MAX861ESA/V+ Datasheet MAX860ISA+T, MAX860ESA+, MAX861ESA+ | P7-P9

MAX860/MAX861

50mA, Frequency-Selectable,

Switched-Capacitor Voltage Converters

_______________________________________________________________________________________ 7

__________Applications Information

Capacitor Selection

The MAX860/MAX861 are tested using 10µF capacitors

for both C1 and C2, although smaller or larger values

can be used (Table 3). Smaller C1 values increase the

output resistance; larger values reduce the output

resistance. Above a certain point, increasing the

capacitance of C1 has a negligible effect (because the

output resistance becomes dominated by the internal

switch resistance and the capacitor ESR). Low-ESR

capacitors provide the lowest output resistance and

ripple voltage. The output resistance of the entire circuit

(inverter or doubler) is approximately:

OUT

= R

+ 4 x ESR

+ ESR

+ 1 / (f

x C1)

where R

(the effective resistance of the MAX860/

MAX861’s internal switches) is approximately 8Ω and f

is the switching frequency. R

OUT

is typically 12Ω when

using capacitors with 0.2Ω ESR and f

, C1, and C2 val-

ues suggested in Table 3. When C1 and C2 are so

large (or the switching frequency is so high) that the

internal switch resistance dominates the output resis-

tance, estimate the output resistance as follows:

OUT

= R

+ 4 x ESR

+ ESR

A typical design procedure is as follows:

1) Choose C1 and C2 to be the same, for convenience.

2) Select f

a) If you want to avoid a specific noise frequency,

choose f

appropriately.

b) If you want to minimize capacitor cost and size,

choose a high f

c) If you want to minimize current consumption,

choose a low f

3) Choose a capacitor based on Table 3, although

higher or lower values can be used to optimize per-

formance. Table 4 lists manufacturers who provide

low-ESR capacitors.

*In addition to Table 3, four graphs in the Typical

Operating Characteristics section show typical output

current for C1 and C2 capacitances ranging from

0.33µF to 22µF. Output current is plotted for inputs of

4.5V (5V - 10%) and 3.0V (3.3V - 10%), and also for

10% and 20% output droop from the ideal -V

value.

ATTRIBUTE

LOWER

FREQUENCY

HIGHER

FREQUENCY

Output Ripple Larger Smaller

C1, C2 Values Larger Smaller

Supply Current Smaller Larger

C1, C2 (µF)NOMINAL FREQUENCY (kHz)

6 68

13 47

50 10

100 4.7

130 4.7

250 2.2

Table 2. Switching-Frequency Trade-Offs

Table 3. Suggested Capacitor Values*

Table 4. Low-ESR Capacitor Manufacturers

MANUFACTURER–Series PHONE FAX COMMENTS

AVX TPS Series (803) 946-0629 (803) 626-3123 Low-ESR tantalum, SMT

AVX TAG Series (803) 946-0629 (803) 626-3123 Low-cost tantalum, SMT

Matsuo 267 Series (714) 969-2491 (714) 960-6492 Low-cost tantalum, SMT

Sprague 595 Series (603) 224-1961 (613) 224-1430 Low-ESR tantalum, SMT

Sanyo MV-GX Series (619) 661-6835 (619) 661-1055 Aluminum electrolytic, through hole

Sanyo CV-GX Series (619) 661-6835 (619) 661-1055 Aluminum electrolytic, SMT

Nichicon PL Series (847) 843-7500 (847) 843-2798 Aluminum electrolytic, through hole

United Chemicon (Marcon) (847) 696-2000 (847) 696-9278 Ceramic SMT

TDK (847) 390-4461 (847) 390-4405 Ceramic SMT

MAX860/MAX861

50mA, Frequency-Selectable,

Switched-Capacitor Voltage Converters

8 _______________________________________________________________________________________

MAX860

MAX861

“n”

MAX860

MAX861

“1”

OUT

C1C1

445

OUT

= -V

OUT

OF SINGLE DEVICE

NUMBER OF DEVICES

…

Figure 2. Paralleling MAX860s or MAX861s to Reduce Output

Resistance

MAX860

MAX861

“n”

MAX860

MAX861

“1”

OUT

445

OUT

= -nV

…

Figure 1. Cascading MAX860s or MAX861s to Increase

Output Voltage

Flying Capacitor, C1

Increasing the size of the flying capacitor reduces the

output resistance.

Output Capacitor, C2

Increasing the size of the output capacitor reduces the

output ripple voltage. Decreasing its ESR reduces both

output resistance and ripple. Smaller capacitance val-

ues can be used if one of the higher switching frequen-

cies is selected, if less than the maximum rated output

current (50mA) is required, or if higher ripple can be

tolerated. The following equation for peak-to-peak rip-

ple applies to both the inverter and doubler circuits.

OUT

RIPPLE

= ———————— + 2 x I

OUT

x ESR

2 x f

x C2

Bypass Capacitor

Bypass the incoming supply to reduce its AC impedance

and the impact of the MAX860/MAX861’s switching

noise. The recommended bypassing depends on the cir-

cuit configuration and where the load is connected.

When the inverter is loaded from OUT to GND or the

doubler is loaded from V

to GND, current from the

supply switches between 2 x I

OUT

and zero. Therefore,

use a large bypass capacitor (e.g., equal to the value

of C1) if the supply has a high AC impedance.

When the inverter and doubler are loaded from V

OUT, the circuit draws 2 x I

OUT

constantly, except for

short switching spikes. A 0.1µF bypass capacitor is

sufficient.

Cascading Devices

Two devices can be cascaded to produce an even

larger negative voltage, as shown in Figure 1. The

unloaded output voltage is nominally -2 x V

, but this is

reduced slightly by the output resistance of the first

device multiplied by the quiescent current of the sec-

ond. The output resistance of the complete circuit is

approximately five times the output resistance of a sin-

gle MAX860/MAX861.

Three or more devices can be cascaded in this way,

but output resistance rises dramatically, and a better

solution is offered by inductive switching regulators

(such as the MAX755, MAX759, MAX764, or MAX774).

Connect LV as with a standard inverter circuit (see Pin

Description).

The maximum load current and startup current of nth

cascaded circuit must not exceed the maximum output

current capability of (n-1)th circuit to ensure proper

startup.

Paralleling Devices

Paralleling multiple MAX860s or MAX861s reduces the

output resistance. As illustrated in Figure 2, each

device requires its own pump capacitor (C1), but the

reservoir capacitor (C2) serves all devices. C2’s value

should be increased by a factor of n, where n is the

number of devices. Figure 2 shows the equation for cal-

culating output resistance. An alternative solution is to

use the MAX660 or MAX665, which are capable of sup-

plying up to 100mA of load current. Connect LV as with

a standard inverter circuit (see Pin Description).

Combined Doubler/Inverter

In the circuit of Figure 3, capacitors C1 and C2 form the

inverter, while C3 and C4 form the doubler. C1 and C3

are the pump capacitors; C2 and C4 are the reservoir

capacitors. Because both the inverter and doubler use

part of the charge-pump circuit, loading either output

causes both outputs to decline towards GND. Make

MAX860/MAX861

50mA, Frequency-Selectable,

Switched-Capacitor Voltage Converters

_______________________________________________________________________________________ 9

sure the sum of the currents drawn from the two out-

puts does not exceed 60mA. Connect LV as with a

standard inverter circuit (see Pin Description).

Compatibility with

MAX660/MAX665/ICL7660

The MAX860/MAX861 can be used in sockets

designed for the MAX660, MAX665, and ICL7660 with

a minimum of one wiring change. This section gives

advice on installing a MAX860/MAX861 into a socket

designed for one of the earlier devices.

The MAX660, MAX665, and ICL7660 have an OSC pin

instead of

–

—

–

. MAX660, MAX665, and ICL7660 nor-

mal operation is with OSC floating (although OSC can

be overdriven). If OSC is floating, pin 7 (

–

—

–

) should

be jumpered to V

to enable the MAX860/MAX861

permanently. Do not leave

–

—

–

on the MAX860/

MAX861 floating.

The MAX860/MAX861 operate with FC either floating or

connected to V

, OUT, or GND; each connection

defines the oscillator frequency. Thus, any of the nor-

mal MAX660, MAX665, or ICL7660 connections to pin 1

will work with the MAX860/MAX861, without modifica-

tions. Changes to the FC connection are only required

if you want to adjust the operating frequency.

MAX860

MAX861

OUT

= (2V

) -

FD1

) - (V

FD2

)

OUT

= -V

D1, D2 = 1N4148

Figure 3. Combined Doubler and Inverter

PART

NUMBER

OUTPUT

CURRENT

(mA)

OUTPUT

RESISTANCE

(Ω)

SWITCHING

FREQUENCY

(kHz)

MAX660 100 6.5 5/40

MAX665 100 6.5 5/40

MAX860 50 12 6/50/130

MAX861 50 12 13/100/250

ICL7660 10 55 5

Table 5. Product Selection Guide

TRANSISTOR COUNT: 101

SUBSTRATE CONNECTED TO V

SHDN

OUT

C1+

GND

C1-

0.084"

(2.13mm)

0.058"

(1.47mm)

___________________Chip Topography

P1-P3 P4-P6 P7-P9 P10-P11

MAX861ESA/V+

Mfr. #:

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Manufacturer:

Maxim Integrated

Description:

Switching Voltage Regulators 50mA f-Selectable Switched-Cap V Conv

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