Low-voltage step-down converters Y Shi Friday, 27 October 2006

Existing low-voltage step-down converter On-Semi: MC34063 Very low cost: ~£0.09; overall cost: 40p Wide operating range: 3~40V I/P Adjustable O/P volatge Up to 1.5A with current limiting http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF Low switching frequency: <100kHz Bulky & costly LC filtering required Hysteresis ripple control: minimum ripple required Low efficiency: ~70%; Bipolar transistor-based switches No input voltage feed forward: bad line regulation No internal thermal protection

Improvement for MC34063 Low ripple configuration: Use ceramic capacitor R5 to create enough ripple: ~1R If Iout constant, Vout being constant NCP3063: a higher freq upgrade Freq operation of 150kHz Precision 1.5% reference. (2% of 34063) New feature: internal thermal shutdown Samples received Similar price to MC34063 http://www.onsemi.com/pub/Collateral/NCP3063%20DATA%20SHEET.PDF

Existing ST part: L5970D A5E00775658 Used in PN £0.36 Wide input range: 4.4V to 36V & adjustable 1.2~35V O/P Up to 1A O/P current High efficiency: DMOS switch with 250mR Rds(on) Precision 2% reference. Fixed freq 250kHz Voltage feed-forward Internal current limit and thermal shutdown

TI part: TPS5420 $1.70 MOQ@1K from TI website http://focus.ti.com/lit/ds/symlink/tps5420.pdf $1.70 MOQ@1K from TI website Wide input range: 5.5V to 36V Up to 2A O/P current (3A peak) High efficiency up to 95%: MOS switch with 110mR Rds(on) Precision 1.5% reference. Fixed 500kHz switching frequency: downsized & cheaper LC filter design Input voltage feed forward Over-current and thermal shutdown EVM received

NS part: LM25007 $1.05 MOQ@1K from NS website http://www.national.com/pf/LM/LM25007.html $1.05 MOQ@1K from NS website Wide input range: 9V to 42V Up to 0.5A O/P current High efficiency: N-Ch MOS switch with 740mR Rds(on) Precision 2% reference. Up to 800kHz switching frequency: downsized & cheaper LC filter design Constant ON time control with input voltage feed forward Current protection and thermal shutdown Chip and EVM received

Allegro part: ALGA8498SLJ-T http://www.allegromicro.com/sf/8498/ £0.42 @40K Wide input range: 8V to 50V & adjustable 0.8~24V O/P Up to 3A O/P current High efficiency: DMOS switch with ~500mR Rds(on) Precision 2% reference. Constant OFF time >100kHz switching frequency Thermal shutdown

Appendix: Linear step-down converters LT3493             3.6 to 36V input range. 1.2A output current. Fixed 750kHz switching frequency. Small cap’s and inductor. Small DFN package. LT3505             3.6 to 36V input range. 1.2A output current. Adjustable switching frequency to 3MHz. Small DFN package. LT3481             3.6 to 36V input range. 2A output current. Adjustable switching frequency to 2.8MHz. Small DFN package. LT3435 3.3 to 60V input range. 2.4A output current. Fixed 500kHz switching frequency. TSSOP package. Excellent performance Easy to hook up Small package Very expensive; normally > $2

Summary For performance: For O/P current: For the single chip cost: TI-TPS5420 > ST-L5970D > NS-LM25007 > Allegro-A8498 > NCP3063 >MC34063 For O/P current: A8498@3A > TPS5420@2A > NCP3063@1.5A = MC34063@1.5A >L5970D@1A > LM25007@0.5A For the single chip cost: MC34063 > NCP3063 > L5970D > A8498 > LM25007 > TPS5420 For the overall converter cost: NCP3063 > MC34063 > LM25007 > A8498 > L5970D > TPS5420 For performance-to-cost: TPS5420 > L5970D > LM25007 > NCP3063 > MC34063 > A8498