Electronic Design
Chipworks Analysis: Linear Technology’s LTC3108 DC-DC Converter For Energy Harvesting Apps

Chipworks Analysis: Linear Technology’s LTC3108 DC-DC Converter For Energy Harvesting Apps

Linear Technology’s LTC3108 dc-dc converter, which Chipworks has analyzed (CAR-1108-802), only requires harvested energy from extremely low-input-voltage sources to operate. It suits wireless sensors that are only powered to make measurements and transmit data periodically at a low duty cycle. Solar-powered battery chargers, traffic signals, home and commercial lighting products, and automotive trickle chargers are other potential applications.

Very interestingly, or very smartly, the LTC3108 starts up and operates from input voltages as low as 20 mV to provide multiple regulated output voltages for powering other circuits. That’s more than 10 times the previous state of the art embodied in the Freescale Semiconductor PC900840 ultra-low-voltage dc-dc converter, which Chipworks also has analyzed (CAR-0906-803 and PPR-0906-801).

Moreover, the LTC3108 is simple, inexpensive, and easy to use. It’s fabricated in a simple, low-cost, double-metal, double-polysilicon 0.8-μm CMOS process with vertical NPN devices and an additional thin-film metal layer. This is significant for applications that require low input voltages. In such situations, design engineers naturally seek a device made on a simple low-voltage process.

For comparison, the PC900840 is made on the sophisticated SMARTMOS 10-W 0.13-μm bipolar CMOS DMOS (BCD) process. It operates by harvesting energy from solar cells with input voltages as low as 0.3 V, which is at least an order of magnitude higher than the LTC3108. The LTC3108 also is smaller at 1.48 by 0.72 mm, versus 1.2 by 2.2 mm for the PC900840.

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1. The functions of Linear Technology’s LTC3108 dc-dc converter can be divided into two groups: an ultra-low-voltage step-up dc-dc converter (left) and a power manager for the rest of the functional blocks (right). [Image courtesy of Chipworks]

The LTC3108’s functions can be divided into two groups: an ultra-low-voltage step-up dc-dc converter and a power manager for the rest of the functional blocks (Fig. 1). The on-chip compound depletion-mode N-channel MOSFET switch works with an external step-up transformer and a small coupling capacitor to form a resonant step-up oscillator, so the LTC3108 can operate from a 20-mV input voltage.

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2. The compound depletion-mode NMOS switch comprises three NMOS transistors of two types. The N550 is the depletion-mode transistor, and the N549 and N534 are normal transistors. [Image courtesy of Chipworks]

The compound depletion-mode NMOS switch consists of three NMOS transistors of two types (Fig. 2). The N550 is the depletion-mode transistor, while the N549 and N534 are normal transistors. The N550 has a longer drain and shorter channel width (4 by 200 µm), while the N549 and N534 have much longer channel widths (64 by 81 µm and 70 by 280 µm) and a typical power switch MOS transistor layout.

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3. The depletion transistor (left) has a longer drain that adds resistance and reduces current during oscillation starts. The large typical power switch MOS transistor (right) has maximum driving capability to ensure high switching efficiency after oscillation starts. [Image courtesy of Chipworks]

Figure 3 shows zoom-in scanning electron microscope (SEM) photographs of the compound depletion NMOS transistors on both the polysilicon and the substrate layers. The depletion transistor on the left has a longer drain, which adds resistance and reduces current during oscillation starts. The large typical power switch MOS transistor on the right has maximum driving capability to ensure high switching efficiency after oscillation starts.

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4. The LTC3108 includes a synchronous rectifier, a shunt regulator, an LDO regulator, the bandgap reference, current-limit charge switches, and trimming circuits. [Image courtesy of Chipworks]

Functionally, the LTC3108 consists of a power converter, a low-dropout (LDO) voltage regulator, a reference generator, an output-voltage circuit (VOUT2), and a charge control circuit. It also includes a synchronous rectifier, a shunt regulator, the bandgap reference, current-limit charge switches, and trimming circuits (Fig. 4). The synchronous rectifier takes over the paralleled diode for higher rectification efficiency when the step-up converter has boosted the very low input voltage to a certain level (Fig. 5).

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5. The synchronous rectifier takes over the paralleled diode for higher rectification efficiency when the step-up converter has boosted the very low input voltage to a certain level.

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