Switched-capacitor charge-pump dc-dc converters have had a fairly bad reputation over the years as wonderful sources of noise and EMI. This new family of TPS601xx parts from Texas Instruments does a lot to beat that reputation and offers an excellent opportunity to go away from inductors, either in operation or as a fix. Previously, even when no external inductor was needed for a charge-pump converter designers often ended up with one to get enough filtering at the output.

The newly-thought technology changes here came with the establishment of a German design group for TI. Those engineers had some ideas about things that could be done with what might be thought of as almost obsolete technology. They made dramatic improvements in the ripple and noise from a charge-pump converter with the simple expedient of having two pumps, working in opposite phases. While one pump is charging, the other is transferring energy so the availability of output current is very nearly continuous. This push-pull operation requires the pumps to be switched at 180ı from one another and gives a much lower ripple on the output than you would expect from a switched-capacitor part.

For the user of a dc-dc converter the "must-haves" tend to be a fairly long list, but the item at the top of the list never changes: Maximum battery life. That means high conversion efficiency when it is working and the lowest possible current consumption when in standby or shut-down modes. Conversion efficiencies from these parts top 90% when the input voltage falls and for loads in the 50 to 200 mA range. For nearly all conditions the efficiency does not drop below 80%.

Working from the absolutely correct assumption that most portable applications would like to make use of two battery cells as the power source, the first part was designed to take a 1.8 V to 3.6 V input and to output 3.3 V at a maximum load current 200 mA. The TPS60100 produces the 3.3 V with a tolerance of ı4%, most of the spread of which is dependent on input voltage, the output edging a little higher with higher input voltages.

The TPS60100 consists (see Fig. 1) of an internal oscillator running at a nominal 600 kHz to produce a nominal 300-kHz switching frequency, a 1.22-V bandgap, a shutdown/start-up circuit, two sets of MOSFET switches and a control circuit.

Fig. 1 Functional Block Diagram TPS60100

When the circuit is started with a switch to logic high on ENABLE, the output rises to 0.8 x the input voltage. After that time -- which is quite short -- the switch operation starts. The oscillator operates on a 50% duty cycle with the two single-ended charge pumps being driven 180ı out-of-phase, charging the external "flying" capacitors. The circuit is a doubler, so that if it was allowed to run uncontrolled the output voltage would rise to a range of 3.6 to 7.2 V across the full input range. The part can either be run in the constant-frequency mode (SKIP at logic low) or can be switched to a pulse-skip mode (SKIP at logic high.)

The constant-frequency mode gives the best results in output ripple specifications, produces a constant ripple frequency and allows the use of the smallest sizes of external components for a specified ripple performance. It operates by the error amplifier feeding the control circuit and controlling the charges to the flying capacitors by driving the FET gates. This drive increases when the output voltage falls putting a higher voltage across the capacitors.

The pulse-skip mode allows the switching stages to be disabled when the output goes higher than 3.3 V. Switching cycles are skipped until the output voltage drops below 3.3 V; then they recommence. The output ripple is a little higher in this mode and the spectrum of the EMI is inconsistent, varying according to the amount and number of the cycles that are being missed. For lighter loads, however, the system shows the highest efficiency when in this skip mode.

The parts can also be driven from an external clock, so that a user can define the frequency for some reason, when a signal at twice the desired switching frequency (up to 800 kHz) can be fed to SYNC.

The converter can be turned off when either ENABLE goes to logic low or the input voltage falls below 1.6 V. In the shut-down mode the output is isolated from the input and the current drawn is typically 0.05 ıA.

A further feature of the TPS60100 is that the two charge pumps can actually be set to work together when the 3V8 pin is taken to logic high. In that setting the oscillator feeds both pumps in synchronism and the output voltage goes to 3.8 V; this could be useful if it is wanted to drive an LDO, or a number of LDOs, sprinkled around the circuit. What is given up is a lot of the lower ripple benefits of the part, although that would be effectively fixed by the LDO action. When operated in that way there is a single, common flying capacitor for both charge pumps.

It is also possible to parallel two TPS60100 parts to provide a 3.3-V output with a capability of 400 mA.

In Circuit

For operation in the constant-frequency mode (see Fig. 2) there is generally a need for just four external parts: An input capacitor, the two flying capacitors, and the output capacitor. The input capacitor reduces the input impedance and improves the system efficiency; it will normally be two to four times the capacitance of the flying capacitors.

Fig. 2 Typical Operating Circuit TPS60100 in Constant Frequency Mode

The output capacitor is five to fifty times the value of the flying capacitors and TI exhorts the user not to use less than 22 ıF in the constant-frequency mode. The capacitors can be either ceramic or tantalum, with the former giving the best ripple performance because of the lower ESR, but if ceramics are used for the flying capacitors they need to be manufactured with either X5R or X7R dielectric.

The output ripple from the part does not exceed 5 mV pk-pk and the resulting EMI is no higher than 70 dBıV for the lowest frequency spur in the constant-frequency mode. Quiescent current is 50 ıA.

The "must haves" from the circuit designer are topped, as noted earlier, by battery life. Then there follows -- in different orders for different users -- the specifications, quantities and sizes of the external capacitors needed; the ripple on the output; the ease of implementing the design; the spurious emissions from the complete design; the total cost of implementing the whole design. In this case TI has come up with a part that allows for just four relatively low-cost and sized capacitors; with extremely low ripple; an easy analog design layout with an extremely logical pin out from the part; an entirely acceptable and controllable spurious output; a good overall system cost.

Other parts in this family are:
TPS60101       100-mA version of the TPS60100
TPS60110       5-V, 300-mA version
TPS60111       100-mA version of TPS60110

All the parts are packaged in TIıs 20-pin TSSOP PowerPAK which has a thermal pad on the underside that can be bonded to an external thermal plane. The TPS60100 and TPS60110 are priced at $2.49 in 1000-piece lots, while the 100-mA parts are priced at $1.99 in 1000-piece lots.