Rainbow Electronics MAX660 - Manual (Page 6)

MAX660
CMOS Monolithic Voltage Converter
6 _______________________________________________________________________________________
______________Detailed Description
The MAX660 capacitive charge-pump circuit either
inverts or doubles the input voltage (see Typical
Operating Circuits). For highest performance, low
effective series resistance (ESR) capacitors should be
used. See Capacitor Selection section for more details.
When using the inverting mode with a supply voltage
less than 3V, LV must be connected to GND. This
bypasses the internal regulator circuitry and provides
best performance in low-voltage applications. When
using the inverter mode with a supply voltage above
3V, LV may be connected to GND or left open. The part
is typically operated with LV grounded, but since LV
may be left open, the substitution of the MAX660 for the
ICL7660 is simplified. LV must be grounded when over-
driving OSC (see Changing Oscillator Frequency sec-
tion). Connect LV to OUT (for any supply voltage) when
using the doubling mode.
__________Applications Information
Negative Voltage Converter
The most common application of the MAX660 is as a
charge-pump voltage inverter. The operating circuit
uses only two external capacitors, C1 and C2 (see
Typical Operating Circuits).
Even though its output is not actively regulated, the
MAX660 is very insensitive to load current changes. A
typical output source resistance of 6.5
Ω means that
with an input of +5V the output voltage is -5V under
light load, and decreases only to -4.35V with a load of
100mA. Output source resistance vs. temperature and
supply voltage are shown in the Typical Operating
Characteristics graphs.
Output ripple voltage is calculated by noting the output
current supplied is solely from capacitor C2 during
one-half of the charge-pump cycle. This introduces a
peak-to-peak ripple of:
V
RIPPLE
= I
OUT
+ I
OUT
(ESR
C2
)
2(f
PUMP
) (C2)
For a nominal f
PUMP
of 5kHz (one-half the nominal
10kHz oscillator frequency) and C2 = 150µF with an
ESR of 0.2
Ω, ripple is approximately 90mV with a
100mA load current. If C2 is raised to 390µF, the ripple
drops to 45mV.
Positive Voltage Doubler
The MAX660 operates in the voltage-doubling mode as
shown in the Typical Operating Circuit. The no-load
output is 2 x V
IN
.
Other Switched-Capacitor Converters
Please refer to Table 1, which shows Maxim’s charge-
pump offerings.
Changing Oscillator Frequency
Four modes control the MAX660’s clock frequency, as
listed below:
FC OSC Oscillator Frequency
Open Open 10kHz
FC = V+ Open 80kHz
Open or External See Typical Operating
FC = V+ Capacitor Characteristics
Open External External Clock Frequency
Clock
When FC and OSC are unconnected (open), the oscil-
lator runs at 10kHz typically. When FC is connected to
V+, the charge and discharge current at OSC changes
from 1.0µA to 8.0µA, thus increasing the oscillator
MAX829 MAX861 MAX1044
Package SOT 23-5
SO-8,
µMAX
SO-8,
µMAX
Op. Current
(typ, mA)
0.15
0.3 at 13kHz,
1.1 at 100kHz,
2.5 at 250kHz
0.03
Output Ω
(typ)
20 12 6.5
Pump Rate
(kHz)
35 13, 100, 150 5
Input (V) 1.25 to 5.5 1.5 to 5.5 1.5 to 10
ICL7662
SO-8
0.25
125
10
1.5 to 10
MAX660
SO-8
0.12 at 5kHz,
1 at 40kHz
6.5
5, 40
1.5 to 5.5
MAX860
SO-8,
µMAX
0.2 at 6kHz,
0.6 at 50kHz,
1.4 at 130kHz
12
6, 50, 130
1.5 to 5.5
MAX828
SOT 23-5
0.06
20
12
1.25 to 5.5
ICL7660
SO-8,
µMAX
0.08
55
10
1.5 to 10
Table 1. Single-Output Charge Pumps