LTC1705 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1705
Linear Technology
'LTC1705' PDF : 28 Pages View PDF
LTC1705
APPLICATIO S I FOR ATIO
converter is converting 15V to 1.3V, requiring just a 9%
duty cycle. Inductor voltages are now 13.7V when QT is on
and 1.3V when QB is on, giving vastly different di/dt values
and correspondingly skewed transient response with posi-
tive and negative current steps. The narrow 9% duty cycle
usually requires a lower switching frequency, which in
turn requires a higher value inductor and larger output
capacitor. Parasitic losses due to the large voltage swing
at the source of QT cost efficiency, eliminating any advan-
tage the 1-step conversion might have had.
Note that power dissipation in the LTC1705 portion of a
2-step circuit is lower than it would be in a typical 1-step
converter, even in cases where the 1-step converter has
higher total efficiency than the 2-step system. In a typical
microprocessor core supply regulator, for example, the
regulator is usually located right next to the CPU. In a
1-step design, all of the power dissipated by the core
regulator is right there next to the hot CPU, aggravating
thermal management. In a 2-step LTC1705 design, a
significant percentage of the power lost in the core regu-
lation system happens in the 5V or 3.3V supply, which is
usually away from the CPU. The power lost to heat in the
LTC1705 section of the system is relatively low, minimiz-
ing the heat near the CPU.
Additionally, with a 1-step converter, the high input battery
voltage requires the MOSFET to operate at high voltage
levels. This imposes stringent requirements on the
MOSFETs selection. Most of the MOSFETs that meet the
high voltage and high current requirements are expensive
and bulky. This makes for an awkward power supply
design, especially in portable applications. The high input
voltage also necessitates higher gate drive, which aggra-
vate switching losses.
LTC1628*
5V/3A LOGIC SUPPLY
3.3V/5A LOGIC SUPPLY
I/O
CORE
LDO
LTC1705
CPU SUPPLY CONTROLLER
1.5V/2A CPU I/O SUPPLY
1.3V/15A CPU CORE SUPPLY
2.5V/0.15A CPU CLOCK SUPPLY
*OR TWO LTC1625s
2-STEP CONVERSION OFFERS
• BETTER TRANSIENT RESPONSE
• SMALLER COMPONENT SIZE
• BETTER THERMAL MANAGEMENT
• LOWER VOLTAGE REQUIREMENT FOR MOSFETS
• SMALLER SWITCHING LOSS
• EQUIVALENT EFFICIENCY
1705 F10
Figure 10. 2-Step Conversion Block Diagram
23
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