Simple Methods Reduce Input Ri

	Q = C.V	, where I = 800mA, T = 3600s (1Hr), and V = 3.4V.
	I.T = C.V

Thus, C = 847 farads and fFILTER = 0.12mHz. The sum of ESR and battery contact resistance (about 100mΩ) limits the attenuation to a maximum of 21dB, assuming the ripple source resistance (RFILTER) equals 1Ω. The model for an actual battery is more complex, with the central bulk capacitance modified by ESR, ESL, and parasitic capacitance. In practice one should add capacitance close to RFILTER, thereby providing high frequency assistance and low ESR above 250kHz ( 50mΩ) to the battery and its interconnect leads. A typical value for the additional CFILTER is 470nF. For the MAX665 circuit of Figure 10, increasing CFILTER to 1500μF lowers the input voltage and current ripple as shown in Figure 11.


Figure 10. Input Voltage and Current Ripple for the RC-filter circuit (Figure 9): CIN = CFILTER = 100μF, and RFILTER = 2.2Ω. Charge pump is a MAX665. Input current ripple (upper trace): 100mA/div. Input voltage ripple (lower trace): 20mV/div, AC coupled.


Figure 11. Input voltage and current ripple for the RC-filter circuit of Figure 7, with 1500μF quasi-battery capacitor: CIN =100μF, CFILTER = 1500μF, RFILTER = 2.2Ω, and MAX665 charge pump. Input current ripple (upper trace): 100mA/div. Input voltage ripple (lower trace): 20mA/div, AC coupled.

Conclusion

Several methods are available for reducing the effect of input power-supply ripple caused by charge pumps. Placing an LC filter in addition to the input capacitor recommended by the data sheet, for instance, (#2) provides excellent voltage-ripple protection to the rest of the system (Figure 10) with minimal effect on the overall conversion efficiency. An effective alternative for battery systems is a simple series resistor (#4), which occupies minimal space. The resistor is also suitable in non-battery applications for which large storage values (> 50μF) are appropriate. Results of a simulated battery application are shown in Figure 11.

An overview of Maxim's charge-pump ICs (Table 1) is included to help the reader choose an appropriate device according to the desired clock frequency, mode of operation, and level of output current required.

Table 1 Product Selection

Part No	MAX660	MAX665	MAX860	MAX861
Package	8-SO	16-wSO	8-μMax/SO	8-μMax/SO
I/P Volts	1.5V to 5.5V	1.5V to 8V	1.5V (inv) or 2.5V to 5.5V	1.5V (inv) or 2.5V to 5.5V
O/P Current	100mA	100mA	50mA	50mA
Pump Rate	10kHz/80kHz	10kHz/45kHz	3kHz/50kHz/130kHz	13kHz/100kHz/250kHz
Mode	-VIN, +2VIN	-VIN, +2VIN	-VIN, +2VIN	-VIN, +2VIN
Regulated	No	No	No	No
Part No	MAX1680	MAX1681	MAX1682	MAX1683
Package	8-SO	8-SO	5-SOT23	5-SOT23
I/P Volts	2.0V to 5.5V	2.0V to 8V	1.5V (inv) or 2.5V to 5.5V	1.5V (inv) or 2.5V to 5.5V
O/P Current	125mA	125mA	45mA	45mA
Clock Freq	125kHz/250kHz	500kHz/1MHz	12kHz	35kHz
Mode	-VIN, +2VIN	-VIN, +2VIN	+2VIN	+2VIN
Regulated	No	No	No	No
Part No	MAX870	MAX871	MAX 1697 R,S,T,U	MAX1720
Package	5-SOT23	5-SOT23	6-SOT23	6-SOT23
I/P Volts	1.4V to 5.5V	1.4V to 5.5V	1.5V to 5.5V	1.5V to 5.5V
O/P Current	25mA	25mA	60mA	25mA
Clock Freq	125kHz	500kHz	12kHz/35kHz/125kHz/250kHz	12kHz
Mode	-VIN	-VIN	-VIN	-VIN
Regulated	No	No	No	No
Part No	MAX1719 /MAX1721	MAX864	MAX865	MAX680
Package	6-SOT23	16-QSOP	8-μMax	8-SO
I/P Volts	1.5V to 5.5V	2.0V to 6.0V	1.5V to 6.0V	2.0V to 6.0V
O/P Current	25mA	±10mA	±10mA	±10mA
Clock Freq	125kHz	7kHz/33kHz/100kHz/185kHz	24kHz	8kHz
Mode	-VIN	+2VIN and -VIN	+2VIN and -VIN	+2VIN and -VIN
Regulated	No	No	No	No
Part No	MAX619	MAX622A	MAX679	MAX682
Package	8-μMax	8-SO	8-μMax	8-SO
I/P Volts	2.0V to 3.6V	4.5V to 5.5V	1.8V to 3.6V	2.7V to 5.5V
O/P Current	60mA	30mA	20mA	250mA
Clock Freq	500kHz	500kHz	330kHz/1MHz	200kHz/1MHz
Regulated	Yes	Yes	Yes	Yes
Part No	MAX683	MAX684	MAX768	MAX840/MAX843/MAX844
Package	8-μMax	8-μMax	16-QSOP	8-SO