White LEDs are usually driven with a constant DC current source in order to maintain constant luminosity. In portable applications with single-cell Li-Ion source, the sum of the voltage drop across the white LED and the current source can be lower or higher than the battery voltage. This means that a white LED requires the battery voltage to be sometimes boosted. The easiest way to boost the battery voltage is to use a step-up DC/DC converter. This method significantly optimizes efficiency at the expense of cost and PCB area. An alternative method of boosting the battery voltage is to use a charge pump, also called switched capacitor converter. Let's analyze in more detail the principle of operation of such a device.
Charge pump basics
A capacitor is a component that stores electrical charge or energy for release at some predetermined rate and at some predetermined time.
Figure 1. Charging a capacitor from a voltage source is shown for both the ideal (a, b) and real (c, d) cases.
If an ideal capacitor is charged with an ideal voltage source VG(Fig. 1a), the charge storage occurs instantaneously corresponding to a Dirac impulse function for the current (Fig. 1b). The total stored charge is given by:
Q = CVG
Real capacitors have equivalent series resistance (ESR) and equivalent series inductance (ESL), neither of which affects the ability of the capacitor to store energy. They have, however, a large effect on the overall efficiency of the switched capacitor voltage converter. An equivalent circuit for the charge of a real capacitor is shown in Fig. 1c, where RSW is the resistance of the switch. The charging current path will have a series inductance, which can be reduced with proper component layout.
As soon as the circuit is energized, transient conditions of exponential nature occur until a steady-state condition is reached. The capacitor parasitics limit the peak charge current and increase the charge transfer time (Fig. 1d). In other words the capacitor charge build-up can not occur instantaneously which means that the initial voltage variation across the capacitor is equal to zero. Charge pumps use this property of capacitors (Fig. 2a).
Figure 2. Charge pump circuit (a) with relevant waveforms (b).
The voltage conversion is achieved in two phases. During the first phase switches S1 and S2 are closed, whereas switches S3 and S4 are open and C1 is charged to the input voltage:
During the second phase switches S3 and S4 are closed, whereas switches S1 and S2 are open. Because the voltage drop across the capacitor cannot change instantaneously the output voltage jumps to twice the value of the input voltage:
In this way voltage-doubling operation is accomplished. The duty cycle of the switching signal is usually 50% as this value generally yields the optimal charge transfer efficiency. Let us examine more in detail the charge transfer procedure and how the switched capacitor converter parasitics influence its operation.