CMOS High efficiency PFM step-up DC/DC converter
ELM93xxxB
3) Capacitor
When choosing capacitor, please select that which is generally used for smoothing power supply circuit,
with comparatively large capacity and whose rated voltage is at least three times larger than rated output
voltage of used ELM93 series.
For ELM93 series, ELM recommends Aluminum electrolytic or Tantalum capacitor.
Under different conditions, such as different input voltage, load current, etc. larger output voltage ripple
may happen and thus result in intermittent switching. To solve this, using capacitors with larger capacity
can prevent output voltage ripple from becoming larger.
Inserting resistor, as shown in the following circuit, may also be effective.
፸፵፷ᔵ
ᎶᎼᎻ
፲
ᎌ᎓᎔ᎀ፺ᎿᎿᎿᎉ
ᎨᎵᎻᎨᎳᎼᎴ
ᎊᎨᎷᎨᎪᎰᎻᎶᎹ
᎓
᎖
■Design of DC/DC converter
In the DC/DC converter which consists of ELM93 series and above-mentioned external parts, the output is
acquired as follows:
1) Output current of DC/DC converter
The current from the output terminal (Vout) of the circuit is:
9Vin2
Iout = K
Duty=75%
*
32 • L • Fosc • (Vout-Vin)
K: the efficiency of whole circuit (≈75% to ≈85%)
2) Remedies for noise
This DC/DC converter may cause electromagnetic noise due to switching of coil under large current. Solu-
tion is necessary especially when the IC is used in wireless devices.
To reduce noise, this IC is designed in consideration of the coil switching characteristics. The following
methods are also effective to reduce noise.
• Use shield-type, or magnetic shield coil.
• Locate coil and diode to the LX terminal of IC as close as possible.
• Select ground wire as thick and short as possible.
• Connect ground wire of circuit to one point.
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ELM-TECH [ ELM Technology Corporation ]
