

impedance (typically 70

Ω), and C1 and C2 are the flying-

capacitor values. As a general rule, choose the lowest-

value flying capacitors that provide the desired output

current in order to minimize output voltage ripple (see the

section

Choosing the Output Capacitor).

Surface-mount ceramic capacitors are preferred, due

to their small size, low cost, and low equivalent series

resistance (ESR). To ensure proper operation over the

entire temperature range, choose ceramic capacitors

with X7R (or equivalent) low temperature-coefficient

(tempco) dielectrics. See Table 1 for a list of suggested

capacitor suppliers.

Choosing the Output Capacitor

The output capacitor stores the charge transferred from

the flying capacitors and services the load between

oscillator cycles. A good general rule is to make the

output capacitance at least ten times greater than that

of the flying capacitors.

The output voltage ripple is dependent upon the

capacitance of the flying capacitor and upon the output

capacitor’s capacitance and ESR. When operating in

closed-loop mode (when the MAX868 is generating a

regulated output voltage), use the following equation to

approximate peak-to-peak output voltage ripple:

open-loop output impedance, typically 70

Ω.

Choose a low-ESR output capacitor for minimum output

ripple. Surface-mount ceramic capacitors are preferred

for their small size, low cost, and low ESR; low-ESR tan-

talum electrolytic capacitors are also acceptable. When

using a ceramic output capacitor, ensure proper opera-

tion over the entire temperature range by choosing a

capacitor with X7R (or equivalent) low tempco dielec-

tric. See Table 1 for a list of suggested capacitor sup-

pliers.

V

2 x V

V

x

1

1

4 x C

C1

C2

R

R

RIPPLE

IN

OUT

OUT

ESR

OUT

|

|

=

−

(

)

+

+

+





















I

2 x V

V

4

f

x C1

C2

+ R

x

10V

V

V

OUT(MAX)

IN

OUT

MAX

OUT

IN

OUT

|

|

|

|

=

−

+

+

(

)

Regulated, Adjustable -2x

Inverting Charge Pump

_______________________________________________________________________________________

7

Table 1. Manufacturers of Surface-Mount, Low-ESR Capacitors

Sprague

TYPE

Matsuo

AVX

Surface-Mount Tantalum

MANUFACTURER

593D, 595D series

267 series

TPS series

PART

(603) 224-1430

(714) 960-6492

(803) 626-3123

FAX

(603) 224-1961

(714) 969-2491

(803) 946-0690

PHONE

X7R type

X7R type

(714) 960-6492

(803) 626-3123

(714) 969-2491

(803) 946-0690

Matsuo

Surface-Mount Ceramic

AVX

MAX868

IN

OPTIONAL

CONNECTION

*OPTIONAL

FEED-FORWARD

CAPACITOR

OUT

FB

R2

R1

Figure 4. Setting the Output Voltage Using Two External

Resistors