When U2 is shut down, the power supply draws a mere 5 µA because of the low shutdown current of regulator U2. It also draws that much because coupling capacitor C4 prevents a dc connection between the battery and the output. In contrast, when a standard step-up (or boost) circuit is shut down, the load remains connected to the battery through the inductor and the catch diode, allowing the load to continue to draw current from the battery and drain it.
Diode D1 is the main source of the supply's losses. A diode with a lower reverse voltage typically comes with a lower forward voltage drop. Substituting the 20-V MBRM120LT3 diode for the 40-V ZHCS1000 diode reduces the forward voltage drop slightly and improves efficiency by 1.1%. Although the reverse capacitance of the former far exceeds that of the latter diode (300 pF versus 180 pF), the smaller forward voltage drop more than compensates for the efficiency loss due to this increased reverse capacitance.
To further reduce the diode losses, place a synchronous p-channel transistor across D1 to effectively short the diode and eliminate the losses caused by its forward voltage drop. This synchronous transistor must switch ON at exactly the same time as diode D1 conducts, making the circuit more complicated.
Since the currents through L1 and L2 are un-equal, making L2 smaller and L1 larger will decrease their total losses. To reduce the inductive losses still further, the two inductors can be wound around the same core.
If the input voltage were guaranteed to be greater than 2.7 V, removing U1 and C2 and connecting the battery directly to U2 would simplify the circuit. However, these components are necessary to accommodate the bottom end of a NiMH battery's voltage range.
Choosing a larger MOSFET would reduce losses at higher output currents. Doing so, though, would de-crease the efficiency at lower output currents, since the switching losses would be higher.
Regulator U2 relies on current-sense resistor R1 to set the peak current through inductor L1. R1 prevents the current from reaching a level that would cause L1 to saturate, which would allow excessively high current to flow through the inductor. Make sure R1 is a noninductive device.
If large pulses of output current discharge reservoir capacitor C7, R1 limits the output current while U2 recovers. With a high current limit, recovery is fast, but efficiency suffers. A lower current limit slows down recovery but improves efficiency.
When operating a GSM power amplifier, the average current is 380 mA, and the peak current can reach 2.63 A for 577 µs, every 4.6 ms. Reservoir capacitor C7 supplies these current peaks. The capacitance of C7 must be about 4000 µF to limit the output-voltage drop to 380 mV during these 577-µs pulses.
For less demanding applications, C7 can be much smaller. In those cases, a capacitor of at least 22 µF produces less than 50 mV of output ripple when the output current reaches 500 mA, assuming its ESR is sufficiently low.
This low-value capacitor should be connected in parallel with a capacitor that's effective at higher frequencies. The 6.8-µF ceramic capacitor C6 made with X7R material serves this purpose while lowering the overall ESR at the output of the regulator. The ESR of the two paralleled output capacitors can be calculated using the complex impedance of each capacitor (i.e., by modeling each capacitor's ESR as the real component and its capacitance as the imaginary). Once the parallel impedance is calculated, the real component of the resulting complex number can be used as the effective ESR at a given frequency.
To simplify the bill of materials, the same Sumida inductor is used for both L1 and L2. This inductor measures 6.7 by 6.7 mm with a height of 4 mm. It's possible to use different inductors to increase efficiency. In this case, the size of one inductor would increase while the other decreases. By using a switching-regulator chip that includes a precise current limit, the inductor need not be rated to handle a larger current than is needed to supply the maximum output current, minimizing the size of the inductors. But the current rating of some inductors should be somewhat larger than the maximum anticipated inductor current, because the ferrite loss of some inductors increases as the current approaches the inductor's saturation point.
Capacitor sizes can be reduced by using capacitors made from different materials, provided the capacitor meets the required specifications. In this design, C4 is made of X7R material. If a capacitor made of X5R material is used instead, the size diminishes, but the X5R capacitor is rated to only 85°C, while the X7R capacitor is rated to 125°C.
Because regulator U2 switches at 500 kHz, the pc-board layout should be treated as a high-frequency circuit. All the high-current paths should be routed first, using larger copper traces to minimize the parasitic inductance and resistance. Capacitors C2 and C1 should be as close as possible to pins 1 and 9 of the U2 chip.
Be sure to allocate enough copper to properly dissipate the power generated by Q1. However, use only enough copper underneath diode D1 to ensure its temperature never exceeds 125°C. Operating the diode at a high temperature reduces its forward voltage drop and increases efficiency. But be sure the losses due to increased reverse current don't outweigh the benefits of reducing the forward voltage drop.