Detailed description – Rainbow Electronics MAX861 User Manual

Page 6

_______________Detailed Description

The MAX860/MAX861 capacitive charge pumps either
invert or double the voltage applied to their inputs. For
highest performance, use low equivalent series resis-
tance (ESR) capacitors. See the

Capacitor Selection

section for more details. The frequency-control (FC) pin
allows you to choose one of three switching frequen-
cies; these three selectable frequencies are different for
each device. When shut down, MAX860/MAX861 cur-
rent consumption reduces to less than 1µA.

Common Applications

Voltage Inverter

The most common application for these devices is a
charge-pump voltage inverter (see

Typical Operating

Circuits

). This application requires only two external com-

ponents—capacitors C1 and C2—plus a bypass capacitor
if necessary (see

Bypass Capacitor

section). Refer to the

Capacitor Selection

section for suggested capacitor types

and values.

Even though the MAX860/MAX861’s output is not actively
regulated, it is fairly insensitive to load-current changes. A
circuit output source resistance of 12

Ω

(calculated using

the formula given in the

Capacitor Selection

section)

means that, with a +5V input, the output voltage is -5V
under no load and decreases to -4.4V with a 50mA load.
The MAX860/MAX861 output source resistance (used to
calculate the circuit output source resistance) vs. tempera-
ture and supply voltage are shown in the

Typical

Operating Characteristics

graphs.

Calculate the output ripple voltage using the formula
given in the

Capacitor Selection

section.

Positive Voltage Doubler

The MAX860/MAX861 can also operate as positive volt-
age doublers (see

Typical Operating Circuits

). This

application requires only two external components,
capacitors C1 and C2. The no-load output is twice the
input voltage. The electrical specifications in the doubler
mode are very similar to those of the inverter mode
except for the Supply Voltage Range (see

Electrical

Characteristics

table) and No-Load Supply Current (see

graph in

Typical Operating Characteristics

). The circuit

output source resistance and output ripple voltage are
calculated using the formulas in the

Capacitor Selection

section.

Active-Low Shutdown Input

When driven low, the

–

—

–

input shuts down the

device. In inverter mode, connect

–

—

–

to V

if it is

not used. In doubler mode, connect

–

—

–

to GND if it

is not used. When the device is shut down, all active
circuitry is turned off.

In the inverting configuration, loads connected from
OUT to GND are not powered in shutdown mode.
However, a reverse-current path exists through two
diodes between OUT and GND; therefore, loads con-
nected from V

to OUT draw current from the input

supply.

In the doubling configuration, loads connected from the
V

pin to the GND pin are not powered in shutdown

mode. Loads connected from the V

pin to the OUT

pin draw current from the input supply through a path
similar to that of the inverting configuration (described
above).

Frequency Control

Charge-pump frequency for both devices can be set to
one of three values. Each device has a unique set of
three available frequencies, as indicated in Table 1.
The oscillator and charge-pump frequencies are the
same (i.e., the charge-pump frequency is not half the
oscillator frequency, as it is on the MAX660, MAX665,
and ICL7660).

*See the Electrical Characteristics for detailed switching-
frequency specifications.

A higher switching frequency minimizes capacitor size
for the same performance and increases the supply
current (Table 2). The lowest fundamental frequency of
the switching noise is equal to the minimum specified
switching frequency (e.g., 3kHz for the MAX860 with FC
open). The spectrum of noise frequencies extends
above this value because of harmonics in the switching
waveform. To get best noise performance, choose the
device and FC connection to select a minimum switch-
ing frequency that lies above your sensitive bandwidth.

Low-Voltage-Operation Input

LV should be connected to GND for inverting operation.
To enhance compatibility with the MAX660, MAX665, and
ICL7660, you may float LV if the input voltage exceeds 3V.
In doubling mode, LV must be connected to OUT for all
input voltages.

MAX860/MAX861

50mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters

_______________________________________________________________________________________

FREQUENCY (kHz)

FC CONNECTION

MAX860

MAX861

FC = V

or open

FC = GND

100

FC = OUT

130

250

Table 1. Nominal Switching Frequencies*