General Description

The MAX660 monolithic, charge-pump voltage inverter converts a +1.5V to +5.5V input to a corresponding -1.5V to -5.5V output. Using only two low-cost capacitors, the charge pump’s 100mA output replaces switching regulators, eliminating inductors and their associated cost, size, and EMI. Greater than 90% efficiency over most of its load-current range combined with a typical operating current of only 120µA provides ideal performance for both battery-powered and boardlevel voltage conversion applications. The MAX660 can also double the output voltage of an input power supply or battery, providing +9.35V at 100mA from a +5V input.
A frequency control (FC) pin selects either 10kHz typ or 80kHz typ (40kHz min) operation to optimize capacitor size and quiescent current. The oscillator frequency can also be adjusted with an external capacitor or driven with an external clock. The MAX660 is a pincompatible, high-current upgrade of the ICL7660.
The MAX660 is available in both 8-pin DIP and smalloutline packages in commercial, extended, and military temperature ranges.

 

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 overdriving OSC (see Changing Oscillator Frequency section). Connect LV to OUT (for any supply voltage) when using the doubling mode.

 

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 6.5Ω 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 duringone-half of the charge-pump cycle.

 

Capacitor Selection

Three factors (in addition to load current) affect the MAX660 output voltage drop from its ideal value:
1) MAX660 output resistance
2) Pump (C1) and reservoir (C2) capacitor ESRs
3) C1 and C2 capacitance

The voltage drop caused by MAX660 output resistance is the load current times the output resistance.Similarly, the loss in C2 is the load current times C2’s ESR. The loss in C1, however, is larger because it handles currents that are greater than the load current during charge-pump operation. The voltage drop due to C1 is therefore about four times C1’s ESR multiplied by the load current. Consequently, a low (or high) ESR capacitor has a much greater impact on performance for C1 than for C2.

Generally, as the pump frequency of the MAX660 increases, the capacitance values required to maintain comparable ripple and output resistance diminish proportionately.  These curves assume 0.25Ω capacitor ESR and a 5.25Ω MAX660 output resistance, which is why the flat portion of the curve shows a 6.5Ω (RO MAX660 + 4 (ESRC1) + ESRC2) effective output resistance. Note:RO = 5.25Ω is used, rather than the typical 6.5Ω, because the typical specification includes the effect of the ESRs of the capacitors in the test circuit.

To reduce the output ripple caused by the charge pump, increase the reservoir capacitor C2 and/or reduce its ESR. Also, the reservoir capacitor must have low ESR if filtering high-frequency noise at the output is important.

Not all manufacturers guarantee capacitor ESR in the range required by the MAX660. In general, capacitor ESR is inversely proportional to physical size, so larger capacitance values and higher voltage ratings tend to reduce ESR.