Simplified Operation
First, consider a charge pump connected as an inverter. In the simplified version (
Figure 1), operation is controlled by 2phase clock signals with 50% duty cycles. The pump capacitor (chargetransfer component) is charged to V
_{IN} via closure of SW1A and SW1B. SW2A and SW2B are open at this time. On the next clock cycle, the closure of SW2A and SW2B connects the pump capacitor to C
_{OUT}, thereby producing V
_{IN} at the output.
Figure 1. Simplified diagram of a charge pump connected as an inverter.
Next, connect the charge pump as a doubler (
Figure 2). As before, operation is controlled by 2phase, 50%dutycycle clock signals. The pump capacitor is the charge transfer device and is charged up to V
_{IN} by closure of SW2A and SW2B (SW1A and SW2B are open at this time). On the next clock cycle, the closure of SW1A and SW1B produces +2V
_{IN} at the output by connecting the pump capacitor to C
_{OUT}.
Figure 2. Simplified diagram of a charge pump connected as a doubler.
Input and output ripple is caused by rapid charging and discharging of the pump capacitor. An inverter circuit (
Figure 3) built around the MAX665 charge pump and producing 5V across 51Ω, illustrates the inputripple artifacts (
Figure 4). (Ripple produced by the highcurrent, lowfrequency (≤ 100kHz) MAX665 is easily measured.)
Figure 3. This chargepump inverter circuit is used for measurements.
Figure 4. Input voltage and current ripple for standard inverter circuit: C_{IN} = C_{PUMP} = C_{OUT} = 100µF, R_{LOAD} = 51Ω, V_{IN} = +5.73V, and V_{OUT} = 5.06V. Input current ripple (upper trace): 100mA/div. Input voltage ripple (lower trace): 200mV/div, AC coupled.
Ripplereduction Methods
To reduce ripple, you must isolate ripple sources from the rest of the circuit. For best conversion efficiency in the charge pump, you should also minimize ESR and ensure that the input, output, and pumpcapacitor values are as close as possible to those recommended in the data sheet. The following discussion covers four techniques for minimizing ripple and its effects.
1. Reducing ESR in the input capacitor implies multiple capacitors connected in parallel: N identical capacitors in parallel reduces the input ripple by N
^{1}. Unfortunately, that approach is not very effective in terms of cost and pcboard area.
2. Instead, add an LC filter at the input supply pin (
Figure 5). The additional filtering prevents ripple from propagating to other circuits via the input supply trace. As a secondorder filter, the LC network minimizes the component count. In addition, its small series inductance produces a minimal voltage drop between the input supply and the charge pump.
Figure 5. Chargepump inverter with input LC filter.
The ripplefrequency fundamental equals the pump frequency (F
_{CLOCK}/2). Secondorder filters attenuate at 40dB/decade, so the ideal filter frequency should be a minimum of one decade below the chosen F
_{CLOCK}/2.
The inductor must handle dc currents greater than 1.5I
_{OUT} without saturation. For critical damping (ie., with no peaking),
The filter should be critically damped or close to it, given the low impedance values of R
_{SOURCE} and R
_{LOAD}. Critical damping is not essential to the circuit operation, however. Filtering remains effective even with some peaking at the point of rolloff. A 10µF filter capacitor and 10µH filter choke together provide a 3dB frequency of 15.9kHz and a critical R
_{SOURCE} of 1Ω.
Figure 6 shows the Figure 5 circuit's amplitude response for various damping ratios, and
Figure 7 shows its lower levels of ripple (vs. the circuit of Figure 3).
Figure 6. Amplitude characteristic for various damping ratios in the LCFilter circuit of Figure 5.
Figure 7. Input voltage and current ripple of LCfilter circuit (Figure 5). C_{IN} = C_{FILTER} = 100µF, and L_{FILTER} = 10µH. Charge pump is MAX665. Input current ripple (upper trace): 100mA/div. Input voltage ripple (lower trace): 50mV/div, AC coupled.
3. Adding a lowdropout linear regulator to the charge pump's input supply (
Figure 8) yields an effective generalpurpose circuit for preventing the effects of ripple on the rest of the system. The input LDO also operates with smaller capacitors than those associated with a passive LC filter: the 300mA MAX8860 LDO (available in an 8pin µMAX® package) requires 2.2µF capacitors at input and output; the MAX8863–MAX8864 family of 120mA linear regulators (available in SOT23 packages) requires only 1µF ceramic capacitors. The LDO must handle at least twice the charge pump's output load current, however. When compared with an equivalent passive filter, the added expense of that extracurrent capability can place the LDO approach out of bounds in terms of cost and performance (pcb area and attenuation).
Figure 8. Charge pump doubler with LDO for inputripple protection.
4. Adding an RC to the input supply (
Figure 9) is a singleorder version of the LCfilter approach. The RC input is not generally recommended, because the low value of R
_{FILTER} required for minimal efficiency loss (< 5Ω) forces a very large C
_{FILTER}.
Figure 10 shows the effect of adding an RC filter at the input of the Figure 9 circuit, in which a MAX665 with 100µF capacitors generates a 5V output with a load resistance of 51Ω.
Figure 9. Battery application featuring a charge pump inverter with input RC ripple filter.
If the input supply is a battery, then the effective bulk capacitance of the battery can serve as C
_{FILTER}. Because C
_{FILTER} is a very large capacitance, the resulting filter is very effective in reducing ripple effects at the battery. An example helps to illustrate the point: the capacitance of an 800mAH Li cell can be derived from:

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

I.T = C.V 
Thus, C = 847 farads and f
_{FILTER} = 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 (R
_{FILTER}) 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 R
_{FILTER}, thereby providing high frequency assistance and low ESR above 250kHz (< 50mΩ) to the battery and its interconnect leads. A typical value for the additional C
_{FILTER} is 470nF. For the MAX665 circuit of
Figure 10, increasing C
_{FILTER} to 1500µF lowers the input voltage and current ripple as shown in
Figure 11.
Figure 10. Input Voltage and Current Ripple for the RCfilter circuit (Figure 9): C_{IN} = C_{FILTER} = 100µF, and R_{FILTER} = 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 RCfilter circuit of Figure 7, with 1500µF quasibattery capacitor: C_{IN} =100µF, C_{FILTER} = 1500µF, R_{FILTER} = 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 powersupply 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 voltageripple 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 nonbattery 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 chargepump 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 
8SO 
16wSO 
8µMax/SO 
8µMax/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 
V_{IN}, +2V_{IN} 
V_{IN}, +2V_{IN} 
V_{IN}, +2V_{IN} 
V_{IN}, +2V_{IN} 
Regulated 
No 
No 
No 
No 
Part
No

MAX1680

MAX1681

MAX1682

MAX1683

Package 
8SO 
8SO 
5SOT23 
5SOT23 
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 
V_{IN}, +2V_{IN} 
V_{IN}, +2V_{IN} 
+2V_{IN} 
+2V_{IN} 
Regulated 
No 
No 
No 
No 
Part
No

MAX870

MAX871

MAX 1697 R,S,T,U

MAX1720

Package 
5SOT23 
5SOT23 
6SOT23 
6SOT23 
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 
V_{IN} 
V_{IN} 
V_{IN} 
V_{IN} 
Regulated 
No 
No 
No 
No 
Part No

MAX1719
/MAX1721

MAX864

MAX865

MAX680

Package 
6SOT23 
16QSOP 
8µMax 
8SO 
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 
V_{IN} 
+2V_{IN} and V_{IN} 
+2V_{IN} and V_{IN} 
+2V_{IN} and V_{IN} 
Regulated 
No 
No 
No 
No 
Part No

MAX619

MAX622A

MAX679

MAX682

Package 
8µMax 
8SO 
8µMax 
8SO 
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 
8µMax 
8µMax 
16QSOP 
8SO 
I/P Volts 
2.7V to 5.5V 
2.7V to 5.5V 
3.0V to 5.5V 
2.5V to 10.0V 
O/P Current 
100mA 
50mA 
±5mA 
4mA 
Clock Freq 
5.0V 
5.0V 
±5V, Adj 
2.0V, Adj 
Mode 
200kHz/1MHz 
200kHz/1MHz 
25kHz/100kHz, 20kHz240kHz ext sync 
20kHz/100kHz 
Regulated 
Yes 
Yes 
Yes 
Yes 
Part
No

MAX850/ MAX851/ MAX852/ MAX853

MAX868

MAX881R

MAX1673

Package 
8SO 
10µMax 
10µMax 
8SO 
I/P Volts 
4.5V to 10.0V 
1.8V to 5.5V 
2.5V to 5.5V 
2.0V to 5.5V 
O/P Current 
5mA 
30mA 
4mA 
125mA 
O/P Volts 
4.1V, Adj 
Adj, 2V_{IN} max 
2V, Adj 
Adj, V_{IN} max 
Clock Freq 
100kHz 50kHz250kHz ext sync 
450kHz 
100kHz 
350kHz 
Regulated 
Yes 
Yes 
Yes 
Yes 
Part
No

MAX1686
/MAX1686H

MAX1730

MAX1759


Package 
8µMax 
10µMax 
8µMax 

I/P Volts 
2.7V to 4.2V 
2.7V to 5.5V 
1.6V to 5.5V 

O/P Current 
12mA 
50mA 
100mA 

O/P Volts 
4.75V/5.0V 
1.8V/1.9V Adj 
3.3V, Adj 

Clock Freq 
1MHz 
450kHz 
1.5MHz 

Regulated 
Yes 
Yes 
Yes 
