Design Note 74: Techniques for Deriving 3.3V from 5V Supplies

Microprocessor chip sets and logic families that operate from 3.3V supplies are gaining acceptance in both desktop and portable computers. Computing rates, and in most cases, energy consumed by these circuits show a strong improvement over 5V technology. The main power supply in most systems is still 5V, necessitating a local 5V to 3.3V regulator.

Linear regulators are viable solutions at lower (IO ≤ 1A) currents, but they must have a low dropout voltage in order to maintain regulation with a worst-case input of only 4.5V. Figure 1 shows a circuit that converts a 4.5V minimum input to 3.3V with an output tolerance of only 3% (100mV). The LT1129-3.3 can handle up to 700mA in surface mount configurations, and includes both 16μA shutdown and 50μA standby currents for system sleep modes. Unlike other linear regulators, the LT1129-3.3 combines both low dropout and low voltage operation. Small input and output capacitors facilitate compact, surface mount designs.

Figure 1. Low Dropout Regulators Delivers 3.3V from 5V Logic Supply.

For the LT1129-3.3, dissipation amounts to a little under 1.5W at full output current. The 5-lead surface mount DD package handles this without the aid of a heat sink, provided the device is mounted over at least 2500mm2 of ground or power supply plane. Efficiency is around 62%.

Dissipation in linear regulators becomes prohibitive at higher current levels where they are supplanted by high efficiency switching regulators. A 2A, 5V to 3.3V switching regulator is shown in Figure 2. This synchronous buck converter is implemented with an LTC1148-3.3 converter. The LTC1148 uses both Burst Mode® operation and continuous, constant off-time control to regulate the output voltage, and maintain high efficiency across a wide range of output loading conditions. Efficiency as a function of output current is plotted in Figure 3.

Figure 2. 94% Efficiency Synchronous Buck Regulator Pumps Out 2A at 3.3V from 5V Logic Supply.

Figure 3. LTC1148-3.3: Measured Efficiency.

All of the components used in the Figure 2 switching regulator are surface mount types, including the inductor and shunt resistor, which are traditionally associated with through hole assembly techniques.

Depending on the application, a variety of linear and switching regulator circuits are available for output currents ranging from 150mA to 20A. Choices in linear regulators are summarized in Table 1. There are some cases, such as in minicomputers and workstations, where higher dissipations may be an acceptable compromise against the circuit complexity and cost of a switching regulator, hence the >1A entries. Heat sinks are required.

Table 1. Linear Regulators for 5V to 3.3V Conversion
Load Current Device Features
150mA LT1121-3.3
Shutdown, Small Capacitors
700mA LT1121-3.3
Shutdown, Small Capacitors
800mA LT1121-3.3
1.5A LT1086
DD Package
 3A to 7.5A LT1083
High Current, Low Quiescent Current at High Loads 
 10A  2 × LT1087  Parallel, Kelvin Sensed

Table 2 summarizes the practical current range of a number of switching regulators for 5V to 3.3V applications, along with their typical efficiencies.

Table 2. Switching Regulators for 5V to 3.3V Conversion
Load Current Device  Efficiency Features
200mA to 400mA
90% Internal P-Channel Switch, 1μA Shutdown
.5A to 2A
92% 8-Pin SO, High Efficiency Converter
1A to 5A
94% Ultra-High Efficiency Synchronous Converter
5A to 20A
91% Ultra-High Current Synchronous Converter

A 5V to 3.3V converter circuit collection is presented in Application Note 55, covering the entire range of currents listed in Tables 1 and 2.



Mitchell Lee