Simple Methods Reduce Input Ripple for All Charge Pumps
Abstract: Charge pumps are a form of DCDC converter that rely on capacitors instead of inductors for energy storage and transfer. The absence of inductors makes them attractive in situations requiring a lowpower auxiliary supply (output currents up to about 150mA). They use less circuitboard area, offer minimal component height, and are easy to use.
Charge pumps can have regulated or unregulated outputs. An unregulated charge pump either doubles or inverts the voltage that powers it and the output voltage is a function of the supply voltage. A regulated charge pump either boosts or inverts the supply voltage. Its output voltage is independent of the supply.
Techniques that reduce capacitor size and optimize output current—fast switching speed and lowonresistance switches—also produce noise and transient ripple at the input supply pin. Noise can propagate back along the input supply pins, creating problems for crystalcontrolled oscillators, VCOs, and other sensitive circuits with poor powersupply rejection. This article focuses on methods for reducing the noise.
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,
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
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





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 





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 





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 





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 





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 





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 





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 





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 