LT1304 - 5V from two NiMH batteries, even with an added value

To obtain 3.3 or 5V from a lower voltage is a typical requirement at battery-powered or portable devices. Naturally, to obtain 5V it is also possible to use 4-5 NiMH cells or 2 Li-Po cells, but then the device is uselessly bigger, heavier and more expensive. The solution is just the usage of a suitable step-up circuit. LT1304 from Linear Technology belongs to „MicroPower“ DC/DC step-up circuits operating already from a very small voltage, 1.5V typically. Thanks to a very low power consumption (10uA/Shutdown) and a built-in independent low battery detector, it is optimized for usage with batteries. LT1304 needs for its operation only a few external components - 2 capacitors, 1 inductor and 1 diode is all that is necessary to create an operating DC/DC circuit.
LT1304 is available in 3 versions - LT1304VCS8-5, LT1304CS8-3,3 with fixed voltages and LT1304CS8 with adjustable output. Versions with fixed output voltages don´t need a divider in a feedback, what further simplifies a circuit.

The input for low battery monitoring - LBI (1,17V) operates even in Shutdown mode (SHDN). LBI can be connected through an appropriate voltage divider, setting a level at which the LBO output will switch (open collector). Maximal current through a coil and an internal switch is limited to 1A. If the current was to exceed this level, LT1304 will shorten the t_on in a given cycle, thus preventing a further increase of current. A big plus is the I_LIM input, where we can adjust (decrease) the max. coil current by adding a single resistor. In this case, LT1304 shortens the t_ON of every cycle. In many cases, this can help prevent coil saturation and alternatively also to decrease DC (Ohmic) losses in a coil.

LT1304 works in a „BurstMode“, i.e. in the mode of continuous operation in so called “bursts” with a fixed length of every cycle - t_ON (6 us) and t_OFF (1.5us). Only when a current limitation is adjusted, the t_ON period gets shortened if necessary. It means, that D (duty cycle = t_ON/(t_ON+t_OFF)) is commonly 0.8 or lower, if the current limitation will act. For a continuous mode of operation of every step-up converter is valid, that D=1-V_IN/V_OUT, i.e. the output voltage depends only from D. That´s why it´s good to keep on mind, that if we want to take a considerable current from the input, we can´t use an excessively low value of coil inductance at the same time, because the inner current limitation will act by shortening t_ON, what will decrease the D value. If we´ll drop below the D value lower than minimum necessary to reach a desired output voltage, the output voltage will decrease. If we only need a small output current but a high output voltage (for example 12V from 2V), it is necessary to use a discontinuous mode, when a coil current drops in every cycle down to zero, as it is described in the datasheet. The disadvantage is bigger ripple, an advantage is a relatively very small inductor necessary in this case.
Choice of an inductor is not critical and the circuit will operate properly in a wie range of inductance values. Guide values are mentioned in the datasheet, however it is worth to pay attention to its choice, because it influences the efficiency of the circuit and the output voltage ripple. Excessively low value of L will cause high current ripple resulting in increased demands for input and output filtering capacitors and it will also worsen the EMI. On the other side, an excessively high value of L will worsen a response of the circuit at a sudden change of load.

Detailed information can be found in LT1304 datasheet and in the documents Design Note 120 and Design note 154. For the development support is also available the LT1304 evaluation board - DC088.

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