ELM94xxB CMOS PFM step-up DC/DC converter
■External parts
To design DC/DC converter with ELM94 series, coil, diode, and capacitor are necessary.
(Refer to standard circuit configuration.)
1) Coil
When choosing choke coil, please select that whose core is not magnetically saturated, DC resistance is
low, and which has sufficient margin for rated current.
For ELM94 series, ELM recommends following coil.
• CM-5/CM-5N (Sumida Electric Co., Ltd.)
2) Diode
When choosing diode, please select that whose forward voltage is small, switching speed is high and which
has sufficient margin for rated current.
For ELM94 series, ELM recommends schottoky diodes.
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 ELM94 series.
For ELM94 series, ELM recommends Aluminum electrolytic or Tantalum capacitor.
■Design of DC/DC converter
In DC/DC converter which consists of ELM94 series and above-mentioned external parts, the output is
acquired as follows:
1) Output current of DC/DC converter
The current from output terminal (Vout) of the circuit is:
Vin 2
Iout = K
Duty=50%
*
8 • 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 coil switching characteristics. The following meth-
ods 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 ]
