UTCUT2306/UT2306-3.3V/UT2306-5.0V

LINEAR INTEGRATED CIRCUIT

7

QW-R103-029,A

DCR is important for best efficiency. Ideally, the inductor DCR should be less than 0.05W, although the physical size

of such an inductor makes its use prohibitive in many space conscious applications. If EMI is a concern, such as

when sensitive analog circuitry is present, a toroidal inductor such as the Coiltronics CTX20-1 is suggested.

A special case exists where the VOUT/VIN differential is high, such as a 2V to 12V boost converter. If the required

duty cycle for continuous mode operation is higher than the UT2306 can provide, the converter must be designed

for discontinuous operation. This means that the inductor current decreases to zero during the switch OFF time. For

a simple step-up (boost) converter, duty cycle can be calculated by the following formula:

where,

If the calculated duty cycle exceeds the minimum UT2306 duty cycle of 76%, the converter should be designed for

discontinuous mode operation. The inductance must be low enough so that current in the inductor reaches the

peak current in a single cycle. Inductor value can be calculated by:

where,

tON = Minimum on-time of UT2306 (4µs)

One advantage of discontinuous mode operation is that inductor values are usually quite low so very small units

can be used. Ripple current is higher than with continuous mode designs and efficiency will be somewhat less.

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors should be used at the output of the UT2306 to minimize output

ripple voltage. High quality input bypassing is also required. For surface mount applications AVX TPS series

tantalum capacitors are recommended. These have been specifically designed for switch mode power supplies and

have low ESR along with high surge current ratings. A 100

µF, 10V AVX TPS surface mount capacitor typically

limits output ripple voltage to 70mV when stepping up from 2V to 5V at a 200mA load. For through hole applications

Sanyo OS-CON capacitors offer extremely low ESR in a small package size. Again, if peak switch current is

reduced using the ILIM pin, capacitor requirements can be eased and smaller, higher ESR units can be used.

Diode Selection

Best performance is obtained with a Schottky rectifier such as the 1N5818. Motorola makes the MBRS130L

Schottky which is slightly better than the 1N5818 and comes in a surface mount package. For lower switch

currents, the MBR0530 is recommended. It comes in a very small SOD-123 package. Multiple 1N4148s in parallel

can be used in a pinch, although efficiency will suffer.

device is turned on. The high current flow can create IR drops along supply and ground lines or cause the input

supply to drop out momentarily. By adding R1 and C1 as shown in Figure 4, the switch current is initially limited to

R1 effectively takes C1 out of the circuit. R1 also provides a discharge path for C1 when SHUTDOWN is brought low

for turn-off.