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

2. 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
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

3. 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

4. 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

5. TI part: TPS5420 $1.70 MOQ@1K from TI website
$1.70 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

6. NS part: LM25007 $1.05 MOQ@1K from NS website
$1.05 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

7. Allegro part: ALGA8498SLJ-T
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

8. 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

9. 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:
> > = >
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