LT1304-5.0V UTC, LT1304-5.0V Datasheet - Page 7

• Manufacturer Part Number: LT1304-5.0V
• Description: MICROPOWER DC/DC CONVERTERS
• Manufacturer: UTC

UTC LT1304/LT1304-3.3V/LT1304-5.0V
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 LT1304 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:
DC = 1 – [(V
where,
V
V
V
V
If the calculated duty cycle exceeds the minimum LT1304 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:
L = (V
where,
tON = Minimum on-time of LT1304 (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 LT1304 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 µ F, 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.
I
The LT1304’s current limit (I
current (about 1A) from the supply to charge the output capacitor. Figure 3 shows V
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
well under 1A as detailed in Figure 5. Current flowing into C1 from R1 and the I
R1 effectively takes C1 out of the circuit. R1 also provides a discharge path for C1 when SHUTDOWN is brought low
for turn-off.
UTC
LIM
IN
SAT
OUT
D
= Diode forward voltage (0.4V)
= Minimum input voltage
Function
= Switch saturation voltage (0.3V)
= Output voltage
IN
– V
SAT
IN
– V
)(t
ON
SAT
/1A)
)/(V
UNISONIC TECHNOLOGIES CO., LTD.
OUT
LIM
+ V
) pin can be used for soft start. Upon start-up, the LT1304 will draw maximum
D
)]
LINEAR INTEGRATED CIRCUIT
LIM
pin will eventually charge C1 and
OUT
and VIN waveforms as the
QW-R103-018,A
7