Many portable devices use LCD displays. These displays need large positive and negative power supplies, but most mobile devices have power rails at 5 V, 3.3 V or even lower. Thus, the challenge is not only to boost the input power supply, typically between 3 V and 5 V, to around +12 V, but also to generate the negative voltage rail, usually -6 V. Typically, the +12-V rail is generated by a boost converter, such as Analog Devices' ADP1611 (Fig. 1).
The ADP1611 consists of a power switch, a PWM controller, a reference voltage source and an error amplifier. When the switch (internal to the ADP1611) turns on, the SW node is grounded, leading to current buildup in the inductor. When the switch turns off, the stored inductor current flows through the diode DOUT+ to the output capacitor and load, generating an output voltage greater than the input voltage. The error amplifier in the ADP1611 measures the output voltage through a voltage divider and creates an error signal that allows the PWM controller to drive the switch with the correct on and off times to maintain the desired output voltage and load current.
The output voltage is set by the ratio of the resistor divider formed by resistors R1 and R2 according to the following equations:
The value of the input and output capacitor are fixed at 10 µF, while the size of the inductor changes with the frequency at which the PWM controller is switching, which is set by the logic pin RT. Also, the diode DOUT+ should be chosen such that it can conduct the average output current.
A boost converter can also be used to generate a negative voltage rail by using a charge-pump configuration (Fig. 2). In this circuit, when the switch turns off, the inductor dumps current into charge-pump capacitor C1 and diode D1 to ground. When the switch turns on, the voltage on capacitor C1 forward biases diode D2. The charge held in C1 transfers through diode D2 to the output capacitor (COUT-), which draws current out of the load. The on and off times of the switch can be modulated to regulate the negative output voltage rail.
In Fig. 1, when the divided output voltage presented at FB increases, the control system decreases the inductor current. This maintains the output voltage at the regulation voltage. In the Fig. 2 circuit, however, the output voltage may not be divided down to control the regulator because the output voltage is negative, but the control circuit regulates the voltage at the FB pin to +1.23 V.
Also, increasing inductor current results in a more negative output voltage, so feeding a divided version of the output voltage would cause positive feedback, and thus an unstable regulation circuit. Instead, an operational amplifier used in an inverting configuration provides level shifting and polarity inversion, thus presenting a ratiometric version of the output voltage at FB and allowing proper control of the output voltage.
A general-purpose op amp — such as the AD8541 shown in Fig. 2 — is sufficient for this application. This amplifier should have a gain bandwidth product above 100 kHz and be able to source a few hundred microamperes of current. Note also that the op amp should support positive single-supply operation with a common-mode input voltage range that includes ground. The ratio of the feedback resistors sets the ratio of the absolute value of the output voltage to the voltage reference of the boost converter. Thus in Fig. 2, to regulate an output voltage of -12 V, the resistor values are R1 = 100 kΩ and R2 = 10 kΩ. The charge-pump capacitor C1 (typically 1 µF) should be large enough to hold the charge that needs to be drawn from the load during a single switching cycle.
While the above solutions show that a boost converter can be used to generate either a large positive or a large negative voltage rail, it is more cost-effective to use the same boost converter to generate both positive and negative voltage rails. The challenge in doing this is that the boost converter can only regulate the output voltage of one rail. Thus, the boost converter regulates one of the voltage rails (positive or negative) while the other rail is post-regulated.
The voltage rail that is being regulated by the boost converter must have a higher magnitude than the other rail. This is the positive rail in most LCD applications. A simple circuit is shown in Fig. 3 where the boost converter is regulating the positive voltage rail (+12 V) while the negative rail (-6 V) is generated by a linear regulator, such as Analog Devices' ADP3331, which post-regulates the unregulated charge-pump output.
When the ADP1611 switch turns off, L1 dumps current through DOUT+ into COUT+, which is regulated to +12 V. The potential on the SW terminal of the ADP1611 will then be one diode drop above +12 V. Unlike the conventional method of post-regulating a switcher with an LDO, the LDO in this circuit is not directly regulating the load. Rather, it is acting as a virtual programmable Zener placed between its IN and OUT terminals that controls the charging of C1 to ensure COUT- maintains a voltage of 6 V.
Thus, if the voltage across COUT- exceeds 6 V, the pass element resistance of the LDO will increase, reducing the charge rate on C1 when the ADP1611 switch is open. This will reduce the amount of replenishing charge transferred from C1 to COUT- through D2 when the switch closes. Conversely, when the voltage across COUT- is below 6 V, the pass-element resistance decreases, and the replenishing charge to COUT- will increase. Because the OUT terminal of the LDO is tied to ground, VOUT- is regulated to -6 V.
This topology offers a simple, cost-effective solution to generate large positive and negative voltages using just a single boost converter, a conventional, low-cost positive supply LDO and a few diodes. It does have a few limitations: the magnitude of the positive rail must be greater than that of the negative rail, and the boost converter must be capable of supplying the total maximum load current of both rails.