Introduction
Two important considerations for the power supplies used in portable electronic products are small overall size and high efficiency for longer battery life. The LTC1779, a constant frequency, current-mode step-down DC/DC converter is designed to provide both.
The LTC1779 is offered in a tiny SOT-23 package. External component count is kept to a minimum as both the high-side 250mA PMOS switching element and the current sense resistor are included on the IC. A high constant operating frequency of 550kHz (typical) allows for the use of a small external inductor.
The LTC1779 consumes only 135µA of quiescent current in normal operation and a mere 8µA in shutdown. The device features Burst Mode™ operation, which reduces switching losses at light load currents, thereby improving efficiency. To further extend battery life, the internal P-channel MOSFET is turned on continuously in dropout (100% duty cycle).
The LTC1779 boasts a ±2.5% output voltage accuracy. The wide input voltage range of 2.4V to 9.8V is suited for single or dual Li-Ion and 9V alkaline applications.
Single Lithium-Ion to 1.8V/250mA Step-Down Converter
Figure 1 shows the LTC1779 in a 1.8V/250mA application powered from a single Li-Ion battery input. Figure 2 shows an oscilloscope photograph of the pin 6 switching node (SW) with VIN = 3.6V and IOUT = 100mA. The switch duty cycle is equal to (VOUT + VD)/(VIN + VD), where VD is the forward voltage drop of the external diode D1 and is approximately 53%. Figure 3 is a plot of efficiency vs load current over the useful life of a single Li-Ion battery (4.2V down to 2.7V). The measured peak efficiency is 87% with VIN = 4.2V and 91% with VIN = 2.7V. For an ideal linear regulator, the corresponding efficiencies would be only 43% and 67%, respectively.
Figure 1. LTC1779 minimum component count, single Li-Ion to 1.8V/250mA step-down converter.
Figure 2. Pin 6 switching node waveform for Figure 1’s circuit.
Figure 3. Efficiency vs load current for Figure 1’s circuit.
Conclusion
A wide input range of 2.4V to 9.8V, low quiescent and shutdown current, high efficiency, low external parts count and small package size make the LTC1779 ideal for portable applications.
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