SP6641B Datasheet, PDF (2/5 Pages) Sipex Corporation – Low Noise Evaluation Board

USING THE EVALUATION BOARD
1) Powering Up the SP6641A/6641B Circuit
The SP6641A/6641B Evaluation Board can be powered from inputs from a +0.9V to +3.3V from
1 or 2 alkaline cells or a power supply. Connect with short leads directly to the “Vbatt” and
“Gnd” posts. Monitor the Output Voltage and connect the Load between the “Vout” post and the
“GND” post.
2) Using the J1 Jumper: Enabling the SP6641A/6641B Output and using the Shutdown
Mode
The SP6641A/6641B output will be enabled if the J1 Jumper is in the upper or pin 1 to 2
position. If J1 is in the pin 2 to 3 position, the Shutdown pin is brought to GND, which puts the
SP6641A/6641B in the low quiescent Shutdown Mode.
3) Selecting the Inductor
In selecting an inductor, the saturation current specified for the inductor needs to be greater
then the SP6641A/6641B peak current to avoid saturating the inductor, which would result in a
loss in efficiency and could damage the inductor. The SP6641A evaluation board uses a
Sumida CDRH5D28 22uH inductor with an Isat value of 0.9A and a DCR of 0.095ohms, which
easily handles the Ipeak of 0.33A of the SP6641A and will deliver high efficiencies. The
SP6641B evaluation board uses a Sumida CDRH5D28 10uH inductor with an Isat value of 1.3A
and a DCR of 0.065ohms, which easily handles the Ipeak of 1.0A of the SP6641B and will
deliver high efficiencies. Other inductors could be selected provided their Isat is greater than the
Ipeak of the SP6641A/6641B.
4) Selecting the Capacitors and Filter Components
For the SP6641A, 22uF ceramic capacitors are used, which work well due to their inherently low
ESR, which will help produce low peak to peak output ripple, and reduce high frequency spikes
as well due to their low impedance. To obtain low peak to peak ripple for the SP6641B, the
output capacitor needs to have ESR as low as 35 to 40 mohms and have a value of at least
100uF, for which the 330uF Sanyo POSCAP was chosen for its 40mohms max. ESR. Other
ultra low ESR capacitors such the radial lead 100uF Sanyo OSCON or one of the new SM low
ESR Polymer capacitors like the 100uF Panasonic UD series could be used. This low ripple is
obtained with no output spikes through the use of a LC filter consisting of small 0805 SM bead
core L2 and 10uF ceramic capacitor C3.
POWER SUPPLY DATA
Power supply data is provided for the “B” version of the SP6641B, which has a peak inductor
current limit of 1A, with the 3.3V output option. Figure 1 shows the switching signals still present
as 1.0V spikes on the 330uF capacitor C2. Note that no output spikes are seen in figures 2 and
3 through the use of a small LC filter after the 330uF capacitor. The SP6641B’s low output ripple
of 45 to 50mVpp is shown for a typical input of 2.0V to 3.0V and an output load of 300mA.