50mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
MAX860/MAX861
_______________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.
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
DD
to OUT draw current from the input
supply.
In the doubling configuration, loads connected from the
V
DD
pin to the GND pin are not powered in shutdown
mode. Loads connected from the V
DD
pin to the OUT
pin draw current from the input supply through a path
similar to that of the inverting configuration (described
above).
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 12Ω (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.
When driven low, the SHDN input —
– —shuts down the
— –
device. In inverter mode, connect SHDN – V
DD
if it is
– — to
— —
not used. In doubler mode, connect SHDN to GND if it
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).
Table 1. Nominal Switching Frequencies*
FREQUENCY (kHz)
FC CONNECTION
MAX860
FC = V
DD
or open
FC = GND
FC = OUT
6
50
130
MAX861
13
100
250
*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.
Active-Low Shutdown Input
–— –
— —
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.
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