Rainbow Electronics MAX1681 - Manual (Page 6)

_______________Detailed Description
The MAX1680/MAX1681 switched-capacitor voltage
converters either invert or double the input voltage.
They have low output resistance (3.5
Ω
) and can deliver
up to 125mA output current. These devices operate at
one of two selectable frequencies: 125kHz/250kHz
(MAX1680) and 500kHz/1MHz (MAX1681). This pro-
vides the flexibility to optimize capacitor size, operating
supply current, and overall circuit efficiency. Frequency
selection also allows for minimizing coupling into other
sensitive circuits. These devices contain no internal
divider; the oscillator frequency equals the switching
frequency. The devices can easily be cascaded to pro-
duce a higher output voltage, or paralleled to deliver
more current.
The MAX1680/MAX1681 feature a shutdown mode that
reduces supply current to <1µA (SHDN = high). OUT,
in the inverter configuration, pulls to ground in shut-
down mode. Shutdown is not available in the doubler
configuration; connect SHDN to OUT.
__________Applications Information
Voltage Inverter
A simple voltage inverter is the most common
MAX1680/MAX1681 application. It requires three exter-
nal capacitors (including the input bypass capacitor)
as shown in the
Typical Operating Circuits (inverter
configuration). Although the output is not regulated, low
output resistance produces a typical drop of only 0.44V
with a 125mA load. This low output resistance makes
the devices fairly insensitive to changes in load (see
the graphs for Output Source Resistance vs.
Temperature and Supply Voltage in the
Typical
Operating Characteristics section).
Voltage Doubler
The MAX1680/MAX1681 can be configured as a volt-
age doubler with two external capacitors as shown in
the
Typical Operating Circuits (doubler configuration).
When loaded, the output voltage drop is similar to that
of the voltage inverter. The minimum input supply range
is slightly higher than in the inverter configuration.
Calculate ripple voltage using the equation in the
Capacitor Selection section.
Frequency Control
A frequency-control pin, FSEL, provides design flexibili-
ty. Each device has two selectable frequencies:
125kHz/250kHz (MAX1680) and 500kHz/1MHz
(MAX1681). This allows optimization of capacitor size
and supply current for a given output load. Table 1
summarizes the frequency options .
Table 1. Nominal Switching Frequencies
MAX1680/MAX1681
125mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
6 _______________________________________________________________________________________
______________________________________________________________Pin Description
Selects operating frequency. MAX1680: 250kHz when FSEL is low, 125kHz when FSEL is high. MAX1681:
1MHz when FSEL is low, 500kHz when FSEL is high.
FSEL 1
Positive Charge-Pump Capacitor Connection CAP+ 2
Negative Charge-Pump Capacitor Connection CAP- 4
Power-Supply Ground Input GND 3
Logic Voltage Input. Connect LV to GND. LV 6
Power-Supply Positive Voltage Input IN 8
SHDN 7
Negative Voltage Output OUT 5
Power-Supply Positive Voltage Input
Power-Supply Ground Connection
Connect LV to OUT.
Not available; connect to OUT.
Shutdown Input. Driving SHDN high disables the
charge pump, and the output goes to 0V. SHDN is a
CMOS input.
Positive Voltage Output
MAX1680
250 FSEL = LV
125 FSEL = IN
FSEL CONNECTION
MAX1681
1000
500
FREQUENCY (kHz)
INVERTER DOUBLER
PIN
FUNCTION
NAME