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MAX868C/D

Regulated, Adjustable -2x Inverting Charge Pump

Maxim Integrated 

Regulated, Adjustable -2x

Inverting Charge Pump

VREF

OPTIONAL

CONNECTION

VIN

CC*

VOUT

R2

R1

FB

MAX868

IN

OUT

*OPTIONAL

FEED-FORWARD

CAPACITOR

Figure 4. Setting the Output Voltage Using Two External

Resistors

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:

IOUT(MAX) =

| | 2 x VIN − VOUT

4

10V

( ) | | fMAX x C1 + C2 + ROUT x VIN + VOUT

where fMAX is the maximum oscillator frequency (typically

450kHz), ROUT is the MAX868 open-loop output

impedance (typically 70Ω), 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:





( ) | | VRIPPLE = 2 x VIN − VOUT



x

 1 +

1

4 x COUT

C1 + C2

+

RESR

ROUT







where C1 and C2 are the flying capacitors, RESR is the

output capacitor’s ESR, and ROUT is the MAX868’s

open-loop output impedance, typically 70Ω.

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.

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

TYPE

MANUFACTURER

PART

PHONE

AVX

TPS series

(803) 946-0690

Surface-Mount Tantalum

Matsuo

267 series

(714) 969-2491

Sprague

593D, 595D series

(603) 224-1961

Surface-Mount Ceramic

AVX

Matsuo

X7R type

X7R type

(803) 946-0690

(714) 969-2491

FAX

(803) 626-3123

(714) 960-6492

(603) 224-1430

(803) 626-3123

(714) 960-6492

_______________________________________________________________________________________ 7

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