Battery powered portable devices are often in standby state for most of their service life. In this standby state, the static current of the internal boost converter still consumes battery energy. The quiescent current during standby may be greater than the actual load current. Although the maximum quiescent current of several inductor based converters is less than 10mA, designers usually prefer or require the use of a regulated charge pump in intrinsically safe and cost sensitive designs. A ready-made regulated charge pump with at least 10 mA output current capacity, with a typical minimum static current of 50 ~ 100mA. If this quiescent current level is unacceptable, you need to increase the circuit to remotely monitor the regulated voltage and switch the charge pump back and forth between entering and leaving the off state, so as to reduce the total average current. However, this method may not achieve the desired quiescent current level lower than 10mA. The appearance of low on resistance analog switch, ultra-low current comparator and ultra-low current reference makes it possible for the charge pump with the maximum quiescent current close to 7ma (Fig. 1).

Design of ultra low quiescent current charge pump circuit using low on resistance analog switch

In Figure 1, this charge pump circuit uses analog switches to obtain ultra-low quiescent current.

Charge pump uses AC coupling technology to transfer energy from transfer capacitor to energy storage capacitor. The transfer capacitor is first charged to the Vbatt level through the analog switch, and then other analog switches transfer the energy to the energy storage capacitor connected to Vout. Then, the transfer capacitor is charged again and goes on again and again. Since the loss of the ideal analog switch is zero, the Vout level is equal to twice that of Vbatt. However, as expected, the output level generated by the limited on resistance of the analog switch decreases with the decrease of the load current. The basic regulated charge pump shown in Figure 1 includes an oscillator, several analog switches, a voltage reference and a comparator. The comparator acts as a voltage monitor and oscillator. When the circuit is stabilized, the output of the comparator is low level, so that the NC switch is closed and C1 is charged to Vbatt. When the voltage of Vout drops below the output voltage stabilization threshold (3.3V in this example), the output of the comparator becomes high. The no switch closes, transferring the charge from C1 to C2. This cycle will be repeated until Vout obtains the voltage stabilizing state again.

Resistors R3 to R5 provide the necessary hysteresis for oscillation. The resistance of these three resistors is 1 m Ω, which can produce considerable hysteresis and minimize the load of batt. When the comparator output changes state, the feedback resistor R5 will change the threshold you add to the positive input of the comparator, resulting in hysteresis. When the resistance value is as shown in the figure, the reference value is the nominal value of IC1 (1.182v) and Vbatt = 3V, the VIN + threshold swings between two approximate values of VIN + (low) = 0.39v and VIN + (high) = 1.39v. When the circuit is stabilizing voltage, VIN – slightly exceeds VIN +, the comparator output is low level, R1-R2 voltage divider detects the voltage of Vout, and the threshold of VIN + is very low (0.39v). When Vin + is 0.39v, you can calculate the resistance of R1 and R2 according to the formula VIN + = Vout [R2 / (R1 + R2)]. In order to minimize the load of Vbatt, the resistance value of R1 + R2 should be greater than 1 m Ω. If Vout = 3.3V and R2 is 2.2 m Ω, R1 is 301 K Ω. The capacitor C3 is connected to the VIN input of the comparator. C3 sets the oscillation frequency together with R1 and R2 according to the following simplified relationship: tdischarge = TLOW = – (r2c3) ln [(VIN + (low)) / (VIN + high))]; tCHARGE=tHIGH=-(R2C3)ln[1-(VIN+(HIGH)-VIN+(LOW))/(VBATT-VIN+(LOW)]; And FOSC = 1 / tperiod, where tperiod = TLOW + tshigh.

In order to maximize efficiency and reduce the impact of the conversion rate of the comparator, you should set a lower frequency. If C3 = 470 PF is selected, the following results can be obtained: TLOW = 178ms, tshigh = 68ms; So, FOSC = 4 kHz. The capacitance values of C1 and C2 are selected to achieve the desired load current and ripple voltage. For this application (Iload = 10 mA), C1 = 10mf. In order to calculate the capacitance value of C2, it is necessary to make an approximation according to the expected ripple voltage: C2 = (Iload × tLOW)/VRIPPLE。 C2 = 12mf when Iload = 10mA and vripple = 150mV.

In the case of the above element values, the maximum static current absorbed by this circuit is 6.9ma, which is much smaller than that of the ready-made charge pump. You can also further reduce the quiescent current by increasing the resistance value. However, since the maximum quiescent current of IC2 is 3.8ma, accounting for most of the total quiescent current, the effect is very small. This circuit enables you to realize an ultra-low static circuit regulated charge pump. Before the ready-made charge pump was bought, it provided an alternative for designers seeking to realize low-cost charge pump without inductors.

Responsible editor: GT

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